H.-D. Reiner

Chemical sputtering yields of carbon based materials at high ion flux densities

Graphite and advanced carbon fiber composites (CFC) are widely used inside the vacuum vessel of magnetic fusion devices. However, erosion by chemical sputtering via hydrocarbon formation might limit their application as target material in future machines like ITER. The first system- atic study of the chemical erosion of graphite and different CFCs (including a silicon-doped one) as a function of ion flux density in the range of 1.4 × 10 21 -5 × 10 22 m -2 s -1 was performed in the plasma generator PSI-1. The results of three different analysis methods agree within about 40%. No differences in the chemical erosion yields between hydrogen and deuterium exposures are found for the various materials. In contrast, the erosion yields differ up to a factor of two for the different CFC-materials. In general, the chemical sputtering yields decrease with increasing ion flux density according to -0.6 reaching levels below 1% at the highest fluxes. Scanning electron microscopy (SEM) and energy dispersive X- ray analysis (EDX) show preferred erosion in the area between the carbon fibers.

Results from a double-sided langmuir probe in T-10, and an extended model of a probe in a streaming magnetized plasma

Abstract By means of a JANUS-type Langmuir probe operated in the scrape-off layer (SOL) in T-10 the temporal evolution and radial profiles of plasma density, electron temperature, and toroidal asymmetry of saturation currents were measured in dependence on discharge parameters. Of particular importance turned out to be the B T field direction and the radial probe position relative to the active limiter structure (rail and aperture limiter). The interpretation of the results is based on the local SOL structure in terms of connection lengths to the active limiter and special flow effects in T-10, and makes use of a theoretical probe model by Hutchinson. As to the latter model, a refinement concerning its basic principles is presented independently.

Deposition probe investigations of the sol of T-10

Abstract Several hundred systematic deposition probe measurements were carried out in the SOL of T-10 to investigate the time development of the Fe-impurity flux at various radial positions. During these investigations the discharge conditions as well as the limiter configuration were varied over a wide range. Some qualitative guiding principles are given for deposition probe measurements and their interpretation. On the basis of the radial dependence of the iron flux during both the plateau and the end phase of the tokamak discharge, the influence of erosion processes as well as of local sources and enhanced transport on the iron deposition is discussed. The experiments show an immediate relation between the direction of the toroidal magnetic field and the evolution of the impurity flux during the plateau phase, no influence of various limiter arrangements could be detected.

Energy flux measurements in a steady-state discharge at PSI-2

A newly developed heat flux detector for direct measurement of the energy flux to a floating or biased target is presented. The detector has been used to measure the plasma sheath transmission factor (γ) for different gases as a function of the electron temperature. The determined values are in the range of 7 < γ < 17; they agree roughly with those theoretical predictions [G.A. Emmert, R.M. Wieland, A.T. Mense, J.N. Davidson, Phys. Fluids 23(4) (1980); H. Kimura, H. Maeda, N. Ueda. M. Seki, H. Kawamura, S. Yamamoto, M. Nagami, K. Odajima, S. Sengoku, Y. Shimomura, Nucl. Fusion 18(9) (1978); R.V. Manos, D.M. Budny, S.A. Cohen, J. Vac. Sci. Technol. A 1 (1983); P.C. Stangeby, J. Phys. D 15 (1982); P.C. Stangeby, Phys. Fluids 27(3) 1984] that include the recombination term.

Characteristics of electric probes in the tokamak SOL: influences of the magnetic field

Abstract The paper describes, discusses and interprets observations made with electric probes in the tokamak SOL to check simple assumptions about the transport of probe current through the plasma, following the theoretical approach of Guenther [7]. Within the framework of the supposed model, it can be concluded that in practice a probe tip expected to operate as a classical single Langmuir probe acts as a double probe. As a consequence, the values for the temperature are generally lower than those obtained by the single-probe formula.

Comparison of chemical sputtering yields for different graphites at high ion flux densities

Graphite is widely used inside the vacuum vessel of magnetic fusion devices and is proposed as target material for future machines like ITER. There are, however, uncertainties concerning the erosion of the material by chemical sputtering via hydrocarbon formation at high ion flux densities. We report on experiments at the plasma generator PSI-1 using a stationary quasi-neutral plasma beam. The ion flux densities used cover the range from 4 · 1020 to 1.2 · 1022 m−2 s−1. They are thus filling the gap between the upper limit of ion beam experiments (1020 mm−2 s−1 and tokamak relevant values (> 1023 m−2 s−1). To suppress impurity-induced erosion the hydrogen discharges were carefully conditioned and checked for possible impurities, especially oxygen. Samples of different advanced carbon fiber composites (CFC) - including a silicon-doped one - were exposed to various plasma conditions. A calibrated mass spectrometer monitored the CxHy-formation in situ and the axial dependence of the CH-band intensities at 431 and 432.4 nm in front of the target was detected. We have studied the temperature dependence (250–700°C) of the erosion yields at ion flux densities up to more than 1022 m−2 s−1 in hydrogen discharges. Weight loss measurements and scans with an optical profilometer were used to determine the mass loss. For Si-doped CFC an erosion yield of 1% was found, which is a factor of two less than for pure CFC.

Studies of impurity recycling by the collector probe technique

Abstract In order to study recycling effects of the nonintrinsic impurity Li discharges with and without LiD-pellet injection were investigated. The observed maximum impurity level of Li in the SOL plasma of discharges without injection reaches less than 10% of that observed in discharges with injection. The measurements offer the possibility to distinguish between influxes from the wall and those which reach the collector probe via the core plasma. The time evolution, orientation and radial dependence of the impurity fluxes are characteristic features of their origin. The consideration of all these features facilitates a better understanding of collector probe measurements in the SOL-plasma.

Studies on plasma-surface interaction in the stellarator W7-AS

Abstract The analysis of long term collector samples has given information on wall erosion by ion sputtering in W7-AS. There is strong indication that this erosion occurred during high - t -discharges which are separatrix-dominated by 5/n island chains. Langmuir and calorimeter probe measurements have demonstrated that in this case particle and heat fluxes leave the plasma locally with high densities. Erosion of limiter surfaces measured post-mortem can be satisfactorily correlated with results from measurements with flush mounted limiter probes. Measurements with a moveable collector probe have shown that the plasma impurity contamination by limiter material is higher after wall boronization in limiter-dominated discharges. This is explained by less contamination of the limiter surface after removal of the boronization layer from the limiter surface. On the basis of a simple 1D-impurity transport model a comparison of Langmuir and collector probe measurements with spectroscopic measurements in the core plasma was made, supporting this hypothesis.

Collector probe measurements in T-10: Results and prospects

Abstract Results of collector probe exposures in the tokamak scrape-off with subsequent surface analysis are presented. Most of the experiments were carried out at T-10 using the WASA-I device. The significance of the measurements was improved by combining the collector probe technique with impurity injection methods as well as with Langmuir probe measurements. The results are discussed in terms of general aspects of probe measurements in tokamaks considering the perturbation of the plasma by the probe and the erosion/deposition balance on the sample surface. Problems arising for tokamak and probe operation from the use of different materials for the inner components of the tokamak vessel or an “all carbon” tokamak are dealt with. The future work is outlined.