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Ion collection by probes in strong magnetic fields with plasma flow
Abstract
Fluid-theory calculations are presented of ion collection by electric probes in strongly magnetized plasmas with parallel flow. In the first calculations the problem is treated in a one-dimensional approximation but the cross-field transport of momentum is included in such a way as to model different ratios of viscosity to diffusivity. The results show that the flow deduced from probe measurements is not particularly sensitive to the assumed viscosity, provided it is finite. However, results with zero viscosity are qualitatively different from those with nonzero viscous momentum transport. The second set of calculations is two dimensional but only for fixed (unity) ratio of viscosity to diffusivity. The results are in remarkably good agreement with the corresponding one-dimensional model.
... Figure 5 and Table 1 show the results of the current densities calculated using Equation (9) with pressure variation, which was used to calculate the ratio of upstream to downstream current densities. Using various models [15][16][17][18][19][20][21] with Equations (8) and (9) for the plasma flow velocity, M ∞ was defined as Te ~ 1 eV was measured by a Langmuir probe, and Ti = 0.1Te was assumed for the calculation of = ( + )/ . The difference in between the approximation condition (Te ~ 1 eV) and the measured results (Te = 0.92-1.09 ...
... Therefore, the fixed condition (Te = 1 eV) was used for the Bohm velocity (1629 m/s). The calibration factors for collisionless conditions have been studied using various kinetic and fluid models [19][20][21]. However, the overestimation of plasma flow velocity in collisional plasmas measured by an MP has been reported because of the effect of ion-neutral collision [22][23][24]. ...
... Therefore, the fixed condition (T e = 1 eV) was used for the Bohm velocity (1629 m/s). The calibration factors for collisionless conditions have been studied using various kinetic and fluid models [19][20][21]. However, the overestimation of plasma flow velocity in collisional plasmas measured by an MP has been reported because of the effect of ion-neutral collision [22][23][24]. ...
To validate the possibility of the developed microwave plasma source with a novel structure for plasma aerosol deposition, the characteristics of the plasma flow velocity generated from the microwave plasma source were investigated by a Mach probe with pressure variation. Simulation with the turbulent model was introduced to deduce calibration factor of the Mach probe and to compare experimental measurements for analyses of collisional plasma conditions. The results show calibration factor does not seem to be a constant parameter and highly dependent on the collision parameter. The measured plasma flow velocity, which witnessed fluctuations produced by a shock flow, was between 400 and 700 m/s. The optimized conditions for microwave plasma assisted aerosol deposition were derived by the results obtained from analyses of the parameters of microwave plasma jet. Under the optimized conditions, Y2O3 coatings deposited on an aluminum substrate were investigated using scanning electron microscope. The results presented in this study show the microwave plasma assisted aerosol deposition with the developed microwave plasma source is highly feasible for thick films with >50 μm.
... Assuming the upstream and downstream far-field plasma has identical n e and T e , that the pins are identical in site and using the substitution expressions for the current ratio of the parallel particle flux. The models correspond to references [114,115,116,107,117,118]. ...
... In Fig. 3.14 (b) some of these models are compared, based on broad discussion in [115], with the result of Eq. (3.13). Despite its simplification, the latter is extremely close to the 1D and 2D fluid result of Hutchinson [116]. It is not immediately obvious why agreement is so good and it is almost certainly at least partly coincidental. ...
... Statistical analysis of time-sequences with 5 ms duration is performed in exactly the same way on data from both experiment and code. The main statistical characteristics of three identical TCV discharges #[24529,24530,24532] from both reciprocations (at medium and high density) are compared in the next subsections with the results of a single ESEL run (116) in terms of density, temperature, particle flux, potential spatial dimensions and detail waveforms. Section 4.2 has already provided a detailed experimental description of discharge #24530. ...
... Mach probes (MP) are electrostatic probes that measure the flow Mach number, M = v f /c s , where v f is the flow velocity and c s is the sound speed. 12 The basic design includes two probe tips separated by an insulator and biased into the ion saturation regime. For a stationary plasma, each probe tip will draw equal ion saturation currents. ...
... However, if plasma is flowing perpendicularly to the collection surfaces (as shown in Figure 5), the upstream collection area will draw more current than the downstream side. The Mach number and flow velocity can then be determined from 12,13 ...
... Each plate is biased into ion saturation at -27 V and the current is measured over 9.96 kΩ sense resistors. Significant analytical and experimental work has been performed to determine exact values of k for different probes in both magnetized 12,14,15 and unmagnetized 13,16,17 plasmas. Since our probe is located far from the source, the magnetic field is negligible and the probe should be analyzed using unmagnetized Mach probe theory. ...
... Approaching the target surface the Mach number M = u/c s increases from values of around 0.5 to 1 on typical scales of λ a = 30 mm and λ b = 5 mm, respectively. In order to explain these very short scale lengths the measured data were compared with a collisional-diffusive model in the case of (a) and with Hutchinson's model [27] in the case of (b). A good agreement was achieved in (a) by assuming a very low neutral gas temperature of about 400 K. ...
... λ b = 5 mm an. Um diese kurzen Längen erklären zu können wurden die Messdaten in (a) mit einem Stoß-Diffusionsmodell und im Falle von (b) mit dem Modell von Hutchinson [27] verglichen. Eine gute Übereinstimmung in (a) wurde erzielt, wenn eine sehr niedrige Neutralgastemperatur von etwa 400 K angenommen wird. ...
... An electrical collector is located in the center of each head, which measures the current-voltage characteristics. The probe then also constitutes a Mach probe [27]. ...
In der vorliegenden Arbeit wurde das Strömungsverhalten eines magnetisierten Argonplasmas beim Auftreffen auf eine neutralisierende Oberfläche untersucht. Mit Hilfe der Laserinduzierten Fluoreszenz wurde dazu nicht-invasiv die Geschwindigkeitsverteilung der Ionen mit einer Ortsauflösung von standardmäßig dz=0.5 mm als Funktion des Abstandes zur Oberfläche gemessen. Zwei Situationen wurden untersucht (a): praktisch das ganze Plasma strömt auf ein großes Target (Durchmesser 100 mm) und (b) die Größe des Targets ist wesentlich kleiner (Durchmesser 15 mm) als der Durchmesser der Plasmasäule. Unmittelbar vor der Oberfläche war in beiden Fällen die Strömungsgeschwindigkeit u mindestens so groß wie die Ionenschallgeschwindigkeit cs, genau wie von Bohm bereits 1949 vorhergesagt[]. Unter fusionsrelevanten Bedingungen ist dies die erste direkte Beobachtung des Bohmkriteriums. Bei Annäherung an die Oberfläche steigt die Machzahl M=u/cs von 0.5 auf 1 auf typischen Skalenlängen lambda_a=30 mm bzw. lambda_b=5 mm an. Um diese kurzen Längen erklären zu können wurden die Messdaten in (a) mit einem Stoß-Diffusionsmodell und im Falle von (b) mit dem Modell von Hutchinson[] verglichen. Eine gute Übereinstimmung in (a) wurde erzielt, wenn eine sehr niedrige Neutralgastemperatur von etwa 400 K angenommen wird. Die Messdaten in (b) werden sehr gut durch das Modell wiedergegeben, wenn ein Transportkoeffizient von D=20 m²/s angenommen wird. Ein derartig hoher Transport kann unmöglich allein durch Diffusion verursacht werden. Teilweise kann dieser Transport anhand der endlichen Gyroradien erklärt werden, vermutlich aber spielen auch zeitabhängige Phänomene, wie z.B. Driftwellen eine wichtige Rolle. Weiterhin wurde die Abhängigkeit von dem Winkel zwischen Flächennormalen und B-Feld untersucht. Die unmittelbar vor der Oberfläche auftretenden Überschallströmungen werden verhältnismäßig gut von dem Modell von Chodura[] beschrieben. Im Gegensatz dazu ist die Größe der Zone in der Machzahlen größer eins auftreten deutlich kleiner, als vom Modell vorhergesagt.
... The probe consists of two tungsten rods and yields current-voltage characteristics, i.e. the relation between the probe current and applied AC bias voltage with its frequency of 60 Hz between the two rods. The rod has a 0.8 mm diameter and 4 mm length [27]. Figure 5(a) shows an example of waveforms of probe current and voltage monitored using the double probe. ...
... where k is a constant depending on plasma magnetism and here, an unmagnetized model k = 1.26 is adopted [27]. We evaluate a diamagnetic azimuthal current j θ_dia . ...
The rotating magnetic field (RMF) acceleration method is a newly proposed approach to enhance the performance of electrodeless radio frequency (RF) plasma thruster. In the previous study, electron current drive in the azimuthal direction was observed using the RMF method, which resulted in the electromagnetic force in the presence of a magnetic nozzle. To further optimize the acceleration effect with the RMF method, we investigated the dependence of spatial profiles of plasma parameters and the driven current density on the RMF field strength. We observed a higher azimuthal-current density in plasma compared to the previous campaign. According to spatial electrostatic probe measurements, the ion Mach number spatially increases with the increase in RMF strength. The ion acceleration on the z-axis can result from the presence of spatial convergence of electron pressure due to radial electron transport. Total thrust composed of a static pressure term and electromagnetic force increases with higher RMF strength under the full penetration condition of the RMF. We clarified the strengthening RMF field contributes to the enhancement of the azimuthal current and spatial ion acceleration effect, leading to the thrust increment. These findings, although the thrust performance is not yet at a practical level, hold significant potential for the future optimization of the RMF acceleration method applied to electrodeless RF plasma thrusters.
