Helium Pumping Strategies for D-T Fusion Devices
- PDF / 997,256 Bytes
- 3 Pages / 604.8 x 806.4 pts Page_size
- 0 Downloads / 191 Views
ULY1990
not only a much higher solubility, but also a much higher diffusion coefficient than hélium in most metals. Consequently, it is to be expected that for some metals, there is a température range in which implanted hydrogen is released much more quickly than implanted hélium. Brooks and Mattas5 proposed using this "self-pumping" effect as the basis for a pump which preferentially removes hélium in the présence of hydrogen. Hélium Trapping Thermal évolution spectrometry6 has shown that for He+ implanted in nickel or stainless steel, the trapping energy dépends on the implantation energy and bombardment rime, as well as target température and crystalline structure. In gênerai, the trapping energy increases with increasing ion energy and dose. Thèse effects are related to the formation of defects which act as stable trapping sites. As the beam energy increases, both the defect production rate and trap depth increase. At least seven différent trapping sites hâve been identified for He+ implanted in nickel, with trap énergies ranging from 1.3 to 4.1 eV.6 For the highest energy trap, corresponding to occupation of a vacancy site, significant thermal desorption of hélium does not occur until the target température approaches 1000°C. Relatively defect-free single crystals hâve a very low initial trapping coefficient which increases with dose as the beam créâtes defect sites. For 1 keV He + incident on single-crystal Ni, the trapping coefficient exceeds 60% only for doses > 6 x 1015 ions/cm2. For cold-worked polycrystalline nickel foil, which has much higher initial defect density, the initial
(zéro dose) trapping coefficient exceeds 60% for incident énergies in excess of only 100 eV. For an evaporated film, the microstructure dépends strongly on the déposition température. It is therefore necessary to choose a déposition plate température low enough to provide a significant number of stable hélium trapping sites, but high enough to provide significant hydrogen diffusion and thereby obtain preferential hélium rétention. If hélium rétention levels >10% can be obtained in a tokamak environment, it will be possible to replace most of the external pumps and ductwork required in current designs with one very compact, completely self-contained pumping module. The implantation energy expected in a tokamak for the plasma species impinging on a limiter or divertor surface is about 50-600 eV for plasma edge températures in the range 20-100 eV. It is therefore important to détermine the low energy trapping properties for He + , especially in the présence of simultaneous hydrogen implantation since hydrogen may successsfully compete with hélium for the low energy trapping sites. It has, for example, been demonstrated7 that 4 keV He+ ions are trapped at 300 K in béryllium with an efficiency of —80% up to a fluence of 2 x 1017 ions/cm2. If however, the béryllium sample is simultaneously bombarded with a hydrogen flux approximating the 20:1 H/He ratio which represents the maximum allowable He accumulation in a long puise D-T burning
Data Loading...
