F. Weschenfelder

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.

Colorimetry of interference colours to investigate thickness changes of protective coatings in TEXTOR

Abstract Colorimetry is introduced at TEXTOR as a technique to observe the carbon erosion from protective coatings on wall elements. It quantifies the interference colours of the transparent carbon and hydrogen containing films (a-C:H) which are mainly depending on thickness. The paper describes the setup and calibration of the facility as a video and an image processing system and how colour coordinates are defined and measured and discusses the influence of other parameters. In neutral beam heated discharges, erosion rates up to - 22 nm/s are measured on a precarbonized test piece exposed in the scrape-off layer plasma. This corresponds to a loss of 1.4 × 10 17 C/cm 2 s taking into account the carbon density of 6.5 × 10 22 C/cm 3 of a-C:H. Part of the eroded carbon is redeposited on the test piece with + 4 nm/s. The thickness pattern is not stationary as time proceeds. Erosion is observed around the tangent point. At locations further apart than 2 cm from the plasma edge, deposition is found influenced by the limiter geometry.

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.

The influence of plasma-edge properties on high confinement discharges with a radiating plasma mantle at the tokamak TEXTOR-94

The radiative improved mode obtained on the limiter tokamak TEXTOR-94 combines the possibility of power exhaust by a radiating plasma boundary (with a fraction of the radiated power with respect to the total input power up to 90% with neon or argon cooling) with improved energy confinement (as good as in the ELM-free H-mode in divertor tokamaks) at high plasma densities (line-averaged central-electron density equal to or even above the Greenwald density limit nGW) in quasi-stationary discharges. An overview is given of the substantial changes in plasma-edge properties occurring at high radiated power levels . These changes are characterized by a reduction of the plasma-edge density and temperature, a reduction of particle transport out of the confined plasma volume and an increase of the penetration depth of deuterium and impurity atoms. As a consequence, the particle confinement time increases and the electron-density profiles steepen. The transition to improved confinement takes place as soon as the density peaking reaches a critical threshold. An internal transport barrier is observed in the bulk of RI-mode plasmas (at r=a6 0:6) characterized by an increase of the pressure gradient and of the shear of the toroidal velocity compared to discharges without additional impurity seeding. The dilution at the plasma boundary is strongly increased by the seeded impurities whereas the central dilution is only weakly affected.

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).

Colorimetric measurements of carbon erosion and deposition rates on extended areas of plasma facing components in TEXTOR

Abstract Colorimetry is introduced at TEXTOR as a technique to observe the carbon erosion on extended areas. It quantifies the interference colours of thin (∼ 200 nm) transparent co-deposits of carbon and hydrogen which are mainly depending on thickness. The paper describes the facility as a video and an image processing system and the thickness determination by means of the colour coordinate hue ( h xy ). Erosion rates up to −22 nm/s are measured on a precarbonized spherical graphite test piece exposed in the SOL for neutral beam heated plasmas. Part of the carbon is redeposited promptly with +4 nm/s, but the transition range between net erosion and deposition is not stationary. Erosion is observed around the tangency point. Beyond 2 cm into the SOL, deposition is found to be influenced by the limiter geometry.

Protection of limiter surfaces by films locally deposited during TEXTOR discharges

Abstract The erosion of first wall material is a serious problem in future fusion devices. A concept is suggested to compensate erosion by the local injection of reactive gases through divertor plates or limiters forming a suitable deposit on their surfaces. The technique can be implemented during plasma operation with feed back control. Initial experiments were performed in TEXTOR in which silane gas (SiD 4 ) was blown through a hole in a graphite test limiter during ohmic discharges. The gas was ionized and partly deposited in the vicinity of the interaction hole, at areas subject to the highest heat load. Without gas puffing these are erosion dominated zones. The deposition of a silicon layer over an area of about 700 mm 2 , peaked at the injection hole with a maximum thickness of about 900 nm and with a poloidal extend of about 20 mm was observed. It contained a fraction of about 4–5% of the total number of injected Si atoms. In addition, about the same quantity of carbon was co-deposited, significantly exceeding the C fluence arriving from the plasma during normal discharges. An additional local carbon source during silane injection must be invoked, caused e.g. by carbon erosion due to Frank-Condon or charge exchange atoms impinging on the limiter itself on nearby wall areas.

In situ measurement of erosion/deposition in the DIII-D divertor by colorimetry

Colorimetry was introduced into the DIII-D tokamak to measure in situ the growth and erosion of transparent wall coatings (a-C:H) on the divertor. The colorimetric measurement system consisting of a halogen light source, a set of three filters and a black/white camera is described together with a first erosion measurement. An insertable graphite sample with a diameter of 4.7 cm was precoated with a 300 nm thick amorphous carbon film and was exposed in the divertor for several discharges with its surface coplanar to the surrounding graphite tiles. For each of the discharges the plasma strike point was moved onto the sample for 1 s to erode the coating. Between the discharges a camera signal with each filter was recorded and the film thickness was evaluated along a radial line across the DIMES sample. Thus it has been possible for the first time to measure erosion and deposition of divertor material in situ and shot-by-shot. The average peak heat flux with the strike point on DIMES was about 110 W . The measurement shows a strong decrease in the film thickness almost over the entire sample with an average erosion rate of .

Non-uniform carbon redeposition on graphite

Abstract Gradual erosion of a thin a-B:D layer on a graphite block is observed in TEXTOR-94 during 28 exposures while carbon out of the SOL has been deposited onto the surface simultaneously. The co-existence of B erosion (−0.5 nm/s) and C deposition (+0.8 nm/s) in the same area is due to non-uniform carbon redeposition of typically 10–30 μm in size. The D content is more than doubled in the co-deposit despite the considerable erosion of boron. Erosion by only neutrals could be discriminated from the erosion by D+ ions. The film lost the transparency due to temperature excursions up to 2000 K during NBI. Observations were made during the exposures (colorimetry, spectroscopy) and post-mortem.

Studies on plasma edge transport and layer deposition by local injection of reactive gases during TEXTOR-94 discharges

Abstract To study local particle transport, silane was injected through a hole in a test limiter surface during discharges in TEXTOR-94. The gas injection lead to the deposition of a thin layer around the injection hole which was close to the area of the highest heat load. The layer was eroded in discharges without silane injection. The layer was not only formed by the injected silicon but also by a large amount of carbon which must originate from other plasma facing components. The preferential direction (symmetry line) of the deposition pattern is tilted by ∼ 25° relative to the magnetic field lines Calculations with the ERO-TEXTOR code recover the general shape of the Si-deposition pattern. To get accordance between the calculations and the experimental results a strong electric field parallel to B and a sticking coefficient much smaller than 1 has to be considered to reproduce the low silicon deposition efficiency, whereas the tilting of the symmetry line can be explained by the existence of a radial electric field.