Differential pumping requirements for the light-ion helicon source and heating systems of Proto-MPEX

The physics of electron and ion heating of high-density deuterium helicon plasmas (>3 × 10 19 m⁻³) in the Proto-Material Plasma Exposure Experiment linear device are under investigation. Theoretical estimates indicate that for efficient heating, discharges with very low neutral gas content ( ≪0.1 Pa) in the heating sections are required to minimize collisional losses and charge exchange interactions with neutrals. However, this requirement is typically not compatible with the neutral gas pressures (1-2 Pa) commonly used in high-density, light-ion helicon sources. To satisfy these competing requirements, differential pumping techniques are needed. In this paper, results are presented that demonstrate the production of high-density discharges (2-6 × 10 19 m⁻³) with very low neutral gas content (<0.01 Pa) and high degree of ionization (>75%) in the heating sections. Results indicate that the best fueling location is upstream of the plasma source. We elaborate on the key aspects that must be considered to produce these discharges: (1) fueling location, radio-frequency pulse length, and magnetic field configuration, (2) flow rate and timing of the gas injection, and (3) use of conductance-limiting elements.