Plasma-surface Interaction Studies Research Articles

Applied Plasma Physics Experiments in Linear (APPEL) device for plasma surface interaction studies

The APPEL device (Applied Plasma Physics Experiments in Linear Device), recently commissioned at the Institute for Plasma Research (IPR), is an intermediate setup towards building a Plasma Material Interaction (PMI) facility. A particular application of this device is to develop and test high-density, low-pressure plasma sources that can be operated under magnetic fields; optimize the plasma sources for working under various magnetic field configurations; and perform physics experiments relevant to electron heating and plasma interactions with external electrodes. The setup consists of 16 electromagnets that provide a flexible, steady-state axial magnetic field in excess of 0.4 T over an axial distance of 3.5 m, with 2 % radial uniformity across 32.0 cm diameter. This paper gives a brief overview of the APPEL device, emphasizes on the APPEL magnet system, and presents the characteristics of a 3.5 m long, high-density (1017–1018 m−3) helium plasma column, using a hollow cathode plasma source obtained in the pressure range below i.e., p < 2.0 Pa. The corresponding ion flux is found to be in the range of 1021 -1022 m−2s−1, similar to other linear systems currently in use for plasma surface interaction studies. A qualitative discussion is given to explain the characteristics of the plasma column.

The CPS's pre-heating effect on the capability to withstand extreme plasma loads

This work presents experimental studies of plasma surface interactions (PSIs) during QSPA plasma impacts on the Sn Capillary Porous Structures (CPSs) in conditions, simulating disruption and ELM-like loads. The 3D tungsten target filled by Sn was exposed to the plasma streams with an energy density of up to 3 MJ/m2 which resulted in tungsten melting and evaporation. The initial temperature of a sample between plasma pulses had two options: remained at a room value for one sample, and reached 300 C which is close to Sn melting point for another one. The number of plasma irradiations reached 100 pulses. It is shown that the particle ejection was observed during PSI for pre-heated as well as cold CPS targets. The difference in values of the number of particles ejected from the targets, their velocities, and the start-time during exposition is discussed. Particles leave the target surface during all expositional pulses for both CPS targets. The threshold character of the ejection is more pronounced for the cold target. After the 100 pulses reduced Sn pool is observed. Moreover, areas without Sn occur on the exposed targets. Nevertheless, the severe damage of the W base of the CPS targets is not revealed during high-cycle powerful plasma loads.

Advances in Magnum-PSI probe diagnosis in support of plasma–surface interaction studies

Magnum-PSI is a linear plasma generator equipped with a superconducting magnet, assuring fusion devices relevant conditions at plasma–surface interface. The plasma column was diagnosed using 64 probes embedded in the target. The cross-sectional distributions of plasma parameters (floating potential, ion saturation current and electron temperature) were measured for hydrogen and deuterium plasmas under various discharge conditions. The radial profile of the floating potential across the plasma column can be described by a reversed Mexican hat-like wavelet, having the most negative potential at the center of the plasma column. The negativity of the floating potential diminishes when the discharge current increases or the magnetic field decreases. The axial gradient of the floating potential is reduced by increasing the magnetic field. The ion saturation current is maximum at the center of the plasma column, increasing with both the discharge current and magnetic field. The ion flux to the target, estimated from Thomson scattering (TS) data, was confirmed by probe measurements. The electron temperature estimated from the ion branch of the probe current–voltage characteristic is few times larger than that obtained from TS. By increasing the gas pressure in the target chamber, the time-dependent ion saturation current measured by probes changes from a constant average current (when the plasma column is attached to the target) to a fluctuating average current with scattered peaks (in a partially detached regime) which vanishes completely in the fully detached regime. With respect to hydrogen, the plasma column is wider in deuterium and is characterized by less negative floating potential distributions.

Comparative analysis of the plasma parameters of ECR and combined ECR + RF discharges in the TOMAS plasma facility

The toroidal magnetized system (TOMAS) plasma facility aims at complementary research on wall conditioning methods, plasma production and plasma–surface interaction studies. This paper explores for the first time the parameters in helium electron-cyclotron resonance (ECR) plasma and combined ECR + radio-frequency (RF) discharges in TOMAS. The ECR discharge in this work, at 2.45 GHz and 87.6 mT, is the main one for creating and maintaining the plasma, while the addition of RF power at 25 MHz allows to broaden the achievable electron temperature and density at a given gas flow, as evidenced by triple Langmuir probe measurements. This effect of the combined ECR + RF discharge provides flexibility to study particular aspects of wall conditioning techniques relevant to larger devices, or to approach plasma conditions relevant to fusion edge plasmas for particular surface interaction studies.

