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

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Physics of Plasmas
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The physics of electron and ion heating of high-density deuterium helicon plasmas (>3 × 1019 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 ×1019 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.