M. Freisinger

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.

Quartz microbalance: a time resolved diagnostic to measure material deposition in JET

Abstract To promote solutions of key fusion problems like tritium retention and wall erosion, more measured data of carbon deposition in remote areas of fusion devices are needed. These data are essential to understand and model local and global particle fluxes and to make predictions for future devices like ITER. In spring 2000 the development of a new diagnostic, quartz microbalance (QMB), was started to measure quantitatively, in situ and time resolved (≤1 s) the material deposition in the sub divertor region of JET. Calibration work of mass and temperature characteristic was done at FZ-Julich giving a layer mass sensitivity of Γmass=7.5×10−9 g/Hz and a temperature sensitivity of Γtemp(100xa0°C)≈0.09xa0°C/Hz. The commercially available QMB technique was adapted to the harsh constraints and requirements of JET. After implementation in March 2002 reliable and successful operation was demonstrated during the C5 campaign (∼1000 plasma pulses). First examples of measurements are presented.

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.

Survey of dust formed in the TEXTOR tokamak: structure and fuel retention

A detailed survey of erosion and deposition on plasma-facing components was performed in the TEXTOR tokamak. Co-deposits and dust particles were collected from graphite limiters and from several lo ...

B4C-limiter experiments at TEXTOR

Abstract In the TEXTOR tokamak the five top and five bottom poloidal carbon limiter blocks have been replaced by inertially cooled copper blocks coated with a 170 μm VPS-B4C layer. Similar limiter blocks have been inserted through lock systems, extensively diagnosed in situ as well as ex situ. During the thermal load by the plasma, the surface temperature rose and decayed extremely fast which can be explained by a different thermal conductivity and heat capacity of the coating. For heat loads below 8 MWxa0m−2 no severe cracking or delamination of the B4C-coating were observed. Due to the insulating behaviour of the layer, distinct craters developed on both limiter types, which reached down to the copper surface and are assumed to be caused by electrical arcs. An oscillation of the evolution of the surface temperature has been observed under certain conditions, which is clearly correlated to the use of the coated test limiter. Particle fluxes as well as hydrogen inventory turned out to be very similar to those from a low-Z surface in a carbon surrounding. No significant impact of the plasma on the coating and vice versa was observed.

Dust Survey Following the Final Shutdown of TEXTOR : Metal Particles and Fuel Retention

The work presents results of a broad TEXTOR dust survey in terms of its composition, structure, distribution and fuel content. The dust particles were collected after final shutdown of TEXTOR in De ...

Fuel re-absorption by thermally treated co-deposited carbon layers

Systematic studies have been conducted to address the fuel re-absorption by carbon deposits under repeated exposure to plasma after cleaning procedures. The investigation was done with graphite tiles from ALT-II (Advanced Limiter Test II), i.e. the main limiter at the TEXTOR tokamak. Pure graphite plates were used as the reference material. The experimental programme comprised the following: pre-characterization of specimens; D desorption by baking the tile at 1273K; surface analyses of the fuel-depleted layers; exposure to deuterium in a laboratory plasma device and in TEXTOR; and quantitative assessment of deuterium re-absorption. The main result is that fuel retention in the re-exposed deposits is 30‐40 times lower than that in the original co-deposit, showing that fuel re-absorption does not lead to an immediate re-saturation of deposits. Annealing at high temperatures enhances layer brittleness, leading eventually to detachment of co-deposits.

ICRF/ECR plasma production for wall conditioning in TEXTOR-94

To develop an alternative scenario for efficient wall conditioning in superconducting fusion devices, comparative studies of ICRF and ECR Discharge Conditioning (ICRF-DC/ECR-DC) have been undertaken on TEXTOR-94 using the present ICRH and ECRH systems without changes in the hardware. The first results clearly indicate essential differences in performance and cleaning between the two types of RF discharges for the same machine parameters {BT=2.0u2009T,pHe=(3÷7)×10−2u2009Pa,PICRF≈PECRH≈0.1u2009MW÷0.2u2009MW,τICRF=τECRH=0.2u2009s}: (i) The ICRF plasma produced by double-strap unshielded antenna (ω=4ωcHe+=2ωcHe++=ωcH) has a low density (ne0≈0.4×1018u2009m−3) and homogeneous distribution in the torus during all phases of the discharge; (ii) The ECR plasma produced by quasi-optical beam launching antenna (ω=2ωce) has a high density (ne0=2.4×1018u2009m−3) and is strongly localized along the equatorial trajectory of the focussed microwave beam; (iii) Hydrogen desorption from the wall looks more pronounced and efficient in the ICRF-DC than i...

TOMAS - A Toroidal Magnetized Plasma Facility for Studying Wall Conditioning of Future Fusion Devices

The TOroidal MAgnetized System (TOMAS) is a simple magnetized torus dedicated to the investigation of wall conditioning methods by microwave-induced plasmas. In the TOMAS facility, an electron cyclotron resonance plasma is produced by microwaves at a frequency of 2.45 GHz and the corresponding resonant magnetic field of 87.6 mT. The facility and the first operational experience of film deposition by means of methane plasmas are described.