... Probes can measure main ion plasma flow by using a Mach tip configuration and appropriate interpretation by theory [79][80][81], technique that has been tested by successful comparison of probe data with standard (C 6+ ) and main ion charge exchange recombination, CER in DIII-D, as we can see in figure 7. where I I , 1 2 are the saturation currents from tips facing opposite directions along the magnetic field, K 1.7 is a constant obtained from a collisionless kinetic model [80] and M is the Mach number. We will consider two relevant types of probes, wall probes, which are fixed to the tokamak plasma facing structures and moving probes, which can be used to produce profiles of the bulk plasma. ...
... The poloidal E B drift, due to the radial electric field, E , r is chiefly caused by the radial gradient in T e and the sheath at the targets, and the radial E B drift due to the poloidal electric field E , q is caused chiefly by parallel T e gradients unless currents to the divertor, due to polarization and diamagnetic currents [169], are involved, which would change a straightforward dependence of the fields on T e gradients. Early work at ALCATOR [79], JT-60U [150] and JET [41] suggested the E B drifts to explain the asymmetries and eventually, drift-enabled simulations [42] proved numerical that the E B drifts were strong enough to explain the observations by producing a circulation pattern as that shown in figures 3 and 4, but direct measurements and quantitative evidence were unavailable. ...
Detached divertor plasmas feature strong radial and parallel gradients of density, temperature, electric fields and flow over the divertor volume and therefore, sampling the divertor plasma directly provides crucial knowledge to the interpretation and modeling efforts. We review the contribution of diagnostics that directly sample the plasma to the advancement of knowledge of the physics of detachment and detached divertors, such as the characteristics of the various regimes, discovery and quantification of drifts and identification of convection of heat and particles. We focus on wall probes, scanning probes, retarding field analyzers and Thomson scattering in the divertor region and also include the contribution of measurements away from the divertor that provide insight on how divertor detachment affects core, edge or pedestal conditions. Wall probes are critical as they can be installed in closed volumes of difficult access to other diagnostics and measure plasma parameters at the divertor structures, which define the plasma boundary conditions and where detachment effects are more likely to be strongest.
... One might argue that treating a two-(2D) or even three-dimensional (3D) situation might limit the accuracy of a 1D approach. Therefore, Hutchinson constructed a 2D code [Hut88a] of which the calculations have shown quite remarkable quantitative agreement with the corresponding results obtained from the 1D code. The agreement between the 1D and 2D approach is based on the fact that the nature of cross-field transport, whether it is cross-field current or coherent (perpendicular) flow, is not affecting the total collected current. ...
... is an assumption identical to the condition of axi-symmetry in a tokamak when treating the flow towards the limiter and divertor surface[Bael91,Chod88,VS98b]. The resulting transport equations in the y -direction are then retransformed back to the ( )//, ⊥ coordinate system.The main assumption here is that v ⊥ remains constant justified from the argumentation in[Hut88a,b]. The unperturbed plasma, i.e. the plasma at infinite distance from the probe, is thus described by the parallel Mach number = . ...
Energie is de levensadem van een moderne maatschappij. Ze is noodzakelijk om aan de menselijke behoeften, de toenemende levensverwachting en de stijgende levensstandaard te blijven voldoen. Thermonucleaire kernfusie is een (bijna) niet-vervuilende, veilige en zo goed als onuitputtelijke energievorm voor de toekomst. Fusie van lichte positief geladen deeltjes (kernen) gebeurt enkel bij extreem hoge temperaturen. De brandstof (het gas) is dan volledig geïoniseerd. We spreken dan van een plasma, ook wel de vierde aggregatietoestand genoemd. Het plasmavolume kan worden beperkt en opgesloten door middel van magnetische opsluiting zoals in een ‘tokamak’. Deze fusiemachine bestaat uit een torusvormige vacuümkamer waarin sterk magnetische velden het plasma opsluit en waarin temperaturen van meer dan 100 miljoen graden kunnen worden bereikt. Het experimentele werk, voorgesteld in dit doctoraatswerk, is uitgevoerd op twee tokamaks: TEXTOR (Jülich, Duitsland) en CASTOR (Praag, Tsjechië). Het onderzoek concentreert zich op de studie van randplasma’s met behulp van elektrische sondes. De belangrijke ongewenste energie- en deeltjesverliezen, die de opsluitingstijd van het hete centrumplasma beperken, staan centraal. Sondes lijden helaas onder aan de empirische ‘wet van diagnostieken’; het ‘gemak’ van interpretatie is omgekeerd evenredig met het gemak aan implementatie. De moeilijkheid bij de interpretatie van de gemeten plasmastromen ligt in het beschrijven ervan. Hoe precies stoort de sonde lokaal het plasma? Hoe zijn de lokale plasmaparameters gerelateerd tot het ongestoorde plasma veraf? Het eerste theoretische gedeelte van deze thesis neemt een ééndimensionaal quasi-neutraal vloeistofmodel onder de loep. Het sondemodel wordt verfijnd, gevalideerd, uitgebreid en het venster van toepasselijkheid wordt nauwkeurig gedefinieerd. In het tweede deel van dit doctoraatswerk staat een nieuwe geavanceerde sonde centraal. Deze gesofisticeerde sonde is ontworpen voor het gelijktijdig meten van allerlei randplasmaparameters in de TEXTOR tokamak. We tonen aan dat deze diagnostiek accuraat en betrouwbaar is. Het simultaan meten van al deze plasmaparameters en hun fluctuaties is een wereldwijd unicum. In het laatste deel kadert in de studie naar mechanismen die verantwoordelijk zijn voor de geobserveerde en ongewenste toename van deeltjestransport naar buiten toe tijdens instabiliteiten in randplasma’s. De experimentele resultaten zijn uitgevoerd op de tokamak CASTOR tijdens toegewijde experimenten van verbeterde plasmaopsluiting. Een belangrijke ontdekking is de geobserveerde dynamische koppeling tussen de parallelle plasmastroming en het radiale transport. Gelijkaardige fenomenen van niet-lineaire dynamische interacties tussen turbulent transport en parallelle stroming zijn gerapporteerd op de veel grotere tokamak JET onder compleet verschillende plasmacondities waarmee wordt gesuggereerd dat de achterliggende fysica van universele aard is. Er wordt bovendien aangetoond dat de fysica van stromingen in het randplasma nood heeft aan een driedimensionale beschrijving waarin de verscheidene componenten die het fenomeen voortbrengt dienen te worden beschouwd.
... One might argue that treating a two-(2D) or even three-dimensional (3D) situation might limit the accuracy of a 1D approach. Therefore, Hutchinson constructed a 2D code [Hut88a] of which the calculations have shown quite remarkable quantitative agreement with the corresponding results obtained from the 1D code. The agreement between the 1D and 2D approach is based on the fact that the nature of cross-field transport, whether it is cross-field current or coherent (perpendicular) flow, is not affecting the total collected current. ...
... is an assumption identical to the condition of axi-symmetry in a tokamak when treating the flow towards the limiter and divertor surface[Bael91,Chod88,VS98b]. The resulting transport equations in the y -direction are then retransformed back to the ( )//, ⊥ coordinate system.The main assumption here is that v ⊥ remains constant justified from the argumentation in[Hut88a,b]. The unperturbed plasma, i.e. the plasma at infinite distance from the probe, is thus described by the parallel Mach number = . ...