Vapour shielding of liquid-metal CPS-based targets under ELM-like and disruption transient loading

This paper presents experimental studies of plasma-surface interactions during powerful plasma impacts of a quasi-stationary plasma accelerator (QSPA) on the Sn capillary porous systems (CPSs) in conditions simulating disruption and edge localized modes (ELM) like loads. Experiments were carried out using two QSPA devices. ELM-like plasma exposures were performed with QSPA-M test-bed facility. A large-scale QSPA Kh-50 device was used to simulate plasma disruptions and giant ELMs. Variation of the plasma stream energy density has been performed to study the onset of vapour shield. It is shown that during plasma exposures of a Sn-CPS target with the QSPA plasma load <1 MJ m−2, single dust particles traces have been registered. A further increase in the heat load leads to the splashing of the eroded material. For ELM-like impacts, a rather weak melt motion was observed on the target surface. A post-mortem analysis has shown that the CPS structure was not destroyed in the course of many repetitive ELM-like pulses. Surface morphology has changed from a smooth surface to corrugation structures with the formation of some cavities in mesh cells due to the influence of the surface tension and capillary effects. Spectral lines of Sn I and Sn II have been identified by optical emission spectroscopy in the near-surface plasma. A plasma shield, that consists mostly of Sn neutrals appears at Q ∼ 0.1 MJ m−2. An increase in the surface heat load resulted in the intensive emission of Sn II lines, which started to be observed at Q ∼ 0.3 MJ m−2. The plasma electron density near the surface increases significantly at Q > 0.5 MJ m−2, which corresponds to the strong vapour shielding of the exposed surface. A comparison between the obtained results on the vapour shielding of Sn CPS and available numerical simulation using the TOKES code has been performed.

The upgraded TOMAS device: A toroidal plasma facility for wall conditioning, plasma production, and plasma-surface interaction studies.

The Toroidal Magnetized System device has been significantly upgraded to enable development of various wall conditioning techniques, including methods based on ion and electron cyclotron (IC/EC) range of frequency plasmas, and to complement plasma-wall interaction research in tokamaks and stellarators. The toroidal magnetic field generated by 16 coils can reach its maximum of 125 mT on the toroidal axis. The EC system is operated at 2.45GHz with up to 6 kW forward power. The IC system can couple up to 6 kW in the frequency range of 10 MHz-50MHz. The direct current glow discharge system is based on a graphite anode with a maximum voltage of 1.5 kV and a current of 6A. A load-lock system with a vertical manipulator allows exposure of material samples. A number of diagnostics have been installed: single- and triple-pin Langmuir probes for radial plasma profiles, a time-of-flight neutral particle analyzer capable of detecting neutrals in the energy range of 10 eV-1000eV, and a quadrupole mass spectrometer and video systems for plasma imaging. The majority of systems and diagnostics are controlled by the Siemens SIMATIC S7 system, which also provides safety interlocks.

RF-heating system of helicon-type for PLM-2 plasma device: design parameters

The PLM-2 linear magnetic plasma device is for steady-state stationary hours-long plasma confinement with parameters providing the relevant fusion reactor plasma loads on materials. The machine is designed both for fundamental studies of plasma-surface interactions under high heat and particle fluxes, and as a high-heat flux facility for the tests of plasma-facing components under realistic plasma conditions. In the PLM-2 plasma device, a plasma heating ICR system will be created using a helicon antenna. Production of plasma by using helicon waves is known as helicon discharge, and the plasma produced by using the helicon discharge has a totally distinct character where the enhancement of the plasma parameters takes place at particular conditions. Helicon discharges produce higher density plasma at a comparatively lower given input power than any other RF or DC discharges, and also the plasma is least contaminated due to the presence of the antenna out-side the chamber.

Uniform propagation of cathode-directed surface ionization waves at atmospheric pressure

The uniform propagation of positive-polarity surface plasmas in air at atmospheric pressure has been achieved using a multi-layer structure, consisting of a silicon wafer covered by a 1 μm thick dielectric SiO2 layer as a propagation surface. Instead of the branched streamers observed in the same conditions when using conventional bulk dielectric surfaces, the plasma exhibits a homogenous ring-shaped structure with a high degree of reproducibility and stability. The plasma is generated by applying nanosecond positive voltage pulses to a tungsten wire touching the dielectric surface. The plasma has been imaged in single shot operation at high spatial resolution with an ultraviolet reflective microscope coupled with a fast intensified charge-coupled device camera. Time- and space-resolved optical emission spectroscopy shows that the homogenous ring corresponds to the propagation of an ionization front with a region of high N2+* emission. We discuss the origin of the ring-shaped ionization wave, considering the role of the Si–SiO2 interface and the effect of illumination by an external light source. The ring ionization wave may result from branching inhibition, due to a photoelectric effect at the interface created by the photons emitted by the plasma. The generation of stable uniform surface ionization waves, in ambient air at atmospheric pressure, could be of interest for further advanced plasma–surface interaction studies or flow control applications.

