Roman Schrittwieser

University of Innsbruck | UIBK · Institute for Ion Physics and Applied Physics

A new cathode system, consisting of two concentric spherical hollow grids with two aligned orifices, is investigated by space-resolved Langmuir probe measurements and non-linear dynamics analysis. Negative biases of this spherical hollow grids arrangement lead to the formation of two complex space charge structures in the regions of the orifices. T...

The ball-pen probe (BPP) technique is used successfully to make profile measurements of the electron temperature on the ASDEX Upgrade (Axially Symmetric Divertor Experiment), COMPASS (COMPact ASSembly), and ISTTOK (Instituto Superior Tecnico TOKamak) tokamak. The electron temperature is provided by a combination of the BPP potential (ΦBPP) and the...

By negatively biasing a metallic grid with a small hole, down to a critical value of the applied potential a complex space-charge structure appears inside and around the grid cathode. The static current–voltage characteristic of the discharge shows one or two current jumps (the number of current jumps depending on the working gas pressure), one of...

This study focuses on producing porous polymer under plasma conditions using two different monomers: ethylene glycol and styrene. A plasma discharge at atmospheric pressure was used to induce polymerisation reactions in order to obtain copolymer films based on the two monomers. The plasma source consists of a dielectric barrier discharge which oper...

We report experimental results and theoretical modeling showing the interaction between two fireballs excited on two positively biased electrodes immersed in a low-temperature plasma. This interaction leads to a synchronized dynamics of the two fireballs, its frequency depending on the plasma density, the voltages applied on the electrodes and the...

In this work we present the development of a heated emissive Langmuir probe with LaB 6 and TiC probe tips to be used in high temperature and dense technical plasmas. The work function, one main parameters for electron emission, of LaB 6 is 2,7 eV that of TiC 3,4 eV. Electrical probes have the advantage of measuring the plasma potential directly wit...

This study focuses on the production of polyethylene glycol-like films (ppEG) under plasma conditions, using plasma discharge polymerization at atmospheric pressure. The plasma source consists of a dielectric barrier discharge (DBD) which operates in helium gas flow with a glass-like dielectric. The films were characterized by Atomic Force Mi-crosc...

In this work we present results of a newly developed highly emissive Langmuir probe with LaB 6 , TiC and pure carbon probe tips for the use in high temperature plasmas and dense plasmas. The work function, the main parameters for electron emission, of LaB 6 is 2,7 eV, that of TiC 3,4 eV, and 4,6 eV for C. Emissive electrical probes have the advanta...

A cavity-hollow cathode, consisting of two specifically formed disks of Ti with an additional cavity intended for enhancement of the pendulum effect of the electrons, was investigated as a low-cost sputtering source. A discharge in Ar gas was produced inside the hollow cathode. Measurements using Langmuir probes yielded evidence for the formation o...

Experimental investigations of the plasma potential by means of a ball-pen probe and self-emitting Langmuir probe technique were performed on the tokamaks COMPASS and ASDEX Upgrade. A spontaneous transition between the cold and self-emitting Langmuir probe has been observed during the deep reciprocation in the vicinity or even inside the last close...

A reciprocating probe head with six pins was used for localized measurements of electric fields and densities in the scrape-off layer (SOL) of ASDEX Upgrade (AUG) up to the edge shear layer (SL) near the Last Closed Flux Surface (LCFS). The edge SL is characterized by a strong sudden change in the poloidal velocity v close to the separatrix. The pr...

Two simultaneous existing fireballs excited in front of two positively biased electrodes immersed in plasma were investigated through emission spectroscopy under different experimental conditions (gas and pressure, plasma density). Experimental results were obtained regarding the spectral composition of the light emitted by the fireballs, as well a...

We report the latest results of turbulence and transport studies in the ASDEX Upgrade scrape-off layer (SOL). Dissimilarity between the plasma and the floating potential fluctuations is studied experimentally and by gyrofluid simulations. Measurements by a retarding field analyser reveal that both, edge-localized mode (ELM) and turbulent filaments,...

