  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.
