Plasma-wall Interaction Studies Research Articles

Determination of chemical composition and charge state distribution of chemical erosion hydrocarbon fluxes

The hydrocarbon fluxes released from a hydrogenated graphite surface due to thermal erosion and chemical sputtering become subject to a number of particle–surface interaction (PSI) processes in the near-surface region. These processes involve an electron transfer between the released particle and the surface and, thus, affect the charge state distribution of released hydrocarbons. The electron capture from the surface by a hydrocarbon ion to an excited dissociative state can lead to prompt dissociation of the formed molecule, producing two (or more) lighter fragments. These PSI processes (of resonant and Auger type) have so far not been included in the plasma–wall interaction studies. It is, however, obvious that they play a decisive role in determination of chemical composition and charge state distribution of hydrocarbon fluxes released from the surface and entering the edge/divertor plasma region. It is equally obvious that the same processes also take place when the hydrocarbon species approach the graphite surface from the interior of the edge/divertor plasma region, thus affecting their sticking to the surface.

JET's contribution to fusion science and ITER

JET is the largest fusion device in the world and is the only tokamak device that can operate with deuterium-tritium plasmas, can confine the 3.5 MeV fusion alphas and can operate with beryllium, one of the wall materials for the next step fusion device ITER. JET has largely contributed to progress in fusion research over the past 20 years. Highlights of fusion research at JET include the first large-scale generation of fusion power (16 MW) from D–T reactions, equivalent to a record fusion gain of 0.7, i.e, very close to break-even, important contributions to the international confinement database, and plasma-wall interaction studies: divertor physics, edge instabilities causing large fluxes of heat and particles (so-called ‘ELMs’), and characterization of the plasma boundary. JET contributes also largely to the development of the operational scenarios foreseen for ITER, the so-called ELMy H-Mode and Advanced Scenarios, including the development of real-time control algorithms for various operational aspects of such discharges. Future upgrades for JET consist in an ITER like wall, installation of an ITER like ICRH antenna and various new diagnostics.

Study of plasma wall interactions in the long-pulse NB-heated discharges of JT-60U towards steady-state operation

Long time scale variation of plasma-wall interactions and its impact on particle balance, main plasma performance and particle behavior have been investigated in ELMy H-mode plasmas by extending the discharge pulse and the neutral beam heating pulse to 65s and 30s, respectively. The wall pumping rate starts to decrease in the latter phase by repeating the long-pulse discharges with 60% of Greenwald density sustained by gas-puffing. After several discharges, the wall inventory is saturated in the latter phase and, consequently, the density increases with neutral beam fuelling only. The edge pressure in the main plasma is reduced and ELMs are close to the type III regime under conditions of wall saturation. The intensities of C II emission near the X-point and CD band emission in the inner divertor start to increase before the wall saturates and continue to increase after the wall is saturated.

High-speed 2-D image measurement for plasma-wall interaction studies

A movable carbon target with a head diameter of 9cm was inserted into the peripheral region of a Heliotron J plasma. High-speed, two-dimensional images of filtered visible light emission near this target were taken by a C-MOS video camera. In high-density ECH discharges (70GHz, 0.3MW, n¯e>1.5×1019 m−3) it was observed that a striped pattern of Hα light moved along the poloidal direction. Simultaneously, other diagnostic data (ECE signals, line integral electron density, ion saturation currents of Langmuir probes) showed plasma oscillation at a frequency of 5–6kHz. Individual signals from pixels of the visible camera images indicate that the oscillation phase is approximately constant along the field line, suggesting that coherent structure phenomena along the field line are present in the high-density plasmas of Heliotron J.

Use of a quartz microbalance for plasma-wall interaction studies

Use of a quartz microbalance for plasma-wall interaction studies

Use of a quartz microbalance for plasma-wall interaction studies

This paper reports on the first use of quartz crystal microbalances as a useful diagnostic for plasma-wall interaction studies in tokamaks. Two quartz microbalance systems were installed recently in the TdeV tokamak. Reliable mass measurements are obtained when a correction is applied to the frequency response of the crystal, owing to its temperature rise during the discharges. Particle fluxes in the scrape-off layer (SOL), perpendicular and parallel to the magnetic field lines along the ion drift or electron drift directions were measured. As expected, a larger erosion rate was measured in the ion drift direction, while deposition was observed on the quartz surfaces facing up and facing down but not facing the plasma (i.e. field lines at grazing incidence). Radial displacement of the set-up permitted the measurement of the radial dependence of the erosion-deposition processes. An increase of the plasma current induces a slight increase of the deposit thickness. A detachment of the plasma in the divertor doubles the thickness of the deposit. The quartz microbalance showed that the deposition rate is strongly increased for a few discharges following a disruption. This diagnostic method is also sensitive to the atomic number of injected impurities.

