Impurity radiation from a beam-plasma neutron source

Abstract

Impurity radiation, in a worst case evaluation for a beam-plasma neutron source (BPNS), does not limit performance. Impurities originate from four sources: (a) sputtering from walls by charge exchange or alpha particle bombardment, (b) sputtering from limiters, (c) plasma desorption of gas from walls and (d) injection with neutral beams. Sources (c) and (d) are negligible; adsorbed gas on the walls of the confinement chamber and the neutral beam sources is removed by the steady state discharge. Source (b) is negligible for impinging ion energies below the sputtering threshold (Ti ⩽ 0.025 keV on tungsten) and for power densities to the limiter within the capabilities of water cooling (30-40 MW/m2); both conditions can be satisfied in the BPNS. Source (a) radiates 0.025 MW/m2 to the neutron irradiation samples, compared with 5 to 10 MW/m2 of neutrons; and radiates a total of 0.08 MW from the plasma column, compared with 60 MW of injected power. The particle bombardment that yields source (a) deposits an average of 2.7 MW/m2 on the samples, within the capabilities of helium gas cooling (10 MW/m2). An additional worst case for source (d) is evaluated for present day 2 to 5 s pulsed neutral beams with 0.1% impurity density and is benchmarked against 2XIIB. The total radiation would increase a factor of 1.5 to ⩽ 0.12 MW, supporting the conclusion that impurities will not have a significant impact on a BPN