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Measurements of HF-Plasma Oscilations by means of a Laser-Heated Emissive Probe
Abstract
Indirectly heated emissive probes have advantages compared to conventional emissive wire probes. We have developed a laser-heated emissive probe consisting of a 1 mm diameter and 2 mm long pin of LaB6, heated by a focussed laser beam of 808 nm wavelength with a power up to 50 W. This probe is smaller and simpler than electrically heated emissive wire probes. Materials of low work function, high temperature stability and longer lifetime such as LaB6 can be used. There is no deformation in a magnetic field and no voltage drop along the probe wire. In this contribution we show the good time resolution of such a probe. Whereas emissive wire probes need two cables and an electric power supply or battery, for our probe one connection to an oscilloscope with high input impedance suffices. Therefore the probe system has a much lower stray capacitance than conventional emissive wire probes.
... In a realistic experiment the procedure to verify that the emission current is sufficient for shifting V fl,em as close as possible to Φ pl is to measure V fl,em as function of the EEP heating power P heat . It is very helpful that for increasing heating power the transition from the cold floating potential V fl towards Φ pl (being more positive than V fl by a few time T e -equation (1)) is usually rather abrupt and that above a certain value of P heat the value of V fl,em almost saturates (see for instance [10,30,35,36]). Also figure 1 illustrates the rather abrupt transition of the floating potential of an unheated, or only slightly heated EEP from that of a CLP (horizontal blue line) to that of an EEP, being close to the plasma potential Φ pl (horizontal red line). This result was obtained with an EEP similar as that one shown below in figure 2, however with a tip of LaB 6 , in an unmagnetized argon plasma with a density of approximately 10 17 m -3 and an electron/ion temperature of around 2eV/0.2eV [37]. ...
We present a new diagnostic tool for the determination of various plasma parameters in the edge region of medium-size tokamaks (MST) and stellarators (specifically Wendelstein 7-X) which is under development under the EUROfusion project task force MST2 and S1. This will be a probe head (called the new probe head-NPH) which will carry two cold Langmuir probes, one electron-emissive probe (EEP), two retarding field analysers (RFA) facing upstream and downstream and two magnetic pickup coils. By various adaptors, the same NPH will be used on all three European MSTs (ASDEX Upgrade, TCV and MAST-U) and on Wendelstein 7-X. For the first time the plasma potential in the edge region of MSTs and comparable toroidal fusion experiments will be directly determined by an EEP that will be permanently heated during the measurements. After the introduction and the theoretical background especially of the EEP, the NPH and its components are described in detail. The NPH will be able to measure electron and ion temperature, electron and ion density, cold floating potential, plasma potential and magnetic fluctuations in all three directions of space at two radial positions.
... However, for frequencies above a few tens of kilohertz, the probe circuit must be carefully built to especially minimize capacitive effects that reduce the cutoff frequency. Another possible approach rests on utilization of an indirectly heated emissive probe [18], [19]. A probe heated by a focused infrared laser beam has a short response time due to the lack of an electrical circuit for heating. ...
A heated emissive probe is a smart diagnostic tool for acquisition of the plasma potential in the discharge and in the plume of a Hall thruster. However, measurements of high-frequency oscillations of the potential require a specific electrical circuit with a low RC time constant. We developed and tested an emissive probe system with a cutoff frequency of 1 MHz. Details about the low-pass filter design as well as first experimental outcomes obtained in the plume of a 200 W permanent magnet Hall thruster are given in this paper.
