J. von Seggern

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.

Short and long range transport of materials eroded from wall components in fusion devices

Abstract Carbon sources and the sinks have been quantified in TEXTOR and are discussed in terms of short and long range transport. The major source (22 g/h) is the graphite belt limiter, but part (10 g/h) of the carbon is directly re-deposited after short range transport. Long range transport causes flake formation on obstacles and neutralisers, but little and deuterium rich (D/C≈0.7) deposition in remote areas. The rest is leaving via the pumps in gaseous form. This behaviour is different from that in JET where large amounts of deuterium rich deposits were found in the louvers. Tungsten is favoured for the ITER divertors because of its low sputtering yield for hydrogen, but melting and erosion by carbon may be an additional concern. The short range transport of tungsten has been investigated in a well defined experiment and quantitatively re-constructed by means of the ERO-TEXTOR code. Code validation is necessary in order to increase the confidence and the applicability to JET and ITER.

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.

Analysis and oxidation of thick deposits on TEXTOR plasma facing components

Abstract Deuterium and hydrogen containing co-deposited layers formed on plasma facing components at the TEXTOR tokamak were characterised by a number of techniques including surface analysis methods and thermal desorption spectrometry. The aim of the investigation was to determine the composition and structure of the layers prior and after their exposure to air at elevated temperatures of 300°C and 550°C. The impact of the oxidation on the deuterium content and distribution in the surface region and in the bulk of PFC was addressed. The initial deuterium-to-carbon concentration ratio was in the range 0.04–0.06 in the flaking layers on top of the PFC and up to 0.17 on their side surfaces. The oxidation at 300°C for 2 h resulted in partial removal of the deuterium, especially from thin or loosely bound layers, but the release was not accompanied by the change in the deposit structure. The content of plasma impurity atoms in the layers was also not influenced by the exposure to air. Following the oxidation at 550°C partial powderisation of thick, flaking films was observed.

Impurity redeposition in the SOL of TEXTOR after boronization

Abstract The material redeposited on the collector probe in the SOL of TEXTOR after boronization has been identified as a-C/B : D, i.e. as almost identical with the virgin coating. A conversion factor of 6.3 × 1017 C/cm2 for 100 nm thickness was found which corresponds to a density ρ = 1.26 g/cm2. Erosion of C and B from the coated graphite limiters causes stationary deposition during the discharge and distributions within the SOL. After 200 discharges the ratio B/C= 0.23 is lower than the original (B/C= 1-0.5). Observations made with the collector probe confirm the reduced C and O contamination of the plasma due to boronization. Carbon rates have been determined shotwise after an integrated exposure by AES-depth profiling of the stratified deposit. A deposition rate of 1 × 1016 C/cm2s typically was found at a + 2 cm and ne = 2.8 × 1013cm−3. It increases at lower densities as do the other impurities.

Long term behaviour of material erosion and deposition on the vessel wall and remote areas of TEXTOR

Abstract Long term erosion/deposition at the plasma facing surfaces as well as the carbon transport to the remote areas are discussed. The erosion of the liner wall and the deposition at the inner walls of the pump ducts were quantified. A small net deposition of carbon was found also at the liner, but only in the vicinity of the toroidal belt limiter. A deposition of polymer like deuterium rich carbon films (D/C ≈ 0.7) took place in remote areas. Despite the minute deposition rate of ∼0.02 gC/h these deposits might be decisive in view of tritium retention in next step fusion devices.

Interaction of energetic oxygen with different boron/carbon materials

Abstract Pure graphite (EK 98), boron-doped graphite (3%B, 15%B, 20%B), thin boron/carbon layers (a-C/B:H films) and B4C were irradiated with energetic oxygen ions of 1–5 keV to study the erosion and retention behaviour of energetic oxygen. The reaction of O+ ions with these materials lead mainly to the formation of volatile CO and CO2 together with physically sputtered carbon, boron and boron oxide species. Compared with graphite, the retention of oxygen in the boron/carbon materials is higher and the reemission of CO and CO2 in delayed after starting the bombardment. The thermal desorption of the implanted oxygen in form of CO occurs in addition to the normal CO peak at 950 K in a second peak at about 1600 K. At these high temperatures also BO, B2O2 and B2O3 (peaked between 1200 K and 1400 K) are desorbed.

JET: Recent results and edge phenomena

Ohmic heating studies in hydrogen and deuterium up to currents of Ip = 5 MA have been completed and additional heating experiments by means of ICRH and NBI are now in process. With ICRH powers up to 7 MW, the global energy confinement time τE is observed to deteriorate with increasing heating power. NBI experiments are at a more preliminary stage, but also show deterioration in τE. Detailed studies of confinement time scaling in ohmically heated discharges show a strong dependence of plasma size, τE ∼ R1.7a1.3, but a much weaker dependence on q and ne than seen in smaller experiments. Boundary phenomena have been studied from early 1984. Data on the impurity coverage of limiters and wall have been collected during three successive experimental campaigns. Substantial amounts of wall material have consistently been found on the limiters and a large inventory of hydrogen isotopes concentrated on the colder edges. The wall in the shadow of protective elements shows non-uniform erosion. Langmuir probe measurements of edge plasma parameters have been taken in OH, RF and NB discharges. Additional heating leads generally to a temperature increase of the scrape-off layer. Model calculations using data from these diagnostics seem to produce a consistent picture of impurity production in the scrape-off layer which agrees reasonably well with spectroscopic observations.

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.