シリコン半導体を用いる新しい重水素負イオン源の構成に関する考察
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概要
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A new type of negative deuterium ion source for the high energy neutral beam injector is proposed. The D<SUP>-</SUP>ions are formed via two steps of processes, which are the dissociation of D<SUB>2</SUB> and the electronic transition. The dissociation occurs when deuteriums are solved into the Si-semiconductor on a side where D<SUB>2</SUB> gas is filled at about one atmospheric pressure. The atomic deuteriums are almost in ionized state at inner sites far from the surface according to the Fermi-Dirac distribution function. However, the deuteriums at just inner sites from the surface and ones emerging from the surface are considered to be almost in neutral state, because the energy gap between the ionized stat and neutral state is enlarged due to the image force on D<SUP>+</SUP> ions in the vicinity of the surface. The D<SUP>-</SUP>ions are formed with the resonance transition of electron to D (ls) emerging from the surface of Si-semiconductor at the vacuum side. The hot electrons for the resonance transition to take place, are produced with the avalanche arising in the p-n junction in the Si-semiconductor. The energy distribution of hot electrons depends on the parameters relating to the junction, which are chosen as to minimizing unfavourable electron flux emerging from the surface. The probability of the resonance transition, D (ls) +e→D<SUP>-</SUP> (ls ls') is obtained with its matrix element which is calculated with use of the simple wave function for the electrons of D<SUP>-</SUP> (ls ls'), φ<SUB>1s1s'</SUB> (r<SUB>1</SUB>, r<SUB>2</SUB>) =α<SUP>3/2</SUP> β<SUP> 3/2</SUP> /π exp (-αr<SUB>1</SUB>-βr<SUB>2</SUB>), where α =1.04/ao, β=O.28/ao, ao is the Bohr radius. The effective transition rate can be calculated with the transition probability and the energy distribution of hot electron in the semiconductor, assuming the suitable forms of potential energy arising from the interaction between particles and the surface of semiconductor. The deuteriums stuck on the surface by the van der Waals force are easily converted to D<SUP>-</SUP> ions with the electronic transition, assuming that D2 formation due to the recombination of D (ls) 's is slower process. Esimations are also made on the deuterium flux diffusing through the D<SUP>-</SUP> ion formation element which consists of a bi-layer of Si-semiconductor and vanadium, V. The calculations are made with use of the following values; the total electron density in conduction band, 1 X10<SUP>19</SUP>/ cm<SUP>3</SUP>; the diffusion coefficients of deuterium in Si-semiconductor and vanadium, Dsi= 1.8×10<SUP>-8</SUP> cm<SUP>2</SUP> /sec and Dv. 4×10<SUP>-5</SUP>cm<SUP>2</SUP>/sec respectively at 150°C, the deuterium concentration in vanadium at the Si-V boundary, 1×10<SUP>22</SUP> /cm<SUP>3</SUP>. The results show that the obtainable current density of D-ion is about 27 mA/cm<SUP>2</SUP> with the D<SUP>-</SUP> ion formation element of which Si-layer thickness is 9 μm and V-layer thickness is 0.1 mm. The neutral component leaving from the surface is about 18 percent of D<SUP>-</SUP> ion flux. On the basis of results, a conceptual design is made on the D<SUP>-</SUP> ion source with output current of about 20 A.
- 社団法人 プラズマ・核融合学会の論文