Plasma probes are well established diagnostic tools. They are not complicated, relatively easy to construct and to handle. The easiest and fastest accessible parameter is their floating potential. However, the floating potential of a cold probe is not very significant. Much more important and relevant is the plasma potential. But in most types of plasmas, consisting mainly of electrons and only positive ions, the floating potential is more negative than the plasma potential by a factor proportional to the electron temperature. Obviously this is due to the much higher mobility of the electrons. We present a review on probes whose floating potential is close to or ideally equal to the plasma potential. Such probes we name Plasma Potential Probes (PPP) and they can either be Electron Emissive Probes (EEP) or so-called Electron Screening Probes (EPS). These probes make it possible to measure the plasma potential directly and thus with high temporal resolution. An EEP compensates the plasma electron current by an electron emission current from the probe into the plasma, thereby rendering the current-voltage characteristic symmetric with respect to the plasma potential and shifting the floating potential towards the plasma potential. Only the simplest case of an EEP floating exactly on the plasma potential is discussed here in which case no sheath is present around the probe. An ESP, principally operable only in strong magnetic fields, screens off most of the plasma electron current from the probe collector, taking advantage of the fact that the gyro radius of electrons is usually much smaller than that of the ions. Also in this case we obtain a symmetric current-voltage characteristic and a shift of the probe’s floating potential towards the plasma potential. We have developed strong and robust EEPs and two types of ESPs, called BUnker Probes (BUP), for the use in the Scrape-Off Layer (SOL) of Medium-Size Tokamaks (MST), and other types of strongly magnetized hot plasmas. These probes are presented in detail.
Emissive probes have been a part of standard diagnostic set in many laboratory plasmas. Due to the possibility of obtaining plasma potential directly from the floating potential of a highly emissive probe it has in the last few years again emerged itself as a possible diagnostics tool in tokamaks and stellarators [ref]. Also, the recent findings of [1] showing the agreement between the plasma potential measurements using a ball-pen probe and a self-emissive Langmuir probe have spurred further interest in this area. With laser heated emissive probe, also the ability to measure high frequency oscillations was demonstrated [2], which is important in measurements of fluctuations in SOL. However, there is still a dispute regarding the actual position of the floating potential of a highly emissive probe relative to the plasma potential. The theory had since [3] persistently suggested, that the floating potential saturates more than 1.5 T e below the plasma potential with increasing heating due to space-charge build-up by the low-energy emitted electrons, however measurement results have been ambiguous. Since also the various Langmuir probe methods also provide a variety of results. Previously, we have made several theoretical models and simulations regarding the position of the point of critical emission (transition between temperature limited and space-charge limited regime) e.g. [4], the saturation potential and the potential dip with regards to temperature and density ratios of involved particle species [5]. It was recently shown, that the ratio between the temperatures of the emitted electron and the bulk electrons is very important in the formation of the potential dip in front of the emitting electrode. While for low temperature ratio the floating potential indeed saturates below the plasma potential, simulations show, that for higher temperature ratios the floating potential tends to surpass the plasma potential. The potential structure in front of the electrode however still remains. Our goal in the present work was to make measurements in plasma with a higher temperature ratio. Since the temperature of the emitted electrons depends solely on the temperature of the emitting electrode, the temperature of emitted electrons is always close to 0.2 eV. Therefore, the only parameter, that could be altered, was the temperature of the bulk
The reliability of Langmuir probe measurements for plasma-turbulence
investigations is studied on GEMR gyro-fluid simulations and compared with
results from conditionally sampled I-V characteristics as well as self-emitting
probe measurements in the near scrape-off layer of the tokamak ASDEX Upgrade.
In this region, simulation and experiment consistently show coherent in-phase
fluctuations in density, plasma potential and also in electron temperature.
Ion-saturation current measurements turn out to reproduce density fluctuations
quite well. Fluctuations in the floating potential, however, are strongly
influenced by temperature fluctuations and, hence, are strongly distorted
compared to the actual plasma potential. These results suggest that
interpreting floating as plasma-potential fluctuations while disregarding
temperature effects is not justified near the separatrix of hot fusion plasmas.
Here, floating potential measurements lead to corrupted results on the ExB
dynamics of turbulent structures in the context of, e.g., turbulent particle
and momentum transport or instability identification on the basis of
density-potential phase relations.
A probe array consisting of three emissive probes and one cold cylindrical probe was developed for edge plasma measurements in ISTTOK. Emissive probes are particularly suitable for turbulence studies as they are able to deliver a more accurate measure of the plasma potential by reducing the effect of temperature fluctuations. The probe array has the advantage of recording the density, the electric field and their fluctuations simultaneously. Radial plasma profiles were recorded with and without negative edge biasing by an emissive electrode. The statistical properties of the poloidal electric field and of the turbulent particle flux, measured with cold and emissive probes, were compared. Both the root mean square of the poloidal electric field and the fluctuation-induced particle flux were found to be significantly larger when measured with the emissive probes, indicating that temperature fluctuations are important for the measurement of the particle flux. The probability distribution of the particle flux was also found to be more peaked and asymmetric when measured with the emissive probes.
