Effects of magnetic mirror on helicon plasma transport in Proto-MPEX

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX), a linear device, at ORNL has a helicon plasma source with additional microwave electron heating and RF ion heating. Proto-MPEX has several magnetic mirrors which could affect the plasma transport. For example, the magnetic field near the helicon source is ~0.07T, and the peak magnetic field in the central chamber is ~1.0T; creating a large magnetic well. In contrast, the magnetic field at the target region is variable, but typically <0.5T. This study is focused on the transport of the helicon generated plasma in the presence of various magnetic mirrors. Experiments have been conducted keeping the helicon source region magnetic field constant and lowering the peak magnetic field downstream towards the target. Results showed expansion and compression of the magnetic field lines which affected the electron density and plasma flow velocity along the magnetic flux tube, but the total ion flux reaching the target was conserved. These experiments revealed that the thermal plasma, streaming along the magnetic field lines in Proto-MPEX behaves like a compressible fluid. Detailed analysis from various diagnostics will be presented and compared to modeling.