The PLM-2 plasma device for full-scale tests of fusion reactor materials with stationary plasma loads: design parameters

The PLM-2 linear magnetic plasma device is designed for the steady-state hours- long plasma confinement. It is the upgraded PLM plasma device. Its parameters will provide the relevant fusion reactor plasma loads on materials. The machine is designed both for fundamental studies of plasma-surface interactions under conditions of high heat and particle fluxes, and for the tests of plasma-facing components under realistic plasma conditions in the high-heat-flux facility. The PLM-2 facility is an advanced linear plasma device uniquely capable of producing plasma conditions similar to those expected in the divertors of the fusion reactors such as ITER, fusion neutron source FNS and DEMO under conditions of steady-state operation. In this article, we describe the PLM-2 design parameters. The PLM-2 device has no analogues in Russia; its parameters are similar to those of the most high-power devices in the world, such as the MAGNUM-PSI, and it is going to be built in Moscow at National Research University “MPEI”.

Operational characteristics of the superconducting high flux plasma generator Magnum-PSI

The interaction of intense plasma impacting on the wall of a fusion reactor is an area of high and increasing importance in the development of electricity production from nuclear fusion. In the Magnum-PSI linear device, an axial magnetic field confines a high density, low temperature plasma produced by a wall stabilized DC cascaded arc into an intense magnetized plasma beam directed onto a target. The experiment has shown its capability to reach conditions that enable fundamental studies of plasma-surface interactions in the regime relevant for fusion reactors such as ITER: 1023–1025 m−2s−1 hydrogen plasma flux densities at 1–5 eV for tens of seconds by using conventional electromagnets. Recently the machine was upgraded with a superconducting magnet, enabling steady-state magnetic fields up to 2.5 T, expanding the operational space to high fluence capabilities for the first time. Also the diagnostic suite has been expanded by a new 4-channel resistive bolometer array and ion beam analysis techniques for surface analysis after plasma exposure of the target. A novel collective Thomson scattering system has been developed and will be implemented on Magnum-PSI. In this contribution, the current status, capabilities and performance of Magnum-PSI are presented.

Overview of the plasma-surface interaction on limiter surfaces in the startup campaign of Wendelstein 7-X

The first operational campaign of Wendelstein 7-X (W7-X) provided an excellent environment for the study of plasma-surface interaction (PSI) in a stellarator. In situ spectroscopic analysis via a combined visible/infrared camera system and a filterscope system revealed that the primary erosion zone was correlated with the high heat flux regions on the limiter. This analysis matched to where the erosion zone was found in the post-mortem analysis, which was done with scanning electron microscopy/focused ion beam/electron dispersive x-ray spectroscopy imaging. Additionally, a region of prompt deposition was found to the inside of these high heat flux zones. A region of far scrape-off layer (SOL) deposition was found at the edges of the limiter tiles. All deposition regions were identified by their homogeneous, increased oxygen content compared to the pure carbon makeup of the limiters. Poloidal variation of the impinging heat flux follow the imprint of the 3D SOL flux tubes. In how far this reflects in the PSI will require further analysis and modeling.

The plasma device for the high-heat plasma testing of refractory metals and inventing of new highly porous materials

A unique plasma device has been constructed at the NRU “MPEI” for the study of plasma-surface interaction and the high-heat plasma testing of refractory metals, such as tungsten, molybdenum, steel and other plasma facing materials used in fusion reactor including the ITER. This plasma device is a multi-cusp linear stationary plasma confinement system. It has power-saving characteristics as well as compactness due to the employment of the 8-pole multicusp magnetic field configuration instead of a strong axial magnetic field. Experiments are planned to develop a novel technology for highly porous surface structure of the refractory metal with a pore size and nanofibers of 50 nanometers including tungsten “fuzz”.