Energie is de levensadem van een moderne maatschappij. Ze is noodzakelijk om aan de menselijke behoeften, de toenemende levensverwachting en de stijgende levensstandaard te blijven voldoen. Thermonucleaire kernfusie is een (bijna) niet-vervuilende, veilige en zo goed als onuitputtelijke energievorm voor de toekomst. Fusie van lichte positief geladen deeltjes (kernen) gebeurt enkel bij extreem hoge temperaturen. De brandstof (het gas) is dan volledig geïoniseerd. We spreken dan van een plasma, ook wel de vierde aggregatietoestand genoemd. Het plasmavolume kan worden beperkt en opgesloten door middel van magnetische opsluiting zoals in een ‘tokamak’. Deze fusiemachine bestaat uit een torusvormige vacuümkamer waarin sterk magnetische velden het plasma opsluit en waarin temperaturen van meer dan 100 miljoen graden kunnen worden bereikt. Het experimentele werk, voorgesteld in dit doctoraatswerk, is uitgevoerd op twee tokamaks: TEXTOR (Jülich, Duitsland) en CASTOR (Praag, Tsjechië). Het onderzoek concentreert zich op de studie van randplasma’s met behulp van elektrische sondes. De belangrijke ongewenste energie- en deeltjesverliezen, die de opsluitingstijd van het hete centrumplasma beperken, staan centraal. Sondes lijden helaas onder aan de empirische ‘wet van diagnostieken’; het ‘gemak’ van interpretatie is omgekeerd evenredig met het gemak aan implementatie. De moeilijkheid bij de interpretatie van de gemeten plasmastromen ligt in het beschrijven ervan. Hoe precies stoort de sonde lokaal het plasma? Hoe zijn de lokale plasmaparameters gerelateerd tot het ongestoorde plasma veraf? Het eerste theoretische gedeelte van deze thesis neemt een ééndimensionaal quasi-neutraal vloeistofmodel onder de loep. Het sondemodel wordt verfijnd, gevalideerd, uitgebreid en het venster van toepasselijkheid wordt nauwkeurig gedefinieerd. In het tweede deel van dit doctoraatswerk staat een nieuwe geavanceerde sonde centraal. Deze gesofisticeerde sonde is ontworpen voor het gelijktijdig meten van allerlei randplasmaparameters in de TEXTOR tokamak. We tonen aan dat deze diagnostiek accuraat en betrouwbaar is. Het simultaan meten van al deze plasmaparameters en hun fluctuaties is een wereldwijd unicum. Het laatste deel kadert in de studie naar mechanismen die verantwoordelijk zijn voor de geobserveerde en ongewenste toename van deeltjestransport naar buiten toe tijdens instabiliteiten in randplasma’s. De experimentele resultaten zijn uitgevoerd op de tokamak CASTOR tijdens toegewijde experimenten van verbeterde plasmaopsluiting. Een belangrijke ontdekking is de geobserveerde dynamische koppeling tussen de parallelle plasmastroming en het radiale transport. Gelijkaardige fenomenen van niet-lineaire dynamische interacties tussen turbulent transport en parallelle stroming zijn gerapporteerd op de veel grotere tokamak JET onder compleet verschillende plasmacondities waarmee wordt gesuggereerd dat de achterliggende fysica van universele aard is. Er wordt bovendien aangetoond dat de fysica van stromingen in het randplasma nood heeft aan een driedimensionale beschrijving waarin de verscheidene componenten die het fenomeen voortbrengt dienen te worden beschouwd.
... With the conversion factors, resistance, and collective area, M ∞ was determined using the voltages measured by each probe. In addition, it was calculated using various models for the MP [25][26][27]. ...
A normalized plasma flow velocity in highly collisional plasma formed by a microwave plasma jet, which is dimensionless unit for plasma flow velocity/ion acoustic velocity, was measured by the parallel Mach probe. To deduce the normalized plasma flow velocity under highly collisional plasma conditions, the collisional model of a Mach probe was proposed. In addition, neutral gas flow velocity which assumed to be plasma flow velocity was calculated by the turbulent model. The results for the two different models were compared with those for the collsionless models of the Mach probe. The turbulent model produced 2–4 times reduced values than by measurements with collsionless models. The measured results with the collisional model were shown as approximately 100–250% lower than those for collsionless models. They were obtained to be in good agreement with difference rate of 10–30% when compared to those for the turbulent model.
... It was equipped with three different probe heads including: (a) A multi-pin probe (named '14-pin probe' and described in detail in [26]), which was used to measure ion saturation current (I sat ) in several Langmuir pins separated radially and poloidally. Part of the discharges also featured a sweeping pin, providing measurements of T e [27], and some featured a Mach probe, yielding measurements of the parallel Mach number, M || [28]. (b) A retarding field analyzer (RFA), used to obtain measurements of T i . ...
In this work we carry out quantitative measurements of particle and heat transport associated to SOL filaments in a tokamak, and relate density shoulder formation to the advection of energy in the far SOL. For the first time, this attempt includes direct measurements of ion and electron temperatures for background and filaments. With this aim, we combine data from a number of equivalent L-mode discharges from the ASDEX Upgrade tokamak in which different probe heads were installed on the midplane manipulator. This approach is validated by a comparison with independent diagnostics. Results indicate an increase of heat transport associated to filaments after the shoulder formation. Several centimeters into the SOL, filaments are still found to carry a substantial fraction (up to one fifth) of the power ejected at the separatrix.
... due to presheath density drop. The real value, however, strongly depends on plasma flow velocity [15]. The last term in (2) is the effective probe collecting area A coll , which is defined by the boundary of the Debye sheath surrounding the probe, since all particles crossing this limit are thought to be collected. ...
The evaluation of probe measurements in the swept regime can be performed via a fit to the ion branch of the current-voltage (I–V) characteristic, providing the most relevant plasma parameters such as the ion saturation current, the electron temperature and the floating potential. In this paper, we present a parametric study based upon the existing flush-mounted probe setup briefly operated at the COMPASS tokamak, aiming to compare relevant I–V formulas widely used for such analysis. Selected four-parametric fit descriptions were applied to synthetic data obtained by simulation of the probe in a particle-in-cell model, SPICE, both in simplified 2D and more complex 3D geometries. Plasma parameters recovered by the fit were compared to the input from the model and the precision of this recovery was mapped to a wide range of parameters, especially with respect to the ratio of the probe size dpin to the Larmor radius rL. Results show that while the electron temperature and the floating potential can be obtained quite precisely regardless of the method, the ion saturation current can be identified incorrectly, mainly due to the fact that the sheath expansion description by optical approximation is not sufficient for almost grazing field line angles of incidence, especially when dpin ∼ rL. We show, however, that operation of flush-mounted probes can be fruitful especially in high-field devices. Additionally, we have explored the sensitivity on the available bias potential range, showing that for proper analysis of I–V characteristic using four-parametric fit, the lower bound of the fit range should be at least 4 k B T e / e below the floating potential. If this condition cannot be satisfied, three-parametric fit in the range close to the floating potential can still produce results with acceptable precision.
... which has been demonstrated theoretically and experimentally. 5,[12][13][14] In these fluid models of the Mach probe, the formula has the same exponential form, while the calibration factor K is varied depending on the various plasma parameters, such as ion temperature, 5 viscosity, 13,14 and collisionality. The magnetic field in the edge region on W7-X is 2.2-2.3 ...
Ion flow velocity measurement in the edge and scraper-off layer region is beneficial to understand the confinement related phenomenon in fusion devices such as impurity transport and plays an important role in impurity control. During the Wendelstein 7-X (W7-X) operation phase 1.2a, a multi-channel (MC) Mach probe mounted on the multi-purpose manipulator has been used to measure radial profiles of edge ion flow velocity. This MC-Mach probe consists of two polar and two radial arrays of directional Langmuir pins (28 pins in total) serving for different aims, of which the polar arrays could obtain a polar distribution of ion saturation current, while the radial arrays can be used to study the dynamic process of a radially propagated event. In this paper, we report the observation of the radially outward propagation of a low frequency mode with a speed of around 200 m/s. The first measurement of the radial ion flow velocity profile using the MC-Mach probe in the boundary plasma of the W7-X with an island divertor will also be presented.
... It was equipped with three different probe heads including: (a) A multi-pin probe (named '14-pin probe' and described in detail in [26]), which was used to measure ion saturation current (I sat ) in several Langmuir pins separated radially and poloidally. Part of the discharges also featured a sweeping pin, providing measurements of T e [27], and some featured a Mach probe, yielding measurements of the parallel Mach number, M || [28]. (b) A retarding field analyzer (RFA), used to obtain measurements of T i . ...
In this work we carry out quantitative measurements of particle and heat transport associated to SOL filaments in a tokamak, and relate density shoulder formation to the advection of energy in the far SOL. For the first time, this attempt includes direct measurements of ion and electron temperatures for background and filaments. With this aim, we combine data from a number of equivalent L-mode discharges from the ASDEX Upgrade tokamak in which different probe heads were installed on the midplane manipulator. This approach is validated by a comparison with independent diagnostics. Results indicate an increase of heat transport associated to filaments after the shoulder formation. Several centimeters into the SOL, filaments are still found to carry a substantial fraction (up to one fifth) of the power ejected at the separatrix.
... For the simulation evaluation, we have used the coefficient k = 0.35 from the Hutchinson theory [24] multiplied by a factor of 2, since in this simulation scenario the probe collects the current from two sources (upstream and downstream the magnetic field direction). ...
Langmuir probes are a widespread tool for measurement of important plasma parameters such as electron temperature Te, plasma electron density ne, ion saturation current Isat and the floating potential Vfl, which are obtained from a fit to the current–voltage (I–V) characteristic of the probe. In magnetized plasmas, the measurements can be affected by sheath expansion due to large negative bias voltages, which is addressed by the introduction of a fourth parameter to the fitting function correcting the values of all measured quantities. In order to derive the plasma density from Isat, the understanding of probe ion collection is needed. In magnetized plasmas, the collecting area may not correspond to the real geometrical probe surface due to ion finite Larmor effects and so the derivation of Isat (and hence n) can be rather complicated. In this work, the influence of magnetic fields on the probe effective collecting area is studied by the means of fully 3D3V particle-in-cell model SPICE3. A parameter scan based on properties of scrape-off layer plasmas at COMPASS tokamak as well as a particular probe pin used on a horizontal reciprocating manipulator is performed. The results reveal that the presence of the probe head has a substantial effect on the outcome of the measurement as it forms a magnetic presheath at the probe location. An approximate formula for addressing the change of effective collecting area is presented and the data from the simulations are compared to measurements of COMPASS reciprocating probes and lithium beam emission spectroscopy.
