K. Ertl

Saturation of deuterium retention in self-damaged tungsten exposed to high-flux plasmas.

Polycrystalline, annealed tungsten targets were bombarded with 12.3 MeV W4+ ions to various damage levels. Deuterium was implanted by high-flux plasmas in Pilot-PSI (>1024 m−2 s−1) at a surface temperature below 525 K. Deuterium retention has been studied by nuclear reaction analysis and by thermal desorption spectroscopy. We found that deuterium retention is strongly enhanced by the tungsten bombardment and that saturation occurs at a W4+ fluence of about 3 × 1017 m−2. The maximum deuterium concentration in the damaged region was measured to be 1.4 at.%. This is in accordance with other experiments that were carried out at much lower fluxes. We therefore conclude that the saturation behaviour and the maximum retention are not affected by the high fluxes used in our experiments.A simple geometric model is presented that assumes that the saturation solely originates in the tungsten irradiation and that explains it in terms of overlapping saturated volumes. The saturated volume per incident MeV ion amounts to 3 × 104 nm3. From our results, we are able to obtain an approximate value for the average occupation number of the vacancies.

Hydrogenic retention in irradiated tungsten exposed to high-flux plasma

Two sets of identical tungsten (W) targets are irradiated at 300 K with 12.3 MeV W4+ ions to peak damage levels ranging from 0.5 to 10 displacements per atom (dpa). This results in a damage profile that is peaked at ~0.8 µm and extends to a depth of ~1.5 µm. Both sets of targets are exposed to high-density (ne,center = 3 × 1020 m−3), low-temperature (Te,center = 1.6 eV) deuterium (D) plasma in Pilot-PSI. One set of irradiated targets is exposed at high surface temperatures (TW = 950–680 K) and the other at low surface temperatures (TW = 480–340 K). The surface temperature is determined by the local plasma conditions. Nuclear reaction analysis (NRA) is used to determine the D depth profiles at specific radial locations, thus giving a surface temperature scan of the D retention in the damaged W. Global retention is determined by thermal desorption spectroscopy, which yields total D retained in the target and also gives information of the different types of lattice defects that are trapping the D in the W lattice. The main results are that there is no measurable difference between the different dpa levels, implying a saturation of the retention enhancement at a level ≤0.5 dpa. For both irradiated and unirradiated tungsten, a peak in the retention is seen at TW = 480 K; however, the W4+ irradiation clearly enhances the retention. This enhancement is also temperature dependent and increases with increasing surface temperature up to an enhancement by a factor of 15–23 at TW = 950 K. At the lowest surface temperatures, a fluence dependence appears since the implanted deuterium is diffusion limited to only a small fraction of the irradiated zone. TDS spectra show an enhancement of both low-energy trap sites and high-energy trap sites. For these conditions, diffusion-limited, low fill fraction trapping determines the hydrogenic retention of the W.

Deuterium retention and lattice damage in tungsten irradiated with D ions

Depth profiles of D atoms and D2 molecules in a W single crystal implanted with 6 keV D ions at 300 K have been determined using secondary ion mass spectrometry (SIMS) and residual gas analysis (RGA) measurements in the course of surface sputtering. Profiles of deuterium and lattice damage in a W single crystal irradiated with 10 keV D ions at 300 K have been investigated by means of nuclear reaction analysis (NRA) and Rutherford backscattering spectrometry combined with ion channelling techniques (RBS/C). There are at least two types of ion-induced defects which are responsible for trapping of deuterium: (i) D2-filled microvoids (deuterium bubbles) localised in the implantation zone; and (ii) dislocations which are distributed from the surface to depths far beyond 1 μm and which capture deuterium in the form of D atoms.

