Mitigation of Beam Gas Bleed Charge-Exchange in C-2W

In TAE Technologies’ current experimental device C-2W (also known as “Norman”), steady-state beam-driven field reversed configuration (FRC) plasmas are sustained by neutral beam injection (NBI), edge biasing, and a robust real-time plasma control system [1]. NBI creates a population of non-Maxwellian energetic ions, known colloquially as fast ions, that act as a heating and current drive mechanism for C-2W FRCs. Mitigation of charge-exchange (CX) collisions with neutrals play an important role in fast ion confinement, and minimization of neutral sources can improve plasma performance. One source of neutral flux into the plasma confinement vessel is the gas bleed from the neutral beams, which can be manipulated within some tolerance without loss of NB performance. In a series of experiments, gas bleeds were varied and measurements of the resulting energy deposition in CX loss footprints were taken. These measurements were achieved through spatially distributed optical thermocouples, which are installed on C-2W to measure the local time-integrated heat flux to the wall. Experimental data is compared with simulation results from Monte Carlo particle codes with DEGAS2 neutral density reconstructions. We found that manipulation of the neutral beam parameters allowed us to minimize the gas bleeds, reducing the energy deposited in the CX loss footprints by approximately 30%.

1. Gota et al., Nucl.  Fusion 61,106039 (2021)