... Similarly, in the field of plasma propulsion scheme, a spatial profile measurement of important plasma parameters is being carried out to estimate the acceleration effect and the thrust performance. As in situ plasma diagnostics, some measurement techniques have been adopted: Langmuir probe, 3 Mach probe, 4,5 and double probe 6 are major methods due to an easy introduction of local values, although these tools may disturb plasma flow and affect the plasma characteristics. On the contrary, electromagnetic and spectroscopy methods, e.g., microwave interferometer, 7 Laser Induced Fluorescence (LIF) method, [8][9][10] and Laser Thomson Scattering (LTS), 11 are effective ways in terms of non-direct contact measurements with a plasma with good measurement accuracy. ...
A two-dimensional scanning probe instrument has been developed to survey spatial plasma characteristics in our electrodeless plasma acceleration schemes. In particular, diagnostics of plasma parameters, e.g., plasma density, temperature, velocity, and excited magnetic field, are essential for elucidating physical phenomena since we have been concentrating on next generation plasma propulsion methods, e.g., Rotating Magnetic Field plasma acceleration method, by characterizing the plasma performance. Moreover, in order to estimate the thrust performance in our experimental scheme, we have also mounted a thrust stand, which has a target type, on this movable instrument, and scanned the axial profile of the thrust performance in the presence of the external magnetic field generated by using permanent magnets, so as to investigate the plasma captured in a stand area, considering the divergent field lines in the downstream region of a generation antenna. In this paper, we will introduce the novel measurement instrument and describe how to measure these parameters.
... Plasma flow can be measured with Mach probes [37]. Probes of this kind were utilized to measure plasma flow speed for magnetized plasmas to study plasma wall interaction [38]. ...
Plasma parameter measurements for negative hydrogen (H⁻) ion sources have been playing an important role in clarifying fundamental physics related to negative ion production and destruction processes. Measured data of beam properties, such as H⁻ ion current density with the co-extracted electron current and the emittance, were correlated to local concentration of charged particles and temperature often characterized by Langmuir probes and optical emission spectrometry. Langmuir probes coupled to pulse lasers quantified local H⁻ ion densities from early days of H⁻ ion source development, while the cavity ring down photodetachment method removed Langmuir probes from contemporary large-size high power density ion sources. Technological progress has made source plasma diagnostics possible during beam extraction, which has thrown light on the transport of H⁻ ions during the application of the extraction electric field. The advancement of plasma diagnostics for high intensity H⁻ ion sources are summarized in this report together with recent results from the research and development negative ion source being operated for collaborative research programs at National Institute for Fusion Science. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
... The ion sound velocity in PANTA is c s $ 2-3 km=s. Another model to evaluate the flow velocity, V z ¼ c 2 s =4v th;i ln½I u =I d , 27 where v th,i is the ion thermal velocity given as v th;i ffiffiffiffiffiffiffiffiffiffiffi T i =m i p , is also examined, and quantitatively similar results are obtained. The radial profile of V z measured with the Mach probe results in a reasonable agreement with that evaluated with the time delay estimation. ...
In this paper, we show the direct observation of the parallel flow structure and the parallel Reynolds stress in a linear magnetized plasma, in which a cross-ferroic turbulence system is formed [Inagaki et al., Sci. Rep. 6, 22189 (2016)]. It is shown that the parallel Reynolds stress induced by the density gradient driven drift wave is the source of the parallel flow structure. Moreover, the generated parallel flow shear by the parallel Reynolds stress is found to drive the parallel flow shear driven instability D'Angelo mode, which coexists with the original drift wave. The excited D'Angelo mode induces the inward particle flux, which seems to help in maintaining the peaked density profile.
... Later work extended and introduced alternatives to that approach [2,3,4], taught in classes on plasma diagnostics to this day [5]. For magnetized plasmas, a fluid theory accounting for diffusive transport across the magnetic field was developed [6], providing a basis for more accurate calculations of how the ion flux to probe surfaces depend on the flow parallel to the magnetic field [7]. Those results were confirmed with a kinetic treatment [8], thus allowing more accurate interpretation of parallel flow measurements with Mach probes [5]. ...
Flow past an obstacle by dense magnetized plasma, having both Debye-length and gyroradii smaller than the obstacle, is explored using particle-in-cell (PIC) simulations. These simulations are relevant to a wide range of physical settings, ranging from the moon in the (supersonic) solar wind to Mach probes in (subsonic) tokamak plasmas. For supersonic flow, the evolution of the resulting elongated wake is captured with high-resolution 1D simulations, using kinetic electrons with realistic mass. This leads to the discovery of a novel wake phenomenon, where electron holes spawned from a narrow dimple in the velocity-distribution grow to large velocity extents, leading to disruption of the ion beams present in the wake. Those beams are the result of shadowing by the obstacle, which also occurs for electrons in what is a less elongated forewake, lying outside the traditional wake. This forewake is explored with 2D simulations, also using kinetic electrons with realistic mass, and it is found that drift-energization near the obstacle can significantly modify the electron distribution in some regions. Most significantly, drift-energization appears to quite robustly generate a slope-reversal of the electron velocity-distribution, which is expected to become unstable; this phenomenon thus provides a novel drive for forewake instability. 2D simulations at subsonic flow are used in an initial investigation of whether kinetic electron effects also impact the stability of wakes at slower flow. It is found that kinetic electrons do trigger disruption of the ion beams in the wake, as in the (supersonic) 1D simulations, but the hole-growth phenomenon cannot be conclusively implicated because a highly artificial electron mass needed to be used. In summary, the understanding of kinetic electron effects as dense magnetized plasma flows past an obstacle is greatly enhanced, uncovering a number of novel phenomena with implications for the stability of the resulting wake and forewake
... Particle tracking and direct probe measurements of the rotating dusty plasmas are also planned. Nevertheless, it is also recognized here that floating potential probes such as Langmuir probes [28], [29] are challenging in plasma flows with magnetic fields [30], [31], and thus diagnostics for dusty plasmas in high magnetic fields is under development at DPLX. Insertable probes are being developed taking existing theories into account, but the availability of a 10 T magnet that can vary in field strength and duration present an excellent opportunity to challenge such theories (e.g. ...
An experimental setup is under construction at the Dusty Plasma Laboratory of the University of Maryland, Baltimore County to study viscous heating and stability in magnetized rotating dusty plasmas. Azimuthal rotation will be imposed on dust, electrons, and ions by having an axial magnetic field B in a vacuum chamber placed inside a 16-cm bore Bitter-type magnet, and a radial electric field E centered at the chamber axis. The resulting E x B rotation is expected to have sheared rotation, which leads to viscous heating from the radial velocity profile. However, heating may also lead to interchange modes that destabilize the rotation. The planned experiments are motivated by the observations of parabolic ion and electron temperature profiles in hydrogen plasmas in a supersonic rotating magnetic mirror, where ohmic and viscous heating were the only mechanisms available for heating the plasma. The goal of the experimental setup is to magnetize and rotate dust with diameter ~1 μm that can be individually captured by particle velocimetry cameras and software. The Bitter-type magnet is planned for a steady field of 10 T with a minimum duration of 10 s per experiment.
... There are many sophisticated models of the Mach probe developed for different geometry, plasma parameters and magnetic field configuration. 26,30,31 In the simplest case, the plasma flow is associated with a difference between the ion saturation currents measured (1) or more precise by an equation based on a free fall model given by 28,29 M $ ...
In this work plasma
acceleration using a RF self-bias effect is experimentally studied. The experiments are conducted using a novel plasma
accelerator system, called Neptune, consisting of an inductively coupled plasma
source and a RF-biased set of grids. The plasma
accelerator can operate in a steady state mode, producing a plasma flow with separately controlled plasma flux and velocity without any magnetic configuration. The operating pressure at the source output is as low as 0.2 mTorr and can further be decreased. The ion and electron flows are investigated by measuring the ion and electron energy distribution functions both space resolved and with different orientations with respect to the flow direction. It is found that the flow of electrons from the source is highly anisotropic and directed along the ion flow and this global flow of accelerated
plasma is well localized in the plasma transport chamber. The maximum flux is about 7.5·1015 ions s−1 m−2 (at standard conditions) on the axis and decreasing to almost zero at a radial distances of more than 15 cm from the flow axis. Varying the RF acceleration voltage in the range 20–350 V, the plasma flow
velocity can be changed between 10 and 35 km/s. The system is prospective for different technology such as space propulsion and surface modification and also interesting for fundamental studies for space-related plasma simulations and investigation of the dynamo effect using accelerated rotating plasmas.
... In order to provide a real-time feedback control of the plasma parameters several diagnostics were integrated in the control system: (i) mirnov coils, (ii) tomography [4], (iii) interferometer, (iv) magnetic sine probe, (v) magnetic cosine probe, (vi) H α radiation bolometer, (vii) loop voltage measurement and (viii) electric probes [5] which are this paper main subject. ...
