Predicting Equilibrium Electron Density Production on Proto-MPEX

The helicon antenna plasma source installed on Proto-MPEX has been able to produce electron densities of n_e >6×10¹⁹ m^-3 in the “helicon-mode”. The existing power availability of the helicon antenna is P≈100 kW at 13.56 MHz driving frequency. The “helicon-mode” plasma on Proto-MPEX is only observed up to a magnetic field of B_H < 0.08 T. This low operating magnetic field strength restricts the plasma diameter at the target, which is typically operated at B₀ ≈ 1.0 T, to d_p ≈ 4 cm. A larger plasma diameter would be beneficial to Proto-MPEX both for electron and ion heating demonstration as well as future PMI experiments. A full-wave electromagnetic model that was used to explain the mechanisms that lead to the observation of the “helicon-mode” plasma on Proto-MPEX is used to predict power deposition in the helicon region. Power losses are calculated and used in a power balance to predict an equilibrium electron density. Comparison with experimentally measured electron density variation with magnetic field strength at a power level of P = 100 kW is presented. This model is then used to predict the expected electron density at higher magnetic field values with an available power of P = 200 kW.