Study of plasma-surface interaction at the GOL-3 facility

The review presents experimental studies of plasma-surface interaction and materials behavior under plasma loads done in the multiple-mirror trap of the GOL-3 facility. In the experiments for the PSI, the energy density in the extracted plasma stream varies from 0.5 to 30MJ/m2. Parameters of near-surface plasma measured by a set of diagnostics are reviewed. Surface patterns of targets exposed to the plasma are analyzed. The erosion depth depends on the energy loads—it rises from 0 to 600μm at 0.5 and 30MJ/m2, correspondingly. Cracking and evolution of graphite and tungsten surface morphology are discussed. The enthalpy of brittle destruction of graphite (10kJ/g), which determines the threshold of bulk damage of targets irradiated with a charged-particle flux with large penetration depth, was determined. Comparison of different facilities for PSI studies are presented. Heat flux play a key role to the target surface erosion.

A dual wavelength imaging system for plasma-surface interaction studies on the National Spherical Torus Experiment Upgrade.

A two-channel spectral imaging system based on a charge injection device radiation-hardened intensified camera was built for studies of plasma-surface interactions on divertor plasma facing components in the National Spherical Torus Experiment Upgrade (NSTX-U) tokamak. By means of commercially available mechanically referenced optical components, the two-wavelength setup images the light from the plasma, relayed by a fiber optic bundle, at two different wavelengths side-by-side on the same detector. Remotely controlled filter wheels are used for narrow bandpass and neutral density filters on each optical path allowing for simultaneous imaging of emission at wavelengths differing in brightness up to 3 orders of magnitude. Applications on NSTX-U will include the measurement of impurity influxes in the lower divertor strike point region and the imaging of plasma-material interaction on the head of the surface analysis probe MAPP (Material Analysis and Particle Probe). The diagnostic setup and initial results from its application on the lithium tokamak experiment are presented.

Laser-based diagnostics applications for plasma-surface interaction studies

Several laser based diagnostics are implemented on to the linear plasmagenerator Magnum-PSI, wherein ITER divertor relevant plasma-wall conditionsare realized. Laser Induced Desorption Quadrupole Mass Spectroscopy(LID-QMS) and Laser Induced Breakdown Spectroscopy (LIBS) are installed tomeasure deuterium retention in plasma facing components. Combined withThermal Desorption Spectroscopy, LID-QMS can be used to measure lateralretention profiles. LIBS is used to measure the surface compositionqualitatively, after plasma exposure. An advanced Thomson Scattering (TS)system measures electron density, neutral density and electron temperatureprofiles (spatial resolution < 2 mm) across the maximum 100 mm plasmadiameter. Very low electron density (9 × 1018 m−3) can bemeasured within seconds with accuracies better than 6%. The minimummeasurable electron density and temperature are ∼ 1 × 1017 m−3 and ∼ 0.07 eV, respectively. By virtue of the high systemsensitivity, single pulse TS can be performed on high density pulsed plasmas(used for replicating ELMs). For measuring the ion temperature and flowvelocity of the plasma a Collective TS system (CTS) is being built: thesmall Debye length of the Magnum-PSI plasma enables application of thismethod at relatively short laser wavelength. In a feasibility study it wasshown that forward CTS with a seeded Nd:YAG laser operating at 1064 nm, canbe applied at Magnum-PSI to measure ion temperature and axial velocity withan accuracy of < 8% and < 15%, respectively. Two high spectralresolution ( ∼ 0.005 nm) detection schemes are applied simultaneously:an Echelle grating spectrometer (enabling profile measurements) and a systembased on a Fabry-Perot etalon that enables wavelength scanning over its freespectral range, by tilting the device. The status and performance of thevarious laser based plasma and surface diagnostics will be reported alongwith experimental results.

GDMT-T: Superconducting Linear Device for PMI Studies

The prospect of a new facility for plasma-surface interaction studies is presented. The unique features of this facility named GDMT-T are ability to generate both stationary plasma flux and transient events which simulate ITER ELM-mode operation and strong magnetic field on the target important for studies of dust and impurity migration. The experimental program of GDMT-T will include testing of plasma-facing components at ITER-relevant conditions and investigations of electron beam generation and beam-plasma interaction for application in the next generation of linear fusion facilities.