The ISTTOK tokamak (Ip = 7 kA, BT = 0.5 T, R = 0.46 m, a = 0.085 m) was recently upgraded with a new multiple-input multiple-output control system, where the user can specify the combination of real-time diagnostics to produce the observed plasma quantities to control any actuator. The present observed quantities are: (i) plasma current, (ii) plasma position and (iii) plasma density, all of which are actively feeded back by the system. Four-quadrant Langmuir probes were installed in ISTTOK on inner, outer, top and bottom parts of a poloidal section at a 75 mm radius. The floating potential signals measured by the four probes are fed to the control system to estimate the plasma position using a linear model. The model is based on a well known characteristic of the ISTTOK plasma, in that the floating potential varies linearly with respect to the radial position for an extension of about 10 mm (from 70 mm to 80 mm). By using only this set of electric probes for plasma position feedback, it was possible to obtain an alternate current (AC) discharge with 40 semi-cycles, surpassing 1 second without loss of ionization during plasma current reversal.
... The mean (fluctuating) parallel velocity, V (Ṽ ), is measured by Mach probe pins (separated by a barrier 3 mm in height) aligned along the co-and counter-direction of the magnetic field. The Mach number is computed as M = 0.4 ln(I sh U/I sh D), where I sh U and I sh D are the sheath currents collected on two Mach probe tips at the up-and down-stream side, respectively [16]. The parallel velocity V = M C s and C s ≈ (2T e /M i ) 0.5 being the sound speed. ...
Direct measurements of residual stress (force) have been executed at the edge of the TEXTOR tokamak using multitip Langmuir and Mach probes, together with counter-current NBI torque to balance the existing toroidal rotation. Substantial residual stress and force have been observed at the plasma boundary, confirming the existence of a finite residual stress as possible mechanisms to drive the intrinsic toroidal rotation. In low-density discharges, the residual stress displays a quasi-linear dependence on the local pressure gradient, consistent with theoretical predictions. At high-density shots the residual stress and torque are strongly suppressed. The results show close correlation between the residual stress and the E-r x B flow shear rate, suggesting a minimum threshold of the E x B flow shear required for the k(parallel to) symmetry breaking. These findings provide the first experimental evidence of the role of E-r x B sheared flows in the development of residual stresses and intrinsic rotation.
... Measurements of the plasma parameters were performed using single Langmuir probe, diamagnetic loop and a poloidally and toroidally oriented Mach probes. Theoretical models have been derived to describe the Mach probes data in un-magnetized and magnetized plasmas, using fluid, particle, and kinetic models (see the Figure (1)) [15][16][17][18][19][20]. Figure (2) shows the time dependence of the ion saturation currents collected by the poloidal Mach probes, the ratio of upstream to downstream current, and the resulting Mach number. The Mach number and thus the poloidal plasma velocity are obtained using the model of Hutchinson. ...
... The ratio between upstream current (I UP ) and downstream current (I down ) is represent of Mach number (M). By using the model of Hutchinson [7], it can be written as equation: ...
We have designed, constructed and installed a multipurpose probe (MPP) for the measurements of plasma density gradient, flow and turbulent transport in edge plasma of IR-T1 tokamak. Moreover, the effects of radial density gradient and parallel flow on turbulent transport have been investigated. The radial gradient density and the parallel flow variation were measured by the electrical part of MPP. The probability distribution function and the expected value between turbulence, density and parallel flow have been computed.
In order to study rotation it is necessary first to measure it reliably. There are several techniques in use on tokamaks which will be summarized anon: Doppler shifts of atomic transitions, MHD mode rotation from frequency analysis, probe measurements and from microwave scattering. For the atomic transitions, the population of the upper levels can either be passive (via collisional excitation or radiative recombination from plasma electrons) or active (from charge exchange recombination with injected neutrals).
•Mechanisms underlying the generation of E×B sheared flows and parallel flows play an important role to improve confinement regimes in fusion plasma machines [1 – 4]
• Comparative studies of the plasma turbulence structure (including fusion and non fusion devices) support the view that the plasma turbulence displays universality [5, 6]
• Recent results emphasize the importance of statistical description of transport processes in plasmas, as an alternative approach to the traditional way to characterize transport
based on the computation of diffusion coefficients and correlation lengths
• Plasma profiles of parallel and poloidal flows and fluctuations have been investigated in a linear plasma machine
•Measurements were performed under different experimental conditions. By biasing an axial electrode an induced radial current is generated
• Radial profiles of Mach number, Plasma potential, electron density as well as its fluctuations are obtained
A technique for making probe measurements of the parameters of a plasma rotating in crossed radial electric and axial magnetic fields is described. Unlike the commonly used method, the processing of readings of the Langmuir triple probe was carried out using the results of measurements based on a Mach plasma probe. When using a magnetic field produced in a solenoid without end-to-end magnetic plugs, a positive potential with respect to the grounded anode and outer metal tube is observed in the plasma that arrives from the electric discharge source with the thermoemission cathode and the annular anode. It is shown that the speed of azimuthal electron drift in the crossed-fields system under investigation is much higher than the ion rotation speed.
In this paper, we will experimentally investigate the power threshold (PL-H) in upper single null plasmas with an ITER-like tungsten divertor under different ∇B drift directions on EAST [F. Ding et al., Commissioning and PSI Behavior of the ITER-Like W/Cu Divertor in EAST 22nd PSI, Rome (2016)]. The power threshold for the low (L) to high (H) confinement mode has a clear and positive toroidal magnetic field, BT, dependence when the ∇B drift points toward the primary X-point (B×∇B↑). A factor of 2–3 increase in PL-H is observed for the ∇B drift away from the primary X-point (B×∇B↓). The edge and core impurities quantified by spectroscopy measurements show comparable levels for the transitions for both drift directions. On the other hand, it is found that the divertor Dα emission just prior to the L-H transition is lower for B×∇B↑, compared with that for B×∇B↓. The upper in-out divertor asymmetry, as manifested by particle fluxes measured by the divertor triple Langmuir probe, is most marked for B×∇B↓, and with significantly more particle flux to the outer divertor. The reversing field increases the particle flux into the upper inner and lower outer divertor, reducing the in-out asymmetry. One important distinction between the two field directions has been observed, with respect to the amplitude of the scrape-off layer (SOL) parallel flow. A dedicated experiment under similar target plasma conditions shows a lower SOL density and thus a steeper density gradient slightly inside the separatrix, where a lower PL-H is found for the B×∇B↑, compared to that for B×∇B↓. We, therefore, conclude that the field-dependent SOL plasma conditions play an important role in the transition physics.
Detailed investigations on the filamentary structures associated with the type-I edge-localized modes (ELMs) should be helpful for protecting the materials of a plasma-facing wall on a future large device. Related experiments have been carefully conducted in the Experimental Advanced Superconducting Tokamak (EAST) using combined Langmuir-magnetic probes. The experimental results indicate that the radially outward velocity of type-I ELMy filaments can be up to 1.7 km s⁻¹ in the far scrape-off layer (SOL) region. It is remarkable that the electron temperature of these filaments is detected to be ∼50 eV, corresponding to a fraction of 1/6 to the temperature near the pedestal top, while the density of these filaments could be approximate to the line-averaged density. In addition, associated magnetic fluctuations have been clearly observed at the same time, which show good agreement with the density perturbations. A localized current on the order of ∼100 kA could be estimated within the filaments. © 2018 Hefei Institutes of Physical Science, Chinese Academy of Sciences and IOP Publishing.
The Advanced Space Propulsion Laboratory (ASPL) of NASA's Johnson Space Center is performing research on a Variable Specific Impulse MagnetoPlasma Rocket (VASIMR). The VASIMR is a high power, radio frequency (RF) driven magnetoplasma rocket, capable of very high exhaust velocities, > 100 km/s. A NASA-led research team involving industry, academia and government facilities is pursuing the development of this concept in the United States. The ASPL's experimental research focuses on three major areas: helicon plasma production, ion cyclotron resonant frequency (ICRF) acceleration and plasma expansion in a magnetic nozzle. The VASIMR experiment (VX-10) performs experimental research that demonstrates the thruster concept at a total RF power on the order of 10 kW. A flexible four-magnet system, with a 1.3 Tesla maximum magnetic field strength, allows axial magnetic field profile shape effects to be studied. Power generated at 10 – 50 MHz with about 3 kW is used to perform helicon plasma source development. A 3 MHz RF transmitter capable of 100 kW is available for ICRF experiments. The primary diagnostics are: gas mass flow controllers, RF input power, Langmuir probes, Mach probe, retarding potential analyzers (RPA), microwave interferometer, neutral pressure measurements and plasma light emission. In addition, many thermocouples are attached inside the vacuum chamber to measure heat loads around the plasma discharge.
Helicon research has been done with hydrogen, deuterium, helium, nitrogen, argon, xenon and mixtures of these gases. Optimization studies have been performed with the magnetic field axial profile shape, antenna geometry, gas flow rate, gas tube geometry and RF frequency. ICRF experiments have begun, primarily using a high density (> 10¹⁸/m³) helium helicon discharge as a target. Over 6 kW of power has been applied using a simple antenna array. The latest results of helicon and ICRF experiments will be presented.