Joint ENEA-ANSALDO design and manufacturing feasibility study of a brazed first wall for net

In collaboration with ANSALDO, ENEA is performing a design and manufacturing feasibility study for the first wall of NET during the physics operation phase. The main design specifications are the average neutron wall load = 1 MW/m 2 , peak surface heat flux = 0.4 MW/m 2 , total number of bum pluses = 1×10 4 , average burn pulse duration = 100 s, structure material = ss 316L S.A., coolant = H 2 O (He) at 50 °C. The ENEA-ANSALDO (reference design is based on the use of flat plates coupled by nicrobrazing to water poloidal cooling tubes. The final aim of the work is manufacturing a representative NET first wall box segment (mockup of indicative dimensions 0.65×0.25×0.15 m 3 ) suitable for testing in the 190 kW Thermal Fatigue Test Facility at JRC-Ispra. In this paper we report on the related design work and the results of the studied performed: technological development (manufacturing test activity, brazed joints characterization tests, mockup design and manufacturing), thermomechanical analysis, plasma-wall interaction studies (tests on protective carbon composite material, theory), and tritium transport analysis.

Conversion electron mössbauer spectroscopy: A new tool for plasma-induced surface modification studies

Conversion electron Mossbauer spectroscopy is an attractive technique for near-surface examination of materials, which has great but as yet unexploited potential in plasma-wall interaction studies, as well as in other materials science elements of the fusion energy program. The CEMS technique has recently been applied for the first time in studying various components of tokamaks, such as in first wall steels, graphite limiters and tritium breeding lithium ceramics. Studies of plasma-induced erosion and redeposition of iron in the ASDEX tokamak, agglomeration of alloy cementite particles on graphite limiters, modifications of iron and stainless steels under deuterium bombardment and analysis of iron impurities in lithium ceramics are presented here as examples.

The T-15 tokamak: Status and program of plasma-wall interaction studies

The main features of T-15 and of some diagnostics intended for it (wall analysis, laser induced fluorescence, and IR-pyrometry) are described.

Plasma-wall interaction and plasma performance in textor — A review

TEXTOR, a tokamak primarily devoted to plasma-wall interaction studies, has been operated during the last 2–3 years under a wide variety of discharge conditions. Discharges with Ohmic heating only and with additional ICR heating in the MW-range were produced over an extended range of plasma parameters. Different limiter materials were used — Inconel, stainless steel, graphite, TiC coated graphite. The surface of the first wall (liner) consisted either of Inconel or of a carbon layer deposited upon the Inconel. Characteristic features of the discharge have been modified by making use of the modular pump limiter ALT-I and by applying a localized perturbation coil for “ergodization” of the magnetic field structure in the boundary. The characteristics of plasma-wall interaction under these conditions and their influence on the global performance of the TEXTOR plasma are described and discussed. Use has been made of novel diagnostics specifically developed for edge plasma investigation.

Plasma performance and plasma-wall interaction in TEXTOR

The performance of the TEXTOR tokamak, as manifested by its density limit, confinement time, behaviour of density and current density profiles, and impurity level, is described. By means of ICRH heating, powers significantly above the Ohmic one are obtained. The deposition of thin carbon layers onto all inner surfaces is shown to be an effective method for achieving low impurity concentrations in the plasma. Results of a systematic study of plasma-wall interaction, with particular emphasis on a quantitative description of the particle sources and of the boundary layer, are presented. Pump limiter experiments resulting in high particle removal rates are described.

Accelerator-based plasma-wall interaction studies on the TEXTOR tokamak

Deposition probes are commonly used to determine plasma edge characteristics in tokamaks. Such probes are frequently analyzed using accelerator-based techniques and can yield information on impurity fluxes, hydrogen fluxes and energies and surface erosion rates in the plasma edge. Several types of deposition probes have been employed to investigate the plasma edge region of the TEXTOR tokamak in Jülich, FRG. TEXTOR is a moderate size tokamak (major radius = 1.75 m) that is capable of 2–3 s discharges and contains a liner that can be heated to 600°C. These capabilities make TEXTOR particularly attractive for the study of plasma-wall interactions. Several countries, including the United States, Sweden and the host nation, are carrying on active research programs in the area of plasma edge studies. This research has included measurements of plasma, impurity and power fluxes at various radii in the scrapeoff layer. The results achieved in characterizing the plasma edge in these recent probe studies on TEXTOR will be reviewed.

Surface deposition probe.

Application of surface deposition probe to the study of plasma-wall interactions in torus devices is reviewed. Various surface analysis techniques have been utilized to detect information from the surface of the deposition probes. Interactions of plasma with first wall surfaces are very complicated, since various particles bombard the surface simultaneously with different energies and fluxes. With use of fine surface probes and analysis techniques we have tried to clarify elementary processes of plasma surface interactions.

C 1s binding-energy shift of pyrolytic graphite bombarded by keV-deuterium ions.

