Spectroscopic study of ion temperature in minimum-B ECRIS plasma

Abstract

Experimentally determined ion temperatures of different charge states and elements in minimum-B confined electron cyclotron resonance ion source (ECRIS) plasma are reported. It is demonstrated with optical emission spectroscopy, complemented by the energy spread measurements of the extracted ion beams, that the ion temperature in the JYFL 14 GHz ECRIS is 5–28 eV depending on the plasma species and charge state. The reported ion temperatures are an order of magnitude higher than previously deduced from indirect diagnostics and used in simulations, but agree with those reported for a quadrupole mirror fusion experiment. The diagnostics setup and data interpretation are discussed in detail to demonstrate adequate understanding of the line broadening mechanisms and instrumental effects, which justifies the use of the measured Doppler broadening of the emission lines to derive the corresponding ion temperature. The ion temperatures of the high charge state plasma of the minimum-B ECRIS are compared to those measured from low temperature singly charged microwave and arc discharge plasmas to exclude systematic errors. The ion temperatures in the minimum-B ECRIS plasmas are shown to increase with the charge state and injected microwave power whereas two-frequency heating and plasma instabilities have only a minor effect on them. Possible mechanisms causing the higher-than-expected ion temperatures are hypothesized and assessed based on the reported experimental data. Finally, the implications of the results are discussed, demonstrating the need for further experiments especially with charge breeder ECR ion sources, and experimental data on ion confinement times.