A plasma flow velocity was measured by using a Much probe in the central cell of Hanbit magnetic mirror device. The Much probe was attached on the fast injection probe system, which can scan the central cell chamber of Hanbit device in the radial direction. The fast injection probe system also has an emissive probe so that the radial profile of the plasma potential is measured simultaneously. Therefore, the flow velocity measured from the Mach probe can be directly compared with Er×B drift calculated from the measured plasma potential profile. The experimental results are analyzed by using existing theories of the Mach probe. The measured flow velocity shows about 3 km/s, and the flow direction and magnitude is approximately the same as the Er×B drift velocity.
A compact apparatus to produce arcjet plasma was fabricated to investigate supersonic flow dynamics. Periodic bright–dark emission structures were formed in the arcjets, depending on the plasma source and ambient gas pressures in the vacuum chamber. A directional Langmuir probe (DLP) and emission spectroscopy were employed to characterize plasma parameters such as the Mach number of plasma flows and clarify the mechanism for the generation of the emission pattern. In particular, in order to investigate the influence of the Mach number on probe size, we used two DLPs of different probe size. The results indicated that the arcjets could be classified into shock-free expansion and under-expansion, and the behavior of plasma flow could be described by compressible fluid dynamics. Comparison of the Langmuir probe results with emission and laser absorption spectroscopy showed that the small diameter probe was reliable to determine the Mach number, even for the supersonic jet.
Antennas operating in the ion cyclotron range of frequency (ICRF) provide a useful tool for plasma heating in many tokamaks and are foreseen to play an important role in ITER. However, in addition to the desired heating in the core plasma, spurious interactions with the plasma edge and material boundary are known to occur. Many of these deleterious effects are caused by the formation of radio-frequency (RF) sheaths. The aim of this thesis is to study, mainly experimentally, scrape-off layer (SOL) modifications caused by RF sheaths effects by means of Langmuir probes that are magnetically connected to a powered ICRH antenna. Effects of the two types of Faraday screens' operation on RF-induced SOL modifications are studied for different plasma and antenna configurations - scans of strap power ratio imbalance, injected power and SOL density. In addition to experimental work, the influence of RF sheaths on retarding field analyzer (RFA) measurements of sheath potential is investigated with one-dimensional particle-in-cell code. One-dimensional particle-in-cell simulations show that the RFA is able to measure reliably the sheath potential only for ion plasma frequencies wpi similar to RF cyclotron frequency wrf, while for the real SOL conditions (wpi > wrf), when the RFA is magnetically connected to RF region, it is strongly underestimated. An alternative method to investigate RF sheaths effects is proposed by using broadening of the ion distribution function as an evidence of the RF electric fields in the sheath. RFA measurements in Tore Supra indicate that RF potentials do indeed propagate from the antenna 12m along magnetic field lines
Plasma diagnostics is a novel branch of fusion science that requires understanding from all the physics fields. For magnetic confinement fusion, it is necessary to create the special experimental techniques for diagnostics. Generally, plasma diagnostic techniques are distinguished into two different classes: passive methods and active methods. Passive methods detect and study radiations that are emitted by the plasma while active methods measure plasma reactions to external stimuli. The performed measurements by former method give averaged information while latter method makes the local measurements. IR-T1 Tokamak is an active Tokamak that has performed extensive researches into plasma diagnostic. There are several different plasma diagnostics on IR-T1 Tokamak for measuring plasma parameters such as magnetic diagnostics, spectroscopic diagnostics, and plasma facing and operational diagnostics. This chapter will provide an overview of different techniques for improving plasma confinement in IR-T1 Tokamak.
Rotation of magnetized plasma between two coaxial electrodes in crossed electric and magnetic fields was studied experimentally. Three regimes of plasma rotation were observed. In the first regime, the radial electric field is created by a beam−plasma discharge due to the charging of the inner axial electrode by electrons, the outer electrode being grounded. Plasma rotation in this case is accompanied by strong high-frequency current oscillations detected by a Mach probe. When a negative voltage was applied to the coaxial electrodes, the second regime was observed, in which weakly perturbed quasi-stationary plasma rotation occurred at a relatively low radial current. The third regime of plasma rotation was observed upon a spontaneous disruption of the second regime. It is characterized by high currents of ~1 kA, sheared plasma rotation, and excitation of high-frequency perturbations.
We present a method to calculate the ion saturation current, Isat, for Langmuir probes at high frequency (>100 kHz) using the harmonics technique and we compare that to a direct measurement of Isat. It is noted that the Isat estimation can be made directly by the ratio of harmonic amplitudes, without explicitly calculating Te. We also demonstrate that since the probe tips using the harmonic method are oscillating near the floating potential, drawing little power, this method reduces tip heating and arcing and allows plasma density measurements at a plasma power flux that would cause continuously biased tips to arc. A multi-probe array is used, with two spatially separated tips employing the harmonics technique and measuring the amplitude of at least two harmonics per tip. A third tip, located between the other two, measures the ion saturation current directly. We compare the measured and calculated ion saturation currents for a variety of plasma conditions and demonstrate the validity of the technique and its use in reducing arcs.
Spatial profile of an ion Mach number for arcjet He and Ar plasmas are measured by using a directional Langmuir probe. The Ar arcjet has bright-dark emission structures, implying that expansion-compression waves are formed, whereas no structure is observed for the He discharge. It is found that for He plasma, the Mach number monotonically decreases along the jet axis, while in the Ar jet the prominent peaks appear. The peak positions are almost the same with the bright emission regions, indicating that there is a strong correlation between the plasma emission structure and the Mach number.
Transports of plasmas in the edge of fusion devices have similarities in terms of formation of a free presheath and unclear explanation on the transport process relating the diffusion coefficient ( ) to characteristic length of perturbing for flux tube ( ). and are investigated by generating perturbations in various free presheaths due to a perturbing object located at the axial center of a linear plasma device, called DiPS (Divertor Plasma Simulator). Free presheaths are generated due to a tungsten perturbing object by changing the magnetic flux density. Bounded presheaths are also formed due to a limiting structure of a magnetic nozzle and due to the given geometry of DiPS. In term of plasma discharge currents, radial plasma profiles were measured by using a fast scanning probe system. and within the free presheath regions were calculated by using the measured plasma parameters and compared with those of bounded presheaths near the chamber walls. Decay length of plasma density was introduced to calculation of . To calculate the perturbation length (L) of free presheaths, a theoretical scale factor K was introduced as using a fluid model. Normalized factor , Bohm diffusion coefficient, were obtained as 8 at free presheaths and 11 at bounded presheaths.
In a magnetized plasma sheath, strong velocity shear exists owing to the three-dimensional nature of ion velocity. Thus, the ion viscosity should have an important effect on the sheath structure, which has not been studied. This article presents the study of the effect of ion shear viscosity on the sheath in an oblique magnetic field within the framework of classical cross-field transport. It is shown that the inclusion of the shear viscosity in the ion momentum equation results in a significant reduction in the sheath thickness. It is also shown that the "generalized Bohm criterion" is not affected by the shear viscosity within the present model. However, additional boundary conditions such as the velocity shear arise in the viscous case. The appropriate boundary conditions are formulated, accounting for E × B and diamagnetic drifts at the sheath edge, which affects the criterion and sheath profiles.
The in-out divertor asymmetry in the Experimental Advanced Superconducting Tokamak (EAST), as manifested by particle fluxes measured by the divertor triple Langmuir probe arrays, is significantly enhanced during type-I edge localized modes (ELMs), favoring the inner divertor in lower single null (LSN) for the normal toroidal field (B-t) direction, i.e. with the ion B x del B direction towards the lower X-point, while the in-out asymmetry is reversed when the ion B x del B is directed away from the lower X-point. The plasma flow measured by the Mach probe at the outer midplane is in the ion Pfirsch-Schluter (PS) flow direction, opposite to both B x del B and E x B drifts, i.e. towards the inner divertor for normal Bt, and the outer divertor for reverse Bt, consistent with the observed in-out divertor asymmetry in particle fluxes. Since the particle source from an ELM event is predominantly located near the outer midplane, this new finding suggests a critical role of the PS flow in driving the in-out divertor asymmetry. The divertor asymmetry during type-III ELMs exhibits a similar trend to that during type-I ELMs. Strong in-out divertor asymmetry is also present during inter-ELM and ELM-free phases for the normal field direction, i.e. with more particle flux to the lower inner divertor target, but the peak particle flux merely becomes more symmetric, or slightly reversed, for reverse B-t, i.e. reversed B x del B drift direction.
Grid biasing is utilized in a large-scale helicon plasma to modify an existing instability. It is shown both experimentally and with a linear stability analysis to be a hybrid drift-Kelvin–Helmholtz mode. At low magnetic field strengths, coherent fluctuations are present, while at high magnetic field strengths, the plasma is broad-band turbulent. Grid biasing is used to drive the once-coherent fluctuations to a broad-band turbulent state, as well as to suppress them. There is a corresponding change in the flow shear. When a high positive bias (10Te) is applied to the grid electrode, a large-scale (ñ/n≈50%) is excited. This mode has been identified as the potential relaxation instability.