Advanced Thomson scattering system for high-flux linear plasma generator

An advanced Thomson scattering system has been built for a linear plasma generator for plasma surface interaction studies. The Thomson scattering system is based on a Nd:YAG laser operating at the second harmonic and a detection branch featuring a high etendue (f/3) transmission grating spectrometer equipped with an intensified charged coupled device camera. The system is able to measure electron density (n(e)) and temperature (T(e)) profiles close to the output of the plasma source and, at a distance of 1.25 m, just in front of a target. The detection system enables to measure 50 spatial channels of about 2 mm each, along a laser chord of 95 mm. By summing a total of 30 laser pulses (0.6 J, 10 Hz), an observational error of 3% in n(e) and 6% in T(e) (at n(e) = 9.4 × 10(18) m(-3)) can be obtained. Single pulse Thomson scattering measurements can be performed with the same accuracy for n(e) > 2.8 × 10(20) m(-3). The minimum measurable density and temperature are n(e) < 1 × 10(17) m(-3) and T(e) < 0.07 eV, respectively. In addition, using the Rayleigh peak, superimposed on the Thomson scattered spectrum, the neutral density (n(0)) of the plasma can be measured with an accuracy of 25% (at n(0) = 1 × 10(20) m(-3)). In this report, the performance of the Thomson scattering system will be shown along with unprecedented accurate Thomson-Rayleigh scattering measurements on a low-temperature argon plasma expansion into a low-pressure background.

Computational study of plasma-surface interaction in plasma-assisted technologies

The influence of the unevenness of substrates immersed into plasma important for plasma-based treatment of materials were studied by computer experiment. The role of both substrate properties and plasma parameters was investigated. For this analysis the combination of multidimensional fluid modelling and particle simulation was used. The fluid part of our model consisted of continuity equations for all charged species, energy balance equation for electrons and Poisson equation. The basic scattering processes were also included. The particle simulation technique was used both for the calculation of electron energy distribution function and for the derivation of quantities characterising plasma-surface interaction. This approach enabled us to study in detail the structure of the sheath and presheath near metal substrates with realistic geometries and finite dimensions. The main attention was devoted to the influence of substrate geometry in both macroscopic and microscopic spatial scales on the local electric fields in plasma.

Studies of plasma surface interactions during short time plasma etching of 193 and 248nm photoresist materials

As the device dimensions scale to 100nm, the use of photoresist materials is suitable for lithographic patterning at 193nm. The molecular structure of 193nm photoresist materials is significantly different from that of 248nm photoresist materials [H. Ito, IBM J. Res. Deu. 45, 683 (2001), T. Kajita et al., Proc. SPIE 4345, 712 (2001)], which leads to a number of undesirable consequences, including pronounced surface and line edge roughness during plasma etching [H. Ito, IBM J. Res. Deu. 41, 69 (1997), [E. Reichmanis et al., J. Vac. Sci. Technol. B 15, 2528 (1997), [L. Ling et al., ibid. 22, 2594 (2004)]. In this article, we present an investigation of the mechanisms for the surface/line edge roughening of photoresist materials during plasma etching using C4F8∕90%Ar discharges. We emphasized in our study short exposure times (the first few seconds) of the photoresist materials and structures to the plasma, a time regime that has not been well studied. Rapid modifications were observed for both 193 and 248nm photoresists during short time exposure. During the first seconds of plasma exposure, photoresist material densification and hydrogen depletion are important processes. It is also found that rough surfaces develop within a few seconds of exposure to the C4F8∕90%Ar discharges. Plasma exposure leads to the formation of rough edges on the top of trench sidewalls in photoresist trench and line structures. During prolonged exposure to the plasma, the roughness is transferred to produce striations on the sidewalls. After an initial stage, the roughening rate remains constant for 193nm photoresist, whereas for 248nm photoresist the roughening rate is negligible. This difference is possibly related to the preferential removal of carbonyl groups for the 193nm photoresist material, which has been revealed by x-ray photoelectron spectroscopy and seconday ion mass spectroscopy.

Plasma–surface interaction in the context of ITER

The decreasing availability of energy and concern about climate change necessitate the development of novel sustainable energy sources. Fusion energy is such a source. Although it will take several decades to develop it into routinely operated power sources, the ultimate potential of fusion energy is very high and badly needed. A major step forward in the development of fusion energy is the decision to construct the experimental test reactor ITER. ITER will stimulate research in many areas of science. This article serves as an introduction to some of those areas. In particular, we discuss research opportunities in the context of plasma-surface interactions. The fusion plasma, with a typical temperature of 10 keV, has to be brought into contact with a physical wall in order to remove the helium produced and drain the excess energy in the fusion plasma. The fusion plasma is far too hot to be brought into direct contact with a physical wall. It would degrade the wall and the debris from the wall would extinguish the plasma. Therefore, schemes are developed to cool down the plasma locally before it impacts on a physical surface. The resulting plasma-surface interaction in ITER is facing several challenges including surface erosion, material redeposition and tritium retention. In this article we introduce how the plasma-surface interaction relevant for ITER can be studied in small scale experiments. The various requirements for such experiments are introduced and examples of present and future experiments will be given. The emphasis in this article will be on the experimental studies of plasma-surface interactions.