The radial turbulent particle flux and the Reynolds stress in the scrape-off layer (SOL) of ASDEX Upgrade were investigated for two limited L-mode (low confinement) and one ELMy H-mode (high confinement) discharge. A fast reciprocating probe was used with a probe head containing five Langmuir probes. One of the probe pins was protruding radially ab...

Knowledge of the electron energy distribution function (EEDF) is of great interest in different branches of plasma physics ranging from laboratory to fusion plasmas. In the frame of this work systematic measurements of the EEDF in low temperature helium plasmas ( T e ≈ 2 eV) at different working gas pressures and discharge currents ( I dis between...

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 wi...

We have tested two cylindrical graphite probe heads coated by a layer of electrically isolating UNCD (Ultra Nano-Crystalline Diamond) using a CVD (Chemical Vapour Deposition) method. The probe heads were mounted on the reciprocating probe manipulator of the Experimental Advanced Superconducting Tokamak (EAST) in Hefei, People's Republic of China. T...

The time and space averaged profiles of temperature, particle density, and momentum are in the scrape off layer determined by the intermittent transport generated at the edge shear layer. The distinction between profiles and fluctuations becomes arbitrary for situations where the transport is highly intermittent and shows long range correlations. W...

Fireballs are luminous regions produced by double layers in front of positively biased electrodes in plasmas. Although fireballs have been investigated previously there are a great variety of unexplained nonlinear phenomena, some of which are addressed in this work. First, it is shown that a fireball is not an isolated local phenomenon but an integ...

Electron inertia causes a delay between high-frequency currents and electric fields. When the transit time of a bunch of streaming electrons equals one RF period, the electrons can enhance the field and produce oscillations. Here, electron transit-time instabilities have been observed in magnetized sheaths and fireballs. Novel fireballs trapped in...

Emissive probes have been used for the direct measurement of plasma potential in many plasma devices. Different approaches have been introduced to measure the plasma potential using emissive probes. But the biggest disadvantage of the emissive probe is short life span due to its self-arrangement and different plasma environment. Few initiations hav...

It is now well known that edge localized modes (ELMs) lead to the ejection of a number of filamentary structures into the scrape-off layer (SOL). ELMs thus generate structures with excess energy and density. Recent results show that ELM filaments also carry current. Furthermore it can be conjectured that ELM filaments leave corresponding holes behi...

This contribution is focused on the magnetic signatures of type I ELM filaments. On JET a limited number of high time resolution magnetic coils were used to derive essential ELM filament parameters. The method uses forward modelling and simultaneous fitting of magnetic pickup coil signals to a simple model, motivated by observations. A new diagnost...

Positively biased probes in magnetized plasmas can create several phenomena which are not described by standard probe theories. These include transient currents for pulsed electrodes in the regime of Electron MHD, relaxation oscillations induced by large probes and high frequency instabilities at the sheath-plasma frequency. Although they may limit...

Experimental results are presented on the nonlinear effects related to the simultaneous excitation of three low-frequency instabilities in the magnetized plasma column of a Q-machine, namely the potential relaxation instability, the electrostatic ion-cyclotron instability and the Kelvin-Helmholtz instability. The influence of electron drift and mag...

The evolution of an electron-rich sheath on a plane electrode has been investigated experimentally. A rapidly rising voltage is applied to a plane gridded electrode in a weakly ionized, low temperature, and field-free discharge plasma. Transient currents during the transition from ion-rich to electron-rich sheath are explained including the current...

Instabilities in an electron-rich sheath on a plane electrode in a discharge plasma have been investigated experimentally. The high-frequency sheath-plasma instability near the electron plasma frequency is observed. With increasing dc voltage, the instability exhibits bursty amplitude and frequency jumps. The electrode current shows spikes and jump...

High-frequency instabilities are observed in connection with unstable fireballs. Fireballs are discharge phenomena near positively biased electrodes in discharge plasmas. They are bounded by a double layer whose potential is of order of the ionization potential. Fireballs become unstable when plasma losses and plasma production are not in balance,...

We describe a novel probe method for ion temperature measurements by using a swept ballpen probe (BPP) with special fitting routines and error analysis. Originally BPPs were developed to directly measure the plasma potential in DC regime. We report results of the first measurements with fast swept BPPs fixed on the midplane manipulator of ASDEX Upg...

