H.G. Esser

Hydrogen inventories in nuclear fusion devices

Hydrogen retention in tokamaks is due to implantation into plasma-facing materials and trapping in deposited layers. In the limiter tokamak TEXTOR-94 hydrogen-rich deposited layers with thicknesses up to 1 mm are observed on recessed parts of the limiters, areas perpendicular to the magnetic field in the scrape-off layer (SOL), neutralizer plates of the pumped limiter and inside the pumping ducts. In the divertor tokamak JET the main deposition is observed in the divertor, additional deposits are observed in the main chamber on the sides of the guard limiters. Codeposition of carbon ions with hydrogen is the major mechanism of layer growth at areas with direct plasma contact. At remote areas without direct plasma contact, sticking of neutral hydrocarbon radicals seems to play an important role for hydrogen trapping.

Borontrimethyl B(CH3)3 — A less hazardous substance for boronization

Abstract Boronization, i.e. plasma induced deposition of amorphous boron containing carbon films a-C/B : H on all inner surfaces of fusion devices, has proven to be a powerful conditioning method to achieve very pure fusion plasmas. The use of the highly toxic and explosive gas diborane (B2 H6) in conventional boronization requires considerable precautions for safe handling. We report on a new technique for boronization, using less hazardous organic boron compounds, namely borontrimethyl B(CH3)3 and borontriethyl B(C2H5)3. First results indicate that films can be produced from borontrimethyl showing a performance in fusion devices similar to that of layers produced in the conventional way. The new technique thus offers an easier and more convenient way for the boronization of surfaces.

Removal of redeposited layers and hydrogen release by oxygen ventilation of TEXTOR-94

Abstract Ventilation of the TEXTOR torus with oxygen at pressures between 0.007 and 0.3 mbar and at wall temperatures between 500 and 700 K has been investigated to remove redeposited carbon material and to release the incorporated hydrogen. It has been observed that a significant part of the injected oxygen adsorbs on the walls due to formation of stable oxygen compounds. Part of the oxygen reacts with CO and CO 2 . The CO reaction rate is independent of the filling pressure whereas the CO 2 production increases with increasing pressure. Plasma operation after the oxygen baking has been achieved after GDC in D 2 and He (15–30 min). The oxygen impurity content was initially about a factor of 2–3 higher than before but also showed a decreasing behaviour shot by shot.

Limiter Lock Systems at TEXTOR: Flexible Tools for Plasma-Wall Investigation

Abstract Limiter lock systems on the top and the bottom of the TEXTOR vessel are essential elements for experimental investigations of plasma-wall interaction in a tokamak. The lock systems are designed as user facilities that allow the insertion of wall elements (limiter) and tools for diagnostic (electrical probes, gas injection) without breaking the TEXTOR vacuum. The specially designed holder on top of the central carrier and a powerful vacuum pump system permit the exchange of components within ~1 h. Up to ten electrical signals, four thermocouples, and a gas supply can be connected at the holder interface. Between discharges, the inserted component can be positioned radially and turned with respect to the toroidal magnetic field. Additionally, the central carrier is electrically isolated to apply bias voltages and currents up to 1 kV and 1 kA, respectively. An important feature of the lock system is the good access for optical spectroscopic observation of the inserted components in the vicinity of the edge plasma. The whole spectrum from ultraviolet to infrared is covered by spectrometers and filters combined with cameras. Toroidally and poloidally resolved measurements are obtained from the view on top of the probes while the tangential poloidal view delivers radially and toroidally resolved information. A programmable logic controller (Simatic S5) that is operated inside the TEXTOR bunker and from remote locations outside the concrete wall drives all possible features of the lock system.

ICRF wall conditioning at TEXTOR-94 in the presence of a 2.25 T magnetic field

To investigate alternative conditioning concepts for future fusion devices with permanent magnetic fields, plasmas produced by the coupling of ICRF power to He and gas mixtures of Helium + silane, have been analyzed in the presence of a 2.25 T toroidal magnetic field at TEXTOR-94. Their qualification for wall conditioning has been investigated for different He-pressures, PHe (1 × 10−3 < PHe (Pa) < 1 × 10−1) and ICRF power, PICRF (100 < PICRF (kW) < 800). Electron densities ne averaged along different radial lines of sight across the vacuum vessel from the top to the bottom have been obtained in the range 5 × 1010 < ne (cm−3) < 3 × 1012. To study quantitatively the efficiency of hydrogen desorption from the first wall at different ICRF plasma conditions in a reproducible way, the first wall was presaturated by RG-glow discharges in H2. The amount and the evolution of the H2 desorption from rf discharge to rf discharge was determined by ion gauge measurements combined with mass spectrometry. To demonstrate the capability of the new method for plasma assisted thin film deposition, different amounts of silane (<50%) were added to the He gas. During the ICRF pulses, the silane molecules were dissociated in the plasma and the Si atoms stick to the wall. A good balance of the amount of Si disappearing from the gas phase and that measured by post mortem surface analyses of collector probes at the wall position was found.

