Conceptual design of 30 kW-NBI injector using a multi-cusp ion source for heating of D-shaped Damavand tokamak plasma

Abstract

A conceptual design is presented for a neutral beam injection system using the parameters of Damavand tokamak plasma. The design is of a multi-cusp ion source, an efficient large-scale multi-aperture system, a high-energy beam neutralizer cell and a bending magnet for the Damavand neutral beam injector. It is shown that, when taking the tokamak parameters and the amount of beam absorption into consideration, the ion source must deliver an ion beam for which the critical energy, ion beam power and current are 4.5 keV, 30 ± 1.5 kW and 6.7 A, respectively. The extractor system has three electrodes, each with a radius of 35 mm, and 69 apertures, each with a radius of 3 mm, for extraction of a proton beam with a radius of 35 mm. The extractor transparency was 50%, magnetic fields near the ion source of electron emission and ion extraction areas was less than 0.0003 T, the ion source dead space thickness was 38.4 mm, acceleration gap was 6 mm and the optimum voltage of the extractor suppressor electrode was -2.5 kV. The results show that the neutralization efficiency increased with an increase in the outgassing flux, which is equivalent to a decrease in the neutralizer length at a fixed input flux rate. The maximum efficiency obtained was about 83% at an input flux rate of 37.5 sccm. Considering the hydrogen gas pressure and density in the charge exchange cell, this was compatible with the theoretical relation and the Damavand tokamak vacuum. Iron magnets, a steel transmission chamber and a deviation angle of 180° were considered. The uniform magnetic field along the path of the beam was 0.03 T, number of coils was 112 and the current of was 18.2 A.