Fireballs are local discharge phenomena on positively biased electrodes in partially ionized plasmas. Electrons, energized at a double layer, heat neutral gas which expands. The gas pressure exceeds the plasma pressure, hence becomes important to the stability and transport in fireballs. The flow of gas moves the electrode and sensors similar to a...

This paper presents turbulence investigations in the scrape-off layer (SOL) of ASDEX Upgrade in ohmic, L-mode and H-mode discharges using electrostatic and electromagnetic probes. Detailed studies are performed on small scale turbulence and on ELM filaments. Simultaneous measurements of floating and plasma potential fluctuations revealed significan...

Magnetically confined plasmas in the high confinement regime are regularly subjected to relaxation oscillations, termed edge localized modes (ELMs), leading to large transport events. Present ELM theories rely on a combined effect of edge current and the edge pressure gradients which result in intermediate mode number (n≅10-15) structures (filament...

Emissive probes offer the possibility for a direct determination of the plasma potential since for sufficient electron emission its floating potential is ideally equal to Phi(pl). By means of two such probes the true electric field can be derived. Emissive probes also deliver reliable results if there are electron drifts and beams in the plasma. A...

A fireball is formed inside a highly transparent spherical grid immersed in a dc discharge plasma. The ambient plasma acts as a cathode and the positively biased grid as an anode. A strong nearly current-free double layer separates the two plasmas. Electrons are accelerated into the fireball, ionize, and establish a discharge plasma with plasma pot...

A new fireball configuration has been developed which produces vircator-like instabilities. Electrons are injected through a transparent anode into a spherical plasma volume. Strong high-frequency oscillations with period corresponding to the electron transit time through the sphere are observed. The frequency is below the electron plasma frequency...

Fireballs are luminous phenomena, which evolve if a positively biased electrode is inserted into a thin background plasma with a sufficiently high neutral pressure. These phenomena show a wide variety of instabilities and nonlinearities and are therefore an interesting topic of fundamental plasma research. Unstable as well as stable fireballs have...

We present direct experimental evidence of the presence of filamentary current structures in turbulent magnetized plasmas. Experiments have been performed in different devices. In the the reversed field pinch RFX-mod device, small scales turbulent intermittent structures, have been interpreted as Drift-Kinetic Alfén vortices, resulting from the non...

For ASDEX Upgrade (AUG) a new probe head was developed for simultaneous measurements of electric and magnetic fluctuations in the edge plasma region. The probe head consists of a cylindrical graphite case. On the front side six graphite pins are mounted. With this arrangement the poloidal and radial electric field components, the ion saturation cur...

Emissive probes have been used for the direct measurement of plasma potential in many plasma devices and different approaches have been introduced to measure plasma potential using emissive probes. But the biggest disadvantage of the emissive probe is its short lifespan due to its self-arrangement and different plasma environment. For example, fila...

Experimental investigations of the plasma potential, poloidal electric field and electron temperature during L-mode and ELMy H-mode were performed on ASDEX Upgrade by means of a probe head containing four ball-pen probes and four Langmuir probes. This allows to measure simultaneously the floating and plasma potential at the same time which are rela...

A cavity-hollow cathode was investigated as low-cost sputtering source for titanium. An argon discharge is produced inside a hollow cathode consisting of two specifically formed disks of titanium. An additional cavity further enhances the pendulum effect of the electrons. Measurements with small Langmuir probes yielded evidence for the formation of...

Positively biased probes in magnetized plasmas can create several phenomena which are not described by standard probe theories. These include transient currents for pulsed electrodes in the regime of Electron MHD, relaxation oscillations induced by large probes and high frequency instabilities at the sheath-plasma frequency. Although they may limit...

Fluid turbulence modeling of the Scrape-Off Layer (SOL) has been an area of promising progress. Here we report on results obtained using codes that relax the usual scale separation paradigm in the SOL and treat fluctuations and time averaged background profiles simultaneous and without separating them. Initial modeling is based on an interchange mo...

Radial transport of poloidal momentum in the scrape-off layer (SOL) of ASDEX Upgrade was investigated. A reciprocating probe was used with a probe head containing six Langmuir probes with which the poloidal and radial electric field components and the density were determined. Separating all quantities into stationary and fluctuating parts, four com...