For over 80 years emissive probes have been used to measure plasma potential and a wide variety of methods for interpreting probe data now exists. Constructions, heating methods and measurement techniques are reviewed in detail and their various strengths and limitations are compared. Additionally, several novel uses for emissive probes, such as measuring electron temperature are presented. This review also includes tables of recommendations for emissive probe design given the type of plasma and desired measurements.
A method is presented for obtaining the temporal evolution of the plasma potential, which is assumed to be given by the floating potential of a simple emissive probe. The construction of the probe is also described. The method avoids the slow time response of the usual technique where the floating potential is measured across a high resistance. During each sweep of the probe voltage, the changing of the sign of the probe current, which is sampled at a specific time, gives rise to a negative pulse, driving the pen-lift of an X-Y recorder. Since the real floating potential is measured where the probe current is zero, the disturbance of the plasma is kept as low as possible.
Reliable diagnostics of the plasma potential is one of the most important challenges in context with the production, control and confinement of a plasma. Emissive probes are readily available as direct diagnostic tools for the plasma potential with a good temporal and spatial resolution in many plasmas, even up to middle-sized fusion experiments. We present the results of investigations on the heating of lanthanum hexaboride and graphite with an infrared diode laser and on the development of a laser-heated emissive probe. Such a probe has a higher electron emission, much longer life time and better time response than a conventional emissive wire probe. We have observed that from both materials electron emission current can be achieved sufficiently strongly even for dense laboratory and experimental fusion plasmas.
Direct measurements of the plasma potential with high temporal and spatial resolution are only possible with special probes whose floating potential becomes equal to the plasma potential. One of these probes is the emissive probe which can be heated to electron emission. This leads to the effect that the plasma electron current towards the probe is compensated by an equal electron emission current flowing from the probe into the plasma which shifts the probe floating potential to the value of the plasma potential. While emissive probes are usually realized by heatable loops of wires of a refractory metal like tungsten or thoriated tungsten, we have developed various types of probes whose tip can be heated by an infrared laser beam. This offers several advantages like higher emission, longer lifetime and faster time response. Here we report on the latest development of a laser-heated emissive probe and on measurements with it in a magnetized helicon plasma. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Emissive electrostatic probes for the use in fusion experiments must be able to sustain significantly higher thermal loads than in low-temperature plasma experiments. Several types of probe design are discussed, the results from the use of such probes in the edge plasma of the Wendelstein 7-AS stellarator are presented and compared with the predictions of emissive and non-emissive probe models. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Whistler wave dispersion measurements are done in a linear magnetized helicon plasma experiment. The waves are excited by an induction loop and detected by movable magnetic probes for a frequency range of 100-800 MHz, corresponding to 0.05-0.9 omega(ce). The dispersion of whistler waves is measured for various plasma densities and magnetic field strengths. A key issue is to study the transition from an unbounded to bounded plasma wave dispersion. A comparison with theoretically derived dispersion relations is made. For small wavelengths, the dispersion can be described with whistler wave theory for unbounded plasmas whereas for larger wavelengths, the bounded geometry must be taken into consideration. The experimental results agree with theoretical dispersion relations derived for the bounded and the unbounded situation. (C) 2002 American Institute of Physics.
Emissive probes are standard tools in laboratory plasmas for the direct determination of the plasma potential. Usually they consist of a loop of refractory wire heated by an electric current until sufficient electron emission. Recently emissive probes were used also for measuring the radial fluctuation-induced particle flux and other essential parameters of edge turbulence in magnetized toroidal hot plasmas [R. Schrittwieser et al., Plasma Phys. Controlled Fusion 50, 055004 (2008)]. We have developed and investigated various types of emissive probes, which were heated by a focused infrared laser beam. Such a probe has several advantages: higher probe temperature without evaporation or melting and thus higher emissivity and longer lifetime, no deformation of the probe in a magnetic field, no potential drop along the probe wire, and faster time response. The probes are heated by an infrared diode laser with 808 nm wavelength and an output power up to 50 W. One probe was mounted together with the lens system on a radially movable probe shaft, and radial profiles of the plasma potential and of its oscillations were measured in a linear helicon discharge.
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