Plasma source development for fusion-relevant material testing

Plasma-facing materials in the divertor of a magnetic fusion reactor have to tolerate steady state plasma heat fluxes in the range of 10 MW/m2 for ∼107 s, in addition to fusion neutron fluences, which can damage the plasma-facing materials to high displacements per atom (dpa) of ∼50 dpa. Materials solutions needed for the plasma-facing components are yet to be developed and tested. The material plasma exposure experiment (MPEX) is a newly proposed steady state linear plasma device designed to deliver the necessary plasma heat flux to a target for testing, including the capability to expose a priori neutron-damaged material samples to those plasmas. The requirements of the plasma source needed to deliver the required heat flux are being developed on the Proto-MPEX device which is a linear high-intensity radio-frequency (RF) plasma source that combines a high-density helicon plasma generator with electron- and ion-heating sections. The device is being used to study the physics of heating overdense plasmas in a linear configuration. The helicon plasma is operated at 13.56 MHz with RF power levels up to 120 kW. Microwaves at 28 GHz (∼30 kW) are coupled to the electrons in the overdense helicon plasma via electron Bernstein waves and ion cyclotron heating at 7–9 MHz (∼30 kW) is via a magnetic beach approach. High plasma densities >6 × 1019/m3 have been produced in deuterium, with electron temperatures that can range from 2 to >10 eV. Operation with on-axis magnetic field strengths between 0.6 and 1.4 T is typical. The plasma heat flux delivered to a target can be >10 MW/m2, depending on the operating conditions. An initial plasma material interaction experiment with a thin tungsten target exposed to this high heat flux in a predominantly helium plasma showed helium bubble formation near the surface, with no indication of source impurity contamination on the target.