PISCES-RF: a liquid-cooled high-power steady-state helicon plasma device

Abstract: Radio-frequency (RF) driven helicon plasma sources can produce relatively high-density plasmas (n > 10^19 m-3) at relatively moderate powers (< 2 kW) in argon. However, to produce similar high-density plasmas for fusion relevant gases such as hydrogen, deuterium and helium, much higher RF powers are needed. For very high RF powers, thermal issues of the RF-transparent dielectric window, used in the RF source design, limit the plasma operation timescales. To mitigate this constraint, we have designed, built and tested a novel liquid-cooled RF window which allows steady state operations at high power (up to 20 kW). De-ionized (DI) water, flowing between two concentric dielectric RF windows, is used as the coolant. We show that a full azimuthal blanket of DI water does not degrade plasma production. We obtain steady-state, high-density plasmas (n > 10^19 m-3, T_e ~ 5 eV) using both argon and hydrogen. From calorimetry on the DI water, we measure the net heat that is being removed by the coolant at steady state conditions. Using infra-red (IR) imaging, we calculate the constant plasma heat deposition and measure the final steady state temperature distribution patterns on the inner surface of the ceramic layer. We find that the heat deposition pattern follows the helical shape of the antenna. We also show the consistency between the heat absorbed by the DI water, as measured by calorimetry, and the total heat due to the combined effect of the plasma heating and the absorbed RF. These results are being used to answer critical engineering questions for the 200 kW RF device (MPEX: Materials Plasma Exposure eXperiment) being designed at the Oak Ridge National Laboratory (ORNL) as a next generation plasma material interaction (PMI) device.