Maximum entropy based reconstruction of soft X-ray emissivity profiles in W7-AS

The reconstruction of 2-D emissivity profiles from soft X-ray tomography measurements constitutes a highly underdetermined and ill-posed inversion problem, because of the restricted viewing access, the number of chords and the increased noise level in most plasma devices. An unbiased and consistent probabilistic approach within the framework of Bayesian inference is provided by the maximum entropy method, which is independent of model assumptions, but allows any prior knowledge available to be incorporated. The formalism is applied to the reconstruction of emissivity profiles in an NBI heated plasma discharge to determine the dependence of the Shafranov shift on beta , the reduction of which was a particular objective in designing the advanced W7-AS stellarator

Retention of ion-implanted deuterium in tungsten pre-irradiated with carbon ions

Abstract Deuterium (D) ion implantation and retention at room temperature was studied in pure and carbon (C) implanted tungsten single crystals. Pre-implantation with C was done at 40 keV and D implantation at 10 keV with the range confined in the carbon modified layer and at 100 keV with the range exceeding the carbon modified layer. The range distributions were investigated in situ using 1 MeV 3 He ions analysing the energy distributions of α particles from the D ( 3 He , p ) α reaction while the total amount of retained D was obtained from the p-integral. The range distribution of carbon was obtained from the backscattered 3 He energy distribution. C pre-impantation influences the D retention only if the range of the D ions is confined within the carbon modified surface layer. In this case, D diffusion beyond the ion range distribution does not occur and the retained D amount is smaller than in the pure W crystal. At D energies exceeding the carbon modified layer the retention occurs in the dislocation zone up to 1 μm and the total retained amount is the same for carbon implanted and pure W samples.

Rutherford backscattering spectroscopy and elastic recoil detection analysis with lithium ions - The better alternative to helium?

Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA) with lithium ions are compared to using helium ions. The availability and accuracy of backscattering cross-section and stopping power data for incident Li ions are reviewed, and energy broadening contributions due to detector resolution and energy loss straggling are discussed. Theoretical calculations of the depth resolution are compared with experimental data for RBS from Nb/Co multilayers and foil-ERDA from amorphous hydrogenated carbon multilayers. In RBS about the same or better depth resolution with Li than with He is achieved, while in ERDA for the detection of hydrogen isotopes the depth resolution is increased by a factor of about 1.5 compared to incident He.

RELEVANCE OF PLASMA-INDUCED ARCS FOR DIVERTOR TOKAMAKS

Plasma-induced arcs and their ignition conditions have been studied in the divertor chamber of the ASDEX tokamak by introducing arc probes and Langmuir probes in front of the divertor plates. Arcs are rare events and are always correlated with unstable phases of the plasma, such as at disruptions or during neutral-beam injection. Arcs require high ignition potentials (100–500 V) and burning voltages of the order of 100 V. In addition, an internal limitation of the rate of current rise to values of about 107 As−1 has been observed. The probability of arcing during short transient voltages is thus reduced. Arcs are generally preceded by diffuse discharges carrying currents of a few amperes. Since diffuse discharges occur more frequently than arcing events, a significant contribution of diffuse discharges to surface erosion is suspected.

Capture from random flux to channeling for protons transmitted through a Si foil

Flux redistribution and formation of channeling peaks have been observed in the geometry of initially wide-angle incidence for protons transmitted through monocrystalline Si membranes. The channeling peaks are generated by the crystalline medium as a result of multiple charge-exchange events due to the impact-parameter dependence of the electron capture and loss cross-sections. This explains successfully the detection of channeling peaks only at a low enough residual proton energy: Epf⩽0.6 MeV, after the transmission.

Heat Flux Measurements in the ASDEX Edge Plasma

Abstract A calorimeter probe used in combination with a Langmuir double probe for studies of the plasma boundary layer of ASDEX is described. The scope and the limitations of the device are discussed. The combined probe is designed for measuring the energy flux parallel to the magnetic field, on the ion and the electron drift side simultaneously. In addition the electron temperature and the density can be determined. First experimental results from discharges employing different heating methods, i.e. OH, LHRH and NBI are presented. Besides yielding decay lengths for P, T e and n , the asymmetry of the energy flux during neutral beam injection indicates a toroidal plasma rotation in the case of tangential neutral beam injection.

Bayesian consideration of the tomography problem

Soft X-ray tomography has become a standard diagnostic equipment to investigate plasma profiles. Due to limitations in viewing-access and detector numbers the reconstruction of the two-dimensional emissivity profile constitutes a highly underdetermined inversion problem. We discuss the principle features of the tomography problem from the Bayesian point of view in various stages of sophistication. The approach is applied to real-world data obtained from the Wendelstein 7AS stellerator.