Preparing for next-step fusion facilities will require developing materials that can withstand the high ion and neutron fluences that will be present in the divertor region. These fluences are inaccessible in current toroidal devices. The Material Plasma Exposure eXperiment (MPEX) is a steady-state linear plasma device, currently undergoing conceptual design, that proposes to reach ion fluences as high as 1031 m−2. It will also be able to receive neutron irradiated samples to examine the multivariate effects of plasma material interactions. A target exchange chamber will be employed so that the MPEX target can be removed and placed in a separate diagnostic station without leaving vacuum. To operate in steady-state, the MPEX plasma will be confined using superconducting magnets, with active cooling for all plasma-interacting and plasma-facing components. The plasma source will be a high-power (200 kW) helicon antenna, which will be placed outside of the vacuum chamber. The radio frequency-transparent window for this antenna must be water cooled and must have a very low dielectric coefficient to limit the dielectric losses. The water-to-vacuum seal should not be an elastomer seal, to limit impurities at the plasma source. It is proposed to use a ceramic-to-metal joint. A prototype water-cooled helicon antenna window and assembly have been manufactured and tested in long-pulse conditions up to 10 kW in the Controlled Shear De-correlation eXperiment at the University of California, San Diego. Thermal results have been correlated with computational fluid dynamics simulation.