Verlag des Forschungszentrums Jülich
JUEL-2995
Refke, Arno
Untersuchung der chemischen Reaktionen von energetischem Sauerstoff mit Graphit, B4C sowie bor- und silisiumhaltigen Kohlenstoffmaterialien
119 S., 1995
Abstract:
One of the major problems in plasma-wall-interaction is the impurity control of the fusion plasma.
In this task the use of carbon as plasma facing material was a major step in reducing the metal and
oxygen impurity concentration in the plasma. The remaining impurities were then carbon and
oxygen, the last one being responsible for the high carbon contamination of the plasma due to
chemical erosion in form of CO and CO2. It has been found that the use of boron/carbon (B/C) and
silicon/carbon (Si/C) materials has significantly reduced the oxygen contamination in the plasma
due to a gettering effect and therefore led to an enhanced plasma performance.
For a better understanding of the underlying reaction mechanisms irradiation experiments have been
performed to investigate the chemical erosion, the retention behaviour, the thermal desorption and
the energy distribution of the reaction products during the bombardment of different pure carbon
as well as B/C- and Si/Cematerials with energetic oxygen ions depending on target temperature.
The experiments were performed with a mass separated 18°2+ ion beam of 2 - 10 keY in the
temperature range between room temperature and 1800 K. The reaction products were detected by
means of mass spectroscopy either with residual gas analysis or by direct detection without hitting
the wall in a "Iine-of-sight" quadrupol mass spectrometer. The energy of the reaction products was
measured by means of "time-of-flight" experiments.
Pure graphite shows a high chemical erosion yield of 0.7 CIO in form of CO and CO2 only slightly
depending on ion energy and target temperature in the investigated parameter range. The retained
oxygen reached a saturation value of 0.25 OIC independent of ion energy and is completely
desorbed in form of CO and CO2 in the temperature range of 700 and 1000 K after degassing with
a linear temperature ramp during thermal desorption measurements. The observed behaviour of the
CO-reemission could be well described with a simple local saturation model. Special experiments
have led to a better understanding of the underlying reaction mechanisms for CO- and COr
production and release: postirradiation of oxygen saturated graphite with Ne+ ions as well as
isotope exchange experiments with 160 /180 and irradiation of 13C-overlaid graphite indicates, that
the release mechanism of chemically eroded reaction products is an ion-induced collision process.
The molecules are formed at the ion implantation depth and will then be released and transported
from the end of ion range to the target surface. Diffusion dominated transport of the molecules can
be excluded. Time-of-flight measurements have shown that the chemically eroded CO molecules
exhibit 110t only a thermal energy distribution but also an over-thermal component with a maximum
at 0.12 eV whereas the CO2 molecules are only thermally released.
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