As a basic study of plasma-wall interactions, chemical state of graphite basal face after deuterium bombardment are studied by means of X-ray photoelectron spectroscopy. As the fluence of deuterium ions with energy of 1 or 4keV is increased from 1014 to 1018 ions/cm2, the C Is line is observed to shift towards the lower binding energy at first, down to the minimum by about 0.2eV and then towards the higher energy up to an asymptotic value of about 0.2eV higher than the initial position. The first negative shift is due to lattice displacement at the target surface, and the subsequent positive shift is due to the deuterium trapping at the graphite surface. The fluence dependence of the observed C 1s shift is explained by a simple saturation model.

Recent Results Obtained by Use of Accelerators on Plasma Edge Properties in Controlled Fusion Devices and on Properties of High Power Neutral Beams

The study of plasma-wall interactions is of primary importance in present fusion devices. Measurements of incident fuel and impurity fluxes, retention and release of fuel atoms, and erosion of internal components are of particular interest. Accelerators in the megaelectronvolt range are being used both to measure the depth profile of fuel atoms implanted in samples placed in the plasma edge by use of nuclear reactions and to measure impurities and film thicknesses by use of elastic scattering reactions. Secondary ion mass spectrometry (SIMS) is used to determine flux and energy distributions of fuel atoms and to measure species composition and impurities in the beams of high power neutral beam injectors. Recent results obtained with these techniques are presented and areas of future study are discussed.

Absolute Density Measurement of Metal Atoms by Laser Resonance Scattering with the Aid of Rayleigh Scattering

The absolute density of metal atoms such as iron and aluminium atoms, the knowledge of which is urgently needed for study of the high temperature plasma-wall interaction, was measured by the laser resonance scattering method within an accuracy of 50%. The calibration of the optical system was performed by using the Rayleigh scattering from argon gas at subatmospheric pressure, and the accuracy of the metal density measurement was confirmed by using accurately adjustable metal vapour from a Knudsen cell as a scattering target.

Application of Laser Resonance Scattering to the Study of High-temperature Plasma-wall Interaction

Application of Laser Resonance Scattering to the Study of High-temperature Plasma-wall Interaction

The application of high-field surface analytical techniques to the study of plasma-wall interactions in PLT

The techniques of imaging atom-probe mass spectroscopy, field-ion microscopy, and transmission electron microscopy have been used to analyze the surface and near-surface regions of field-emitter samples exposed to the plasma of the Princeton Large Torus (PLT) tohamak. The experiments were carried out to determine the extent of damage to the emitter surfaces resulting from plasma exposure, the composition and thickness of films deposited on the sample surfaces during irradiation, and the depth distribution of implanted plasma and impurity species in the near-surface region of the specimens. The analyses indicate that very little structural damage occurs to the wall of PLT as a result of impinging particles travelling in a direction perpendicular to the toroidal magnetic field lines within the tokamak, but that a considerable amount of limiter material is deposited on the wall surface during operation. The deposited layer was found only when the sample was directly exposed to the plasma; control specimens which were in the reactor at the same time but shielded from direct plasma exposure remained free of deposits.

ISX — A Tokamak for surface and impurities studies

The ISX (Impurity Study Experiment) is a moderate size tokamak slightly larger than the ORMAK tokamak. ISX is being built explicitly for the study of impurities and plasma-wall interactions. It is scheduled to begin experiments in the spring of 1977. Several features have been deliberately designed into the ISX which make it particularly adaptable to surface studies. The first is a welded stainless steel vacuum system, bakeable to 400°C, with aprojected base pressure ≳ 2 × 10 −9 torr. Another feature is that of “easy” demountability of the vacuum system. Replacement of the entire vacuum system should take about two weeks. A third feature is diagnostic access to the edges of the plasma. The initial surface physics question to be answered is how best to keep surfaces clean. Later experiments will involve using wall materials other than stainless steel to determine their effect on the plasma.

Introduction to proceeding of second international conference on surface effects in controlled fusion devices

Deposition probes are commonly used to determine plasma edge characteristics in tokamaks. Such probes are frequently analyzed using accelerator-based techniques and can yield information on impurity fluxes, hydrogen fluxes and energies and surface erosion rates in the plasma edge. Several types of deposition probes have been employed to investigate the plasma edge region of the TEXTOR tokamak in Jülich, FRG. TEXTOR is a moderate size tokamak (major radius = 1.75 m) that is capable of 2–3 s discharges and contains a liner that can be heated to 600°C. These capabilities make TEXTOR particularly attractive for the study of plasma-wall interactions. Several countries, including the United States, Sweden and the host nation, are carrying on active research programs in the area of plasma edge studies. This research has included measurements of plasma, impurity and power fluxes at various radii in the scrapeoff layer. The results achieved in characterizing the plasma edge in these recent probe studies on TEXTOR will be reviewed.