A plasma flow velocity was measured by using a Mach probe in the central cell of Hanbit magnetic mirror device. The Mach probe was attached on the fast injection probe system, which can scan the central cell chamber of Hanbit device in the radial direction. The fast injection probe system also has an emissive probe so that the radial profile of the plasma potential is measured simultaneously. Therefore, the flow velocity measured from the Mach probe can be directly compared with ErXB drift calculated from the measured plasma potential profile. The experimental results are analyzed by using existing theories of the Mach probe. The measured flow velocity shows about 3 km/s, and the flow direction and magnitude is approximately the same as the ErXB drift velocity.
The Advanced Space Propulsion Laboratory (ASPL) of NASA's Johnson Space Center is performing research on a Variable Specific Impulse MagnetoPlasma Rocket (VASIMR). The VASIMR is a high power, radio frequency (RF) driven magnetoplasma rocket, capable of very high exhaust velocities, > 100 km/s. A NASA-led research team involving industry, academia and government facilities is pursuing the development of this concept in the United States. The ASPL's experimental research focuses on three major areas: helicon plasma production, ion cyclotron resonant frequency (ICRF) acceleration and plasma expansion in a magnetic nozzle. The VASIMR experiment (VX-10) performs experimental research that demonstrates the thruster concept at a total RF power on the order of 10 W A flexible four-magnet system, with a 1.3 Tesla maximum magnetic field strength, allows axial magnetic field profile shape effects to be studied. Power generated at 10-50 MHz with about 3 kW is used to perform helicon plasma source development. A 3 MHz RF transmitter capable of 100 kW is available for ICRF experiments. The primary diagnostics are: gas mass flow controllers, RF input power, Langmuir probes, Mach probe, retarding potential analyzers (RPA), microwave interferometer, neutral pressure measurements and plasma light emission. In addition, many thermocouples are attached inside the vacuum chamber to measure heat loads around the plasma discharge. Helicon research has been done with hydrogen, deuterium, helium, nitrogen, argon, xenon and mixtures of these gases. Optimization studies have been performed with the magnetic field axial profile shape, antenna geometry, gas flow rate, gas tube geometry and RF frequency. ICRF experiments have begun, primarily using a high density (> 10(18)/m(3)) helium helicon discharge as a target. Over 6 kW of power has been applied using a simple antenna array. The latest results of helicon and ICRF experiments will be presented.
Some components in electric probe diagnostics (EPDs) are improved in order to investigate characteristics of edge plasmas in the upstream scrape-off-layer (SOL) region and to measure wall and divertor fluxes during L-mode and H-mode plasma discharges in the Korea Superconducting Tokamak Advanced Research (KSTAR). From the upgrades in the EPDs, the measured error of the elapsed distance for the evaluation of the SOL profiles can be reduced up to 1% and the ion saturation current of up to 1.0 A near an outer strike point (OSP) can be measured at the divertor region. In the SOL profile measurements during L-mode and inner wall limited plasma (BT = 2.0 T, Ip = 0.4 MA), the e-folding lengths in the main SOL region λTe and λne are evaluated as 3.5 cm and 2.1 cm, respectively. From particle flux measurement at the far SOL region during a diverted ELMy H-mode discharge (BT = 1.8 T, Ip = 0.65 MA), peaked heat flux toward to outboard wall during ELM bursts is estimated up to ∼20 k Wm−2, which may be less than 1% of the peaked divertor heat flux expected for the neutral beam (NB) heating power PNB of ∼2.66 MW and stored energy WTOT of ∼0.38 MJ. In addition, the movement of the OSP during a diverted H-mode plasma (BT = 2.4 T, Ip = 0.5 MA) can be detected from the divertor probe measurement and peaked heat flux near the OSP is estimated as few MW m−2 for PNB = 3.0 MW, WTOT = ∼0.25 MJ. In this work, the technical details of the EPDs including its upgrades and some experimental results from EPD measurements are presented.
The mechanisms underlying the generation of plasma flows play a crucial role to understand transport in magnetically confined plasmas1,2. When the shearing rate approaches the characteristic frequency of the turbulence, a reduction in the turbulence amplitude is predicted. The best performance of existing fusion plasma devices has been obtained in plasma conditions where E×B shear stabilization mechanisms are likely to play a key role: both edge and core transport barriers are related to a large increase in the E×B sheared flows. Although sheared flows play a key role in reducing plasma transport in fusion plasmas, recent experiments in Q - devices3 have investigated the development instabilities excited by parallel sheared flow velocity.
The possible link between turbulence and flow generation have been discussed both from the theoretical and experimental point of view. Sheared flow can be generated when turbulence driven flows play a dominant role in the momentum balance. In this case, sheared perpendicular flow is expected to increase when the turbulent energy is large enough to overcome the flow damping.
Recent experiments4,5, have suggested that turbulence can drive parallel flows which might (at least partially) explain the magnitude of the measured flows. This paper reports experimental evidence of parallel flows dynamically coupled to radial turbulent transport in a linear device.
Multidirectional plasma flow measurements by using Gundestrup Probe in the scrape-off layer of ADITYA tokamak are presented. The ADITYA Gundestrup Probe-head consists of eight plates arranged around the ceramic rod and three pins normal to side plates. Plates are used to measure both parallel and perpendicular flows simultaneously and pins are used to measure plasma density and floating potential. A comparison of direct perpendicular flow measurement and by two other plates of Gundestrup Probe is presented. Possible causes of perpendicular flows are identified and compared with the measured flows. It is observed that the mechanism of the parallel flow and the perpendicular flow is different only at high parallel Mach number. A puff of the working gas is used to study its effect on the perpendicular flows and its reversal with the gas puff is observed.
This manuscript is devoted to the experimental investigation of particle transport in the edge region of the tokamak Tore Supra. The first part introduces the motivations linked to energy production, the principle of a magnetic confinement and the elements of physics essential to describe the dynamic of the plasma at the edge region. From data collected by a set of Langmuir probes and a fast visible imaging camera, we demonstrate that the particle transport is dominated by the convection of plasma filaments, structures elongated along magnetic field lines. They present a finite wave number, responsible for the high enhancement of the particle flux at the low field side of the tokamak. This leads to the generation of strong parallel flows, and the strong constraint of filament geometry by the magnetic shear.
Analytic solutions are derived from one‐dimensional fluid equations with arbitrary shear viscosity for the free and bounded presheaths in the strongly magnetized flowing plasma.Plasma density and flow velocity are obtained analytically in terms of position along the field line, Mach number (M ∞), normalized viscosity (α), and size of bounded presheath. Simple analytic relations between the ratio (R) of sheath current and the flowMach number [M ∞ ≡ V d /√(ZT e +T i )/m i ] are derived as R=exp(M ∞/M c ), where M c = 1/[1 +√α2/1+α arctan√1+α], with less than 5% accuracy for 0≤M ∞≤0.5. By generating two free and two bounded presheaths by two Mach probes within two free presheaths generated by a larger object than Mach probes, one can measure α and M ∞ simultaneously. The allowed range of α in terms of M ∞ is obtained, and comparisons with previous numerical analyses are also made.
Parallel plasma flows in the scrape-off layer of ADITYA tokamak are measured in two orientations of total magnetic field. In each orientation, experiments are carried out by reversing the direction of the toroidal magnetic field and the plasma current. The transport-driven component is determined by averaging flow Mach numbers, measured in two directions of the toroidal magnetic field and the plasma current for the same orientation. It is observed that there is a significant transport-driven component in the measured flow and the component depends on the field orientation.
The plasma-sheath equation for a collisionless plasma with arbitrary ion temperature in plane geometry is formulated. Outside the sheath, this equation is approximated by the plasma equation, for which an analytic solution for the electrostatic potential is obtained. In addition, the ion distribution function, the wall potential, and the ion energy and particle flux into the sheath are explicitly calculated. The plasma-sheath equation is also solved numerically with no approximation of the Debye length. The numerical results compare well with the analytical results when the Debye length is small.
Analytic and two-dimensional computational solutions for the plasma parameters near a toroidally symmetric limiter are illustrated for the projected parameters of a Tokamak Fusion Core Experiment (TFCX). The temperature near the limiter plate is below 20 ev, except when the density 10 cm inside the limiter contact is 8 multiplied by 10**1**3 cm** minus **3 or less and the thermal diffusivity in the edge region is 2 multiplied by 10**4 cm**2/s or less. Extrapolation of recent experimental data suggests that neither of these conditions is likely to be met near ignition in TFCX, so a low plasma temperature near the limiter should be considered a likely possibility.
A one‐dimensional fluid theory of Langmuir probe operation in strong magnetic fields is presented. Cross‐field diffusion of ions both into and out of the the collection region is consistently accounted for, in effect taking momentum and particle diffusivity to be equal. The results differ by significant factors from previous analyses, which did not account for outward diffusion but in effect set momentum diffusivity to zero. The differences are especially large when parallel flow of the external plasma is present. It is thus clear that the value assumed for the momentum diffusivity strongly affects the interpretation of recent probe measurements. It is argued that the present results offer a more reliable basis for this interpretation.
Plasma drift or rotation is caused at the edge of tokamaks by various mechanisms including flow to surfaces and neutral injection. These drift velocities can be measured using electrostatic probes and a theory is presented providing a method of probe analysis to deduce drift velocity, plasma density, and temperature. Initial probe experiments using this probe analysis yield credible values of drift velocity, but comparison with an independent technique, e.g., spectroscopic Doppler shifts, is sought.
