Tungsten is a candidate plasma facing material for next generation magnetic fusion
devices such as ITER and there are major operational and safety issues associated with hydrogen
(tritium) retention in plasma facing components. An ion gun was used to simulate plasmamaterial
interactions under various conditions in order to study hydrogen retention characteristics
of tungsten thus enabling better predictions of hydrogen retention in ITER. Thermal Desorption
Spectroscopy (TDS) was used to measure deuterium retention from ion irradiation while
modelling of TDS spectra with the Tritium Migration Analysis Program (TMAP) was used to
provide information about the trapping mechanisms involved in deuterium retention in tungsten.
X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) were
used to determine the depth resolved composition of specimens used for irradiation experiments.
Carbon and oxygen atoms will be among the most common contaminants within ITER.
C and O contamination in polycrystalline tungsten (PCW) specimens even at low levels (~0.1%)
was shown to reduce deuterium retention by preventing diffusion of deuterium into the bulk of
the specimen. This diffusion barrier was also responsible for the inhibition of blister formation
during irradiations at 500 K. These observations may provide possible mitigation techniques for
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problems associated with tritium retention and mechanical damage to plasma facing components
caused by hydrogen implantation.
Deuterium trapping in PCW and single crystal tungsten (SCW) was studied as a function
of ion energy and substrate temperature. Deuterium retention was shown to decrease with
decreasing ion energy below 100 eV/D+. Irradiation of tungsten specimens with 10 eV/D+ ions
was shown to retain up to an order of magnitude less deuterium than irradiation with 500 eV/D+
ions. Furthermore, the retention mechanism for deuterium was shown to be consistent across the
entire energy range studied (10-500 eV) with the shallow penetration depth of low energy ions
being the major factor in the reduction in retention. A change in retention mechanism was
observed as tungsten temperature during irradiation was increased from 300 to 500 K.
Modelling of deuterium retention in 300 and 500 K SCW specimens revealed that two traps, 1.0
and 1.3 eV, are involved in retention for irradiations performed at 300K while a single 2.1 eV
trap is present for 500 K irradiations. Experiments suggest that the 2.1 eV trap is created during
irradiation of tungsten at 500 K and this process also involves the annihilation of the 1.3 and 1.0
eV traps.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/34868 |
| Date | 19 December 2012 |
| Creators | Roszell, John Patrick Town |
| Contributors | Haasz, Anthony A., Davis, James W. |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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