G. Staudenmaier

Trapping of deuterium implanted in carbon and silicon: A calibration for particle-energy measurements in the plasma boundary of tokamaks

Measurements have been made of the number of deuterons retained in carbon and silicon as a function of fluence, for incident energies between 50 eV and 1 keV. Three independent techniques were used for measuring the retained D: (1) by probing the trapped deuterons with 790 kV 3He+, and counting the total proton yield from the D(3He, H)4He nuclear reaction, (2) by measuring the re-emitted deuterium during and after implantation using mass spectrometry, (3) by thermal desorption mass spectrometry. Initially, the amount of trapped hydrogen increases proportionally to the fluence, while at high doses a saturation value is reached. The quantity of hydrogen trapped at saturation as a function of particle energy follows a power law. The data have particular significance in the estimate of the mean energy of particles to the near-wall region in tokamaks by observing the build-up of trapped hydrogen in a carbon probe during cumulative discharges. As an aid to the interpretation of such data, a multi-energy implant simulating a Maxwellian energy distribution has been made, and the trapping characteristics have been investigated as a function of incident ion fluence.

Wall Analysis in Pulsator_1

Abstract The amount of impurities deposited during the discharge on samples introduced to the liner of PULSATOR I has been measured using a combination of several surface physics techniques (AES, SIMS, EID, RIBS). The amounts are absolutely calibrated and are about 2 × 1013 atoms/cm2 for oxygen, 5 × 1012 atoms/cm2 for Fe, Cr, Ni (liner material) and 2 × 1012 atoms/cm2 for Mo (limiter material). They depend strongly on the kind of discharges and on the position of the sample. First results using a shutter for time resolved exposure of the sample indicate that deposition of impurities takes place during the plateau of the discharge.

Conditioning of asdex by glow discharge

Based on previous experience with conditioning of vessels for plasma experiments by a dc glow discharge, a very simple, safe, and effective technique was developed for ASDEX. It is characterized by a low working pressure of 5 × 10−3mb of H2 at which no violent arcing or damaging parasitic discharges occur and it utilizes anodes in a fixed position well within the limiter shadow. The efficiency of the glow discharge is monitored by a differentially pumped mass spectrometer. The results show a high efficiency in removing carbon from the vessel. Total amounts pumped out mainly as CO and CH4 are of the order of grams corresponding to hundreds of monolayers. Surface analysis of samples yield less than 30% of a monolayer after several hours glow discharge conditioning, but recontamination is seen to be significant. We also find a water production rate corresponding to the removal of one monolayer per hour during the glow discharge. An investigation of the current and pressure dependence of the impurity removal rates show that the chosen conditions are quite optimal.

Impurity fluxes in the scrape off layer of TFR 400 and TFR 600

Abstract In situ surface analysis in TFR 400 and TFR 600 have been performed by QWAASS (Quantitative Wall Ananalysis by Auger and Secondary Ion mass Spectroscopy). New results were obtained for the scrape-off region of the limiter by varying the sample position. Two processes, deposition and erosion, determined the molybdenum deposited on a carbon sample after a few discharges. A simple model based on these two processes is developed which explains the main features of the experimental results. Impurity fluxes in this region can be determined. It is further assumed that both deposition and erosion decrease exponentially with increasing distance between plasma and sample. Erosion decreases faster than deposition. The characteristic decay length for Mo-deposition is about 0.5 cm. Further elucidation of the results presented is obtained from a simple model of plasma-limiter interaction which is based on thermal flow parallel and Bohm diffusion perpendicular to the magnetic field. Assuming an edge temperature of about 10 eV the calculated decay length is of the same order of magnitude as that found experimentally. The results are discussed together with other processes that may contribute to the deposition of metal impurities on samples in the limiter shadow region.

Determination of ion temperatures in the edge plasma from ion flux transmission of apertures

Abstract The flow of ions through an aperture perpendicular to a strong magnetic field has been calculated for a Maxwellian velocity distribution. The results obtained are needed for the interpretation of time resolved analysis of fluxes using the collecting probe technique and also for in situ mass, energy, and charge state analysis with spectrometers. Moreover, the energy dependent transmission of the aperture can be used to determine ion temperatures. Deuterium, carbon and metal impurity ion temperatures in the limiter shadow plasma of TFR 600 have been evaluated from measured ion flux transmission. For deuterium a temperature of 100 eV was found on the electron side of the collector. At the ion side the temperatures are higher by a factor of two. Impurity ion temperatures are ambiguous to the extent of the knowledge on the charge state. Assuming singly charged ions for Ni + a temperature of 10 eV on the ion side and 4 eV on the electron side was found during high density discharges with an Inconel limiter. In case of a carbon limiter the C + ion temperature on the ion side was about 35 eV.