An asymptotic analysis of the Langmuir‐probe problem in a quiescent, fully ionized plasma in a strong magnetic field is performed, for electron cyclotron radius and Debye length much smaller than probe radius, and this not larger than either ion cyclotron radius or mean free path. It is found that the electric potential, which is not confined to a sheath, controls the diffusion far from the probe; inside the magnetic tube bounded by the probe cross section the potential overshoots to a large value before decaying to its value in the body of the plasma. The electron current is independent of the shape of the body along the field and increases with ion temperature; due to the overshoot in the potential, (1) the current at negative voltages does not vary exponentially, (2) its magnitude is strongly reduced by the field, and (3) the usual sharp knee at space potential, disappears. In the regions of the C‐V diagram studied the ion current is negligible or unaffected by the field. Some numerical results are presented. The theory, which fails beyond certain positive voltage, yields useful results for weak fields, too.
The plasma equation for a warm collisionless plasma with a Maxwellian particle source is solved in plane parallel geometry. The generalized Bohm criterion is used to identify the plasma--sheath boundary. This kinetic treatment, in common with fluid and cold-ion kinetic models, results in an infinite electric field at the sheath edge. This is in sharp contrast to results from a previous warm-ion kinetic model, by Emmert et al. (Phys. Fluids 23, 803 (1980)), which gave a finite electric field at the sheath edge. Also, the presheath potential given by the present model is greater than that given by Emmert and is in better agreement with fluid results.
The utilization of elementally-sensitive surface techniques as plasma diagnostics is discussed with emphasis on measuring impurity fluxes, charge states, and energy distributions in the plasma edge. A model of plasma flow to the probe is presented and applied to the interpretation of data. Limits on time and energy resolution, and sensitivity are given. The overlap of these techniques with conventional plasma diagnostics is described.
An experimental study is presented of the electron saturation current arriving at a plane probe situated in a magnetized plasma. Using a simplified Bohm's model, the equations of electron probe current in a magnetized plasma are derived and compared with the experimental results. The equations show that the electron temperature is determined for a sufficiently large range of negative probe potentials and an apparent space potential is lowered by the presence of the magnetic field.
Data derived from a probe inserted into the limiter scrape-off layer of a tokamak do not correspond to unperturbed plasma conditions if the length of the shadow cast by the probe along the magnetic field is greater than the probe-limiter connection length. Measurements have been made in DITE using small sized, Mach number probes located close to the limiter. The magnitude of the downstream flux of ions to the probe corresponds to that expected if the length of the downstream shadow is shorter than the probe-limiter connection length. This short length of shadow cannot be explained on the basis of the observed cross field diffusion in the unperturbed scrape-off layer.
We describe measurements of Mach number, M, and electron density and temperature, and , in a limiter scrape-off layer. The aim was to investigate boundary transport when the ratio of electron-electron mean free path to the toroidal connection length between limiters (the toroidal Knudsen number, ) varied widely. The experiment was in the DITE tokamak in a beam-heated, deuterium discharge, , and M being measured at various poloidal and radial positions, using electrostatic probes. The range of was from 1.5 at the largest radii to 2.5 × 10−2 at 0.25 m radius. The values of M were consistent with a simple parallel flow model, including gas puffing and recycling. The scaling of the density e-folding thickness, was over the range . The radial diffusion coefficient inferred from these results ranges from 1 to 8 m2 s−1 and the scaling of is discussed.
This book provides a systematic introduction to the physics behind measurements on plasmas. It develops from first principles the concepts needed to plan, execute, and interpret plasma diagnostics. The book is therefore accessible to graduate students and professionals with little specific plasma physics background, but is also a valuable reference for seasoned plasma physicists. Most of the examples are taken from laboratory plasma research, but the focus on principles makes the treatment useful to all experimental and theoretical plasma physicists, including those interested in space and astrophysical applications. This second edition is thoroughly revised and updated, with new sections and chapters covering recent developments in the field. Specific areas of added coverage include neutral-beam-based diagnostics, flow measurement with mach probes, equilibrium of strongly shaped plasmas and fusion product diagnostics.
Analysis is presented for the particle and energy flow to an electrically biasable probe immersed in the magnetic field of the boundary plasma of a Tokamak or other magnetic confinement device. The analysis is appropriate to the operation of the probe in the voltage-current (Langmuir) probe mode, the thermal power (bolometer) mode, hydrogenic ion trapping mode or impurity deposition mode. The formulation of the analysis permits the use of any cross-field diffusion model, i.e. classical, semi-classical, empirical. The principal conclusion of the analysis is that a probe should be operated simultaneously in at least two modes, e.g. Langmuir and bolometer modes. In this way it is possible to measure plasma density and electron and ion temperatures separately with several consistency checks also available. Under some circumstances it is also possible to measure the hydrogenic cross-field diffusion coefficient and impurity ion density, temperature and charge state. It is necessary to operate a bolometer in a biased mode in order to infer heat loads to non-floating surfaces, e.g. limiters and divertor targets, since heat flux is a strong function of the potential difference between the plasma and surface.
Visual inspection of the full poloidal ring limiters of Alcator C identifies two directionally asymmetric damage regions at the high field side of the plasma: at the top location on the electron side (as defined by plasma current) and at the bottom location on the ion side. We report direct measurements of edge plasma parameters in the SOL plasma on the top location using Janus, a two-sided, multi-diagnostic edge probe. Higher ion and electron temperatures and densities at the probe location occur on the electron side under standard operating conditions. The degree of asymmetry cannot be explained by limiter configuration alone, but the direction of the toroidal field and variation of the plasma horizontal position, change the magnitude of asymmetry. Mechanisms that may cause the directional asymmetries include parallel plasma flow and asymmetric perpendicular transport into the collecting flux tubes. Measurements made by DENSEPACK, a full poloidal array of Langmuir probes, show that strong poloidal asymmetries in both plasma density and electron temperature in the scrape-off region of Alcator C exist. Such asymmetries may arise from asymmetric perpendicular transport and/or act to drive plasma flows along field lines.
A pair of asymmetric electrostatic double probes has been used in the boundary layer and divertor chamber of DITE. The electrode configuration is such that while one double probe collects ions moving towards the divertor target, the other collects ions moving away from it. The probes are used to measure electron temperature, density and Mach number of the flowing plasma. Although the interpretation of Mach number is model dependent, it is concluded that the Mach number within 6–10 cm from the divertor target ranges from about one to two while in the scrape-off layer it varies from 0.3 to 0.45.
The conception of random positive ion velocities corresponding to ion temperatures in a plasma has serious theoretical difficulties and is lacking in direct experimental verification. It is more reasonable to assume that each ion starts from rest and subsequently possesses only the velocity which it acquires by falling through a static electric field which is itself maintained by the balance of electron and ion charges. This new viewpoint thus ascribes motions to the positive ions which, for long free paths, are ordered rather than chaotic, each negative body in contact with the discharge collecting ions from a definite region of the plasma and from it only. The resulting integral ? the plasma-sheath potential distribution have been set up for plane, cylindrical, and spherical plasmas, for long, short and intermediate length ion free paths, and for both constant rate of ionization throughout the plasma and rate proportional to electron density, and these equations have been solved for the potential distribution in the plasma in all important cases. The case of short ion free paths in a cylinder with ion generation proportional to electron density gives the same potential distribution as found for the positive column by Schottky using his ambipolar diffusion theory, with the advantages that ambipolarity and quasineutrality need not appear as postulates. The calculated potential distribution agrees with that found experimentally. The potential difference between center and edge of plasma approximates Te11,600 volts in all long ion free path cases. The theory yields two equations. One, the ion current equation, simply equates the total number of ions reaching the discharge tube wall to the total number of ions generated in the plasma, but it affords a new method of calculating the density of ionization. The second, the plasma balance equation, relates rate of ion generation, discharge tube diameter (in the cylindrical case), and electron temperature. It can be used to calculate the rate of ion generation, the resulting values checking (to order of magnitude) those calculated from one-stage ionization probabilities. The potential difference between the center of the plasma and a non-conducting bounding wall as calculated from the ion current equation agrees with that found experimentally.
A theory is presented for a cylindrical electrostatic probe in a collisionless plasma in the case where the probe axis is inclined at an angle to a uniform magnetic field. The theory is applicable to electron collection, and under more restrictive conditions, to ion collection. For a probe at space potential, the theory is exact in the limit where probe radius is much less than Debye length. At attracting probe potentials, the theory yields an upper bound and an adiabatic limit for current collection. At repelling probe potentials, it provides a lower bound. The theory is valid if the ratios of probe radius to Debye length and probe radius to mean gyroradius are not simultaneously large enough to produce extrema in the probe sheath potential. The numerical current calculations are based on the approximation that particle orbits are helices near the probe, together with the use of kinetic theory to relate velocity distributions near the probe to those far from it. Probe characteristics are presented for inclination angles from 0 to 90 deg and for probe-radius mean-gyroradius ratios from 0.1 to infinity. For an angle of 0 deg, the end-effect current is calculated separately.