Helicon plasmas are being considered as plasma sources for wakefield accelerators, subject to strict density requirements. We present various mechanisms to increase axial density homogeneity in a helicon plasma for implementation in such an accelerator. We consider various background neutral flow configurations for helicons generated with first one antenna and then two identical antennas in a 2-m-long, 52-mm-diameter plasma chamber with homogeneous magnetic field. In the case of a single antenna, the ionization source rate and density profiles are not significantly influenced by the background neutral flow. The use of a second antenna expands the plasma axially along the device and results in an increased dependence of the axial density profile on the background neutral flow. We find an increase in axial homogeneity by a factor of two when there is no background neutral flow compared to when there is flow in either direction relative to the plasma. The minimum axial density deviation accomplished by optimization of RF antenna spacing and neutral flow was 5%. This is still a factor of 20 above the nominal homogeneity requirement, but means to further improve this have been established in this study.
Zepp et al. (Thu,) studied this question.
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