Mechanism of Beam-Plasma Discharge Development in the Gas Dynamic Trap

Abstract

The work studies how a plasma discharge in the Gas Dynamic Trap (GDT) is ignited in a volume significantly exceeding the volume of the injected electron beam. This property of a beam-plasma discharge makes it attractive to use relatively low-power electron beams to create starting plasma in open traps with parameters sufficient for its further effective heating by the neutral injection. Despite the fact that effective ionization of plasma far beyond the electron beam in open traps has been observed experimentally for more than 60 years, the detailed mechanism of this phenomenon is still not clear. Particularly many questions arise in regimes that are implemented in such large fusion facilities as GDT, where the relaxation length of the beam turns out to be significantly less than the length of the trap, and the turbulence excited by the beam is localized near the entrance magnetic mirror. Based on recent experiments [E.I. Soldatkina et al. Nucl. Fusion 62, 066034 (2022)], we have proposed a scenario for the development of a discharge in GDT, according to which a compact region of intense plasma turbulence first rapidly expands radially, ionizing the gas outside the beam tube in the vicinity of a magnetic mirror, and then a slower ionization process begins in the rest trap volume both due to the tail of suprathermal particles that are formed in the turbulence zone, and due to thermal electrons that receive energy from the pumping region via the longitudinal electron thermal conductivity. To assess how well this scenario explains the experimentally observed dynamics of plasma density growth in different parts of the facility, we carry out particle-in-cell simulations of the radial expansion of the turbulence zone and propose a simplified model of impact ionization in the entire volume of the trap by both thermal and suprathermal electrons.