Surface Effects and Impurity Production in Tokamak Machines

Abstract Plasma-wall interactions are presently investigated in two ways: a) The a priori assumption of a mechanism responsible for impurity release. Relevant experimental data can be used in a model and calculations made in order to understand the observed impurity behaviour in plasma. b) The comprehensive investigation of samples exposed to a plasma. Recent investigations have confirmed the earlier assumption that the interactions occur only in the topmost atomic layers of the wall, and so emphasize the major role of surface physics on this field. These investigations have further shown that besides atomic processes, such as desorption or sputtering, other processes occur, extending on a microscope rather than an atomic scale. These are for example evaporation, arcing, and mechanical stress. Both aspects are discussed as far as possible in a quantitative way. The contribution of most probable processes is estimated using data available on flux and energy of particles and yields of single processes. We reach the conclusion that no process can be disregarded. Several processes seem to contribute to the impurity release and are different at different phases of the discharge. An interdependence between these processes is likely.

Spatial distribution of limiter material and impurities on the first wall of TFR 400

Abstract After two years of operation the tokamak TFR 400 was shut down. About 50 samples of the stainless steel liner were cut out at the outer circumference and at several places on the small circumference and analyzed by elastic backscattering of 2.5 MeV protons, by ion induced X-rays and by high resolution scanning electron microscopy combined with X-ray analysis. Deposits of all materials are found which have been used for limiters and probes positioned close to the plasma boundary during the history of the machine, i.e. B, C, Mo and W. Additionally oxygen is present. On the side of the limiter upon which the ion current is incident more impurities were deposited on samples with an azimuthal position between two main toroidal coils than on samples positioned in line with the coils indicating deposition as ions. On the small circumference the deposits varied in magnitude up to a factor of ten but no systematic positional dependence was found.

Low energy neutral particle fluxes to the walls of ASDEX during He and D2 discharges

Neutral particle fluxes onto the walls of ASDEX have been investigated using a time-of-flight (TOF) method. The energy distributions of the neutrals could be determined in the range of 10–1000 eV/amu. Ohmic divertor and limiter discharges with equal plasma currents and densities have been compared for He and D2. The He0 outflux at ∼2000 eV from He discharges is 110 of the corresponding D0 flux in D2 discharges. At lower energies this difference is much smaller. In all cases many more He neutrals were observed than was anticipated from the CX rate-coefficients for He2+. The impurity fluxes due to sputtering by the CX-neutrals show no significant difference for He and D2 discharges. For divertor discharges CX-sputtering can fully account for the Fe impurity content determined spectroscopically.

Impurity Transport in the ASDEX Edge Plasmas Studied by a Combination of Passive and Active Probes with SIMS Analysis

Abstract Radial profiles of impurity fluxes in the edge plasma of ASDEX are measured with collecting probes. Rutherford backscattering (RBS) and secondary ion mass spectrometry (SIMS) are used to analyse the collectors. The good agreement between the two methods indicates that the SIMS can be used as a new rapid analysis technique for large collectors. The impurity flux is found to decay exponentially with an e-folding length corresponding to the length of the flux tube in front of the probe, indicating that the flow is free streaming. Impurity ion temperatures are measured by means of an E × B analyser. A preliminary evaluation yields Fe ion temperatures of about 10 to 20 eV. Hence the cross field diffusion coefficient is estimated to be about 3 × 10 3 cm 2 /s .

Volume expansion and oxygen incorporation in deuteron-bombarded silicon

Abstract In the course of SIMS depth profiling studies using argon primary ions we have found that the erosion rate of silicon heavily doped with 0.5–4 keV deuterium is up to a factor of about three larger than for undoped silicon. Talysurf investigations showed a pronounced elevation of the irradiated portion of the surface relative to the surrounding unirradiated material. The maximum surface elevation amounted to ~40 nm/keV, accompanied by a drop in the average silicon density by a factor of three. Electron micrographs revealed the presence of microbubbles in highly doped and heavily damaged samples. Comparison with trapping curves suggests that bubble formation and volume expansion result from heavy nuclear damage rather than from mere accumulation of deuterium. At high deuteron fluences enhancement in the D + yield in SIMS analysis was observed which is caused by the build-up of a buried oxygen distribution. It is likely that the oxygen is incorporated from the ambient gas present during implantation.