S. Möller

Linear Plasma Device PSI-2 for Plasma-Material Interaction Studies

Abstract The linear plasma device PSI-2 serves as a pilot experiment for the development of components, operational regimes and control systems for the linear plasma device JULE-PSI, which will be located in the nuclear environment allowing studies of radioactive and toxic samples. PSI-2 is also used for fusion reactor relevant plasma-material interaction studies. This contribution describes the PSI-2 layout and parameters and summarizes the recent scientific and technical progress in the project, including the installation of a target station for the sample manipulation and analyses.

First studies of ITER-diagnostic mirrors in a tokamak with an all-metal interior: results of the first mirror test in ASDEX Upgrade

In ITER, mirrors will be used as plasma-viewing elements in all optical and laser diagnostics. In the harsh environment, mirror performance will degrade hampering the operation of associated diagnostics. The most adverse effect on mirror performance is caused by the deposition of impurities. It is expected that the most challenging situation will occur in the divertor. With the envisaged changes to all-metal plasma-facing components (PFCs) in ITER, an assessment of mirror performance in an existing divertor tokamak with all-metal PFCs is urgently needed. Molybdenum and copper mirrors were exposed for nearly nine months in ASDEX Upgrade which has all-tungsten PFCs. Mirrors were located at the inner wall, under the dome and in the pump duct. During exposure, the mirrors were heated to temperature in the range 145?165??C. This was made to approach the expected level of heating due to absorption of neutrons and gammas on mirrors in the ITER divertor. After exposure, degradation of the reflectivity was detected on all mirrors. The highest reflectivity drop was measured on mirrors under the dome facing the outer strike point, reaching ?55% at 500?nm. The least degradation was detected on mirrors in the pump duct, where the reflectivity was preserved in the range 500?2500?nm and the largest decrease was about ?8% at 250?nm. On all contaminated mirrors carbon fraction did not exceed 50 at% while the major contaminants were metals and oxygen. The degradation of exposed mirrors underlines the necessity for urgent R&D on deposition mitigation and in situ mirror cleaning in ITER.

Impact of ion cyclotron wall conditioning on fuel removal from plasma-facing components at TEXTOR

Ion cyclotron wall conditioning (ICWC) is based on low temperature and low density plasmas produced and sustained by ion cyclotron resonance (ICR) pulses in reactive or noble gases. The technique i ...

Overview of nitrogen-15 application as a tracer gas for material migration and retention studies in tokamaks

Experimental and analytical procedures related to the application of nitrogen-15 isotope for material migration studies have been developed and used for tracer experiments in the TEXTOR and ASDEX-Upgrade tokamaks in order to assess the retention of nitrogen in plasma-facing components made of graphite and tungsten. The surface study was performed by time-of-flight heavy ion elastic recoil detection analysis and by means of nuclear reaction analysis based on the N-15(p, gamma alpha)C-12 process. In both tokamaks nitrogen retention has exceeded 10% of the injected gas. In ASDEX-Upgrade the largest fraction of N-15 has been detected on protruding parts near the injection port, while around 4% has been found in the divertor. The ASDEX-Upgrade results have also been modeled. Helium trapping has been measured in deposits containing tungsten and nitrogen.

Morphology and composition of Fe–W coatings after deuterium plasma exposure as a model system for RAFM steels

A model system representing the RAFM steel EUROFER-97 is produced by magnetron sputter deposition of iron and 1.5 at% tungsten and investigated in order to study the consequences of plasma exposures. The alloy is deposited as coatings with a thickness of 400 nm on polycrystalline, high purity iron substrates. To understand the erosion mechanisms and morphology changes the coatings were exposed to a linear plasma device with an ion flux of 3×1021 D+ m−2 s−1 and an electron temperature of 13 eV. Samples were exposed at sample temperatures of about 420 and 770 K at incident ion energy of 30 eV (floating potential), 70 and 190 eV. Additionally, the effect of ion fluence was investigated. The coatings before and after plasma exposure were investigated by electron microscopy and glow discharge optical emission spectroscopy (GD-OES). Microstructure observation revealed a complex morphology with distinct sharp spikes formed under the plasma exposure at incident ion energies of 70 and 190 eV. The tungsten enrichment by a factor of 3 in the spikes was visualized by backscatter electron observation and confirmed by both energy-dispersive x-ray spectroscopy and GD-OES. No visible erosion and, by that, tungsten enrichment was observed after the plasma exposure at an incident ion energy of 30 eV, as expected since it is below the threshold energy for sputtering of iron.