Direct measurements of current density perturbations associated with non-linear phenomena in magnetized plasmas can be carried out using in situ magnetic measurements. In this paper we report such measurements for three different kinds of phenomena. Current density fluctuations in the edge density gradient region of a fusion plasma confined in reve...

Magnetically confined plasmas are often subject to relaxation oscillations accompanied by large transport events. This is particularly the case for the high confinement regime of tokamaks where these events are termed edge localized modes (ELMs). They result in the temporary breakdown of the high confinement and lead to high power loads on plasma f...

ASDEX Upgrade was operated with a fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronization. The boundary conditions set by the full W wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plat...

Experimental investigations of the plasma potential and electric field were performed for ELMy H-mode plasmas in the vicinity of the limiter shadow of ASDEX Upgrade. A fast reciprocating probe with a probe head containing four ball-pen probes (BPPs) [J. Adamek et al., Czech. J. Phys. 54 (2004) C95 – C99.] was used on the midplane manipulator. Diffe...

Complex space charge structures bordered by electrical double layers were spectrally investigated in argon plasma in the domain 400-1000 nm, identifying the lines corresponding to the transitions from different excited states of argon. The electron excitation temperature in the argon atoms was estimated from the spectral lines intensity ratio.

An array of four heatable floating probes and one cylindrical probe staggered on the same poloidal meridian was used for fluctuation measurements in the ISTTOK edge plasma. The array permits the simultaneous measurement of density and poloidal electric field, and thereby of the radial turbulent flux. We have found that emissive probes deliver more...

Experimental investigations of the plasma potential and electric field during ELMs were performed in the limiter shadow of ASDEX Upgrade by means of a probe head containing four ball-pen probes [1,2] and four Langmuir probes. The measurements of the gradient of both the plasma and floating potentials show the difference between these two quantities...

Potential formation in a two electron temperature plasma with two species of positive ions has been studied analytically by a one-dimensional, collision-less fully kinetic model. The results have been verified in a PIC simulation. It has been found that addition of heavy ions and energetic electrons lowers the floating potential of an electrode in...

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 para...

Fireballs are localized discharge phenomena on positively biased small additional electrodes in thin plasmas with a sufficiently high background gas pressure. They appear as sharply bounded spherical or cylindrical space charge structures with a higher luminosity, density and plasma potential than the ambient plasma. Originating from a normal elect...

Fireballs are discharge phenomena on positively biased small electrodes in plasmas. The discharge arises from electron energization at a double layer. Fireballs can collect relatively large electron currents from the ambient plasma. Fireballs can become unstable to relaxation oscillations. This paper addresses the space–time evolution of pulsed fir...

Summary form only given. By positively biasing an electrode immersed into low-temperature plasma up to a certain threshold value, a complex space charge structure appears in front of it, in form of a luminous quasi-spherical plasma body. This structure was sometimes called anode glow, ball of fire or fireball. Experimental investigation shown that...

We report a study of electron saturation current variations at varying probe heating that were found to be closely related to probe wire contamination. The study was performed in three types of low temperature argon plasma – the weakly magnetized plasma of a cylindrical magnetron, the non-magnetized plasma of a double plasma machine and a hollow ca...

The diffusion coefficient in the edge plasma of fusion devices is measured by a new technique. The method is based on studying the decay of the plasma fluctuation spectrum inside the so-called “Ball-pen probe”: a small ceramic tube having its mouth flush with a magnetic surface and a movable electrode inside the tube. When the electrode protrudes f...

Simultaneous measurements of the ion temperature by the segmented tunnel and Katsumata probe in the CASTOR tokamak are reported. The segmented tunnel probe measures two ion saturation currents Iseg 1, Iseg 2 towards two segments of the tunnel. The axis of the tunnel is aligned with the total magnetic field. The value of the ratio of these two curre...

An analysis of the accurateness of determining the plasma space potential from the probe current-voltage characteristics have been performed to improve the accurateness of the consequently determining plasma parameters from it. The problem is analyzed as an inverse and ill-posed one. Tikhonov's regularization method was introduced to solve such pro...

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...