Active control of type-I edge localized modes on JET

The operational domain for active control of type-I edge localized modes (ELMs) with an n = 1 external magnetic perturbation field induced by the ex-vessel error field correction coils on JET has been developed towards more ITER-relevant regimes with high plasma triangularity, up to 0.45, high normalized beta, up to 3.0, plasma current up to 2.0 MA and q95 varied between 3.0 and 4.8. The results of ELM mitigation in high triangularity plasmas show that the frequency of type-I ELMs increased by a factor of 4 during the application of the n = 1 fields, while the energy loss per ELM, ΔW/W, decreased from 6% to below the noise level of the diamagnetic measurement (<2%). No reduction of confinement quality (H98Y) during the ELM mitigation phase has been observed. The minimum n = 1 perturbation field amplitude above which the ELMs were mitigated increased with a lower q95 but always remained below the n = 1 locked mode threshold. The first results of ELM mitigation with n = 2 magnetic perturbations on JET demonstrate that the frequency of ELMs increased from 10 to 35 Hz and a wide operational window of q95 from 4.5 to 3.1 has been found.

The dynamic ergodic divertor in the TEXTOR tokamak: plasma response to dynamic helical magnetic field perturbations

Recently, the dynamic ergodic divertor (DED) of TEXTOR has been studied in an m/n = 3/1 set-up which is characterized by a relatively deep penetration of the perturbation field. The perturbation field creates (a) a helical divertor, (b) an ergodic pattern and/or (c) excitation of tearing modes, depending on whether the DED current is static, rotating in the co-current direction or in the counter-current direction. Characteristic divertor properties such as the high recycling regime or enhanced shielding have been studied. A strong effect of the ergodization is spin up of the plasma rotation, possibly due to the electric field at the plasma edge. Tearing modes are excited in a rather reproducible way and their excitation threshold value, their motion and their reduction due to the ECRH/ECCD have been studied. The different scenarios are characterized by strong modifications of the toroidal velocity profile and by a reduced or enhanced radial transport.

Impurity release and deposition processes close to limiter surfaces in TEXTOR-94

Abstract Measurements on the formation of hydrocarbons on plasma exposed surfaces performed by mass- and optical emission spectroscopy in TEXTOR is reported. The temperature dependence of hydrocarbon formation and the contribution of the hydrocarbon source to the CII ion densities near the limiter has been observed using a graphite limiter which is externally heatable up to 1400 K. It has been found that hydrocarbon formation occurs in a broad temperature region decreasing only for target temperatures above 1300 K and that hydrocarbons contribute to about 40% to the CII light. Strong methane release has been observed on copper and stainless steel limiters positioned at the LCFS while it is negligible on molybdenum and tungsten limiters under similar plasma edge conditions. Local transport and redeposition of molecules have been studied by gas injection of methane and silane through holes in the limiter surfaces and by local Monte Carlo calculations. Local deposition efficiencies between 4 and 7% have been measured for injected methane and silane. Monte Carlo calculations show, in general, a larger redeposition probability depending only little on local plasma parameters but significantly on the assumptions of the sticking and release properties of redeposited ions and radicals on the surface. For higher surface temperatures possible carbon release by radiation enhanced sublimation (RES) has been investigated. No increase of carbon release could be observed demonstrating that carbon release from RES is negligible under these conditions. Possible reasons for this behavior are discussed.

Behaviour of carbon and boron-carbon materials at high temperatures in TEXTOR

Test limiters made from graphite and boron-doped graphite have been exposed in TEXTOR to high heat loads up to about 30 MW/m2. Maximum surface temperatures up to 3000°C have been reached. Boron sublimes thermally from the boron-doped graphite exceeding the boron sputter-fluxes at about 2000°C. At the same temperature the carbon release increases sharply. The carbon release from the pure graphite shows no measurable increase up to surface temperatures of about 2200°C and increases then steeply. The results are compared with the expected carbon and boron release by physical sputtering, radiation enhanced sublimation and normal thermal sublimation.

Properties of thin a-C/Si:H coatings applied to TEXTOR

Thin, hydrogenated silicon and carbon containing films have been deposited by the siliconization procedure on targets made from some metal alloys, pure metals, graphite and Si single crystal. The deposits were investigated by electron microprobe and surface analysis techniques combined with ion sputtering, infrared spectroscopy, mass spectrometry, ellipsometry and by nuclear reaction and backscattering techniques. The stoichiometry of the layers were controlled by particle balance. They are amorphous, semitransparent, and homogeneous throughout the layer. They are hard, non abrasive, and adhere firmly to the substrate. Their density is approximate to 1.5 g cm(-3) for a-C/Si:H and approximate to 2.0 gcm(-3) for a-Si:H, the refractive index n = 2 +/- 0.2, the extinction coefficient k << 0.01. Carbon and silicon form carbidic Si-C bonds, hydrogen is attached both to carbon and to silicon. The deposits are chemically inert to molecular oxygen, but they strongly getter O-ions. Chemical erosion rates of a-C/Si:H films by H+ are a factor 30 less than those of pure carbon films (a-C:H).