Impact on the deuterium retention of simultaneous exposure of tungsten to a steady state plasma and transient heat cycling loads

The impact on the deuterium retention of simultaneous exposure of tungsten to a steady-state plasma and transient cyclic heat loads has been studied in the linear PSI-2 facility with the main objective of qualifying tungsten (W) as plasma-facing material. The transient heat loads were applied by a high-energy laser, a Nd:YAG laser (λ = 1064 nm) with an energy per pulse of up to 32 J and a duration of 1 ms. A pronounced increase in the D retention by a factor of 13 has been observed during the simultaneous transient heat loads and plasma exposure. These data indicate that the hydrogen clustering is enhanced by the thermal shock exposures, as seen on the increased blister size due to mobilization and thermal production of defects during transients. In addition, the significant increase of the D retention during the simultaneous loads could be explained by an increased diffusion of D atoms into the W material due to strong temperature gradients during the laser pulse exposure and to an increased mobility of D atoms along the shock-induced cracks. Only 24% of the retained deuterium is located inside the near-surface layer (d<4 μm). Enhanced blister formation has been observed under combined loading conditions at power densities close to the threshold for damaging. Blisters are not mainly responsible for the pronounced increase of the D retention.

Active control over carbon deposition by gas feeding for the protection of diagnostic the mirrors in ITER

Deposition of impurities can drastically change optical properties of metallic mirrors used for diagnostics of fusion plasmas. Specifically in ITER, this can become critical for the reactor operation by affecting the quality and reliability of detected signals. A series of experiments was performed in TEXTOR where local gas feeding was used for mitigation of carbon deposition on the mirror surface. A prototype of a diagnostic duct with molybdenum mirrors equipped with a gas feeding system was exposed in scrape-off layer plasma under deposition-dominated conditions. In the case of helium feeding, suppression of deposition was observed, while feeding of deuterium not only demonstrated full suppression of deposition but even favored complete removal of initially pre-deposited a-C:D layers. This paper presents results of experiments along with dedicated modeling of plasma–gas interaction inside the diagnostic duct.

In situ deuterium inventory measurements of a-C:D layers on tungsten in TEXTOR by laser induced ablation spectroscopy

Laser induced ablation spectroscopy (LIAS) is a diagnostic to provide temporally and spatially resolved in situ measurements of tritium retention and material migration in order to characterize the status of the first wall in future fusion devices. In LIAS, a ns-laser pulse ablates the first nanometres of the first wall plasma-facing components into the plasma edge. The resulting line radiation by plasma excitation is observed by spectroscopy. In the case of the full ionizing plasma and with knowledge of appropriate photon efficiencies for the corresponding line emission the amount of ablated material can be measured in situ. We present the photon efficiency for the deuterium Balmer -line resulting from ablation in TEXTOR by performing LIAS on amorphous hydrocarbon (a-C:D) layers deposited on tungsten substrate of thicknesses between 0.1 and 1.1µm. An experimental inverse photon efficiency of [ D XB ] a-C:D ! LIAS D D (EXP) = 75.9±23.4 was determined. This value is a factor 5 larger than predicted values from the ADAS database for atomic injection of deuterium under TEXTOR plasma edge conditions and about twice as high, assuming normal wall recycling and release of molecular deuterium and break-up of D2 via the molecular ion which is usually observed at the high temperature tokamak edge (Te > 30eV).

RF Physics of ICWC Discharge at High Cyclotron Harmonics

Recent experiments on Ion Cyclotron Wall Conditioning (ICWC) performed in tokamaks TEXTOR and ASDEX Upgrade with standard ICRF antennas operated at fixed frequencies but variable toroidal magnetic field demonstrated rather contrasting parameters of ICWC discharge in scenarios with on-axis fundamental ion cyclotron resonance (ICR) for protons,ω=ωH+, and with its high cyclotron harmonics (HCH), ω=10ωcH+⋅ HCH scenario: very high antenna coupling to low density RF plasmas (Ppl≈0.9PRF-G) and low energy Maxwellian distribution of CX hydrogen atoms with temperature TH≈350 eV. Fundamental ICR: lower antenna-plasma coupling efficiency (by factor of about 1.5 times) and generation of high energy non-Maxwellian CX hydrogen atoms (with local energy E⊥H ≥1.0 keV). In the present paper, we analyze the obtained experimental results numerically using (i) newly developed 0-D transport code describing the process of plasma production with electron and ion collisional ionization in helium-hydrogen gas mixture and (ii) earli...

Reactions of garnet-based solid-state lithium electrolytes with water — A depth-resolved study

Abstract Garnet Li6.4La3Zr1.6Ta0.4O12 thin films prepared by magnetron sputtering were analysed by secondary ion mass spectrometry, nuclear reaction analysis and Rutherford backscattering to identify, localize and quantify the reactions associated with the presence of low amounts of water and carbon dioxide. Samples in a pristine state and after storage in an Argon-filled glove box for months were compared. Both, lithium hydroxide and lithium carbonate were detected, with carbon-containing species and hydrogen-containing having surprisingly different depth profiles.