Dependence of the Bremsstrahlung Spectral Temperature in Minimum-B Electron Cyclotron Resonance Ion Sources

Abstract

The spectral temperature $T_{s}$ obtained from bremsstrahlung spectra emitted from electron cyclotron resonance (ECR) ion sources (ECRISs), in which the plasma is confined in a minimum-B magnetic field, is used as a relative indication of the temperature of the plasma hot electrons. Past bremsstrahlung measurements taken on ECRISs indicate that $T_{s}$ is strongly dependent on the magnetic field gradient at the resonance zone or $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ . However, this dependence was never fully proven or explained. To further our understanding a more detailed study of the bremsstrahlung radiation for X-rays above 10 keV is underway using VENUS, a third-generation ECRIS at Lawrence Berkeley National Laboratory. Initial analysis of previous and new data has revealed that $T_{s}$ appears to be dependent solely on the minimum magnetic field $B_{\mathrm{ min}}$ rather than $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ and the microwave frequency $\omega$ . Decoupling $T_{s}$ from $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ , mainly $B_{\mathrm{ ECR}}$ , implies that $T_{s}$ does not depend on the magnitude of the heating frequency $\omega$ . While it certainly appears to be true that plasma density $n_{e}(\propto \omega ^{2}\propto B^{2})$ increases with heating frequency, as was postulated by Geller in 1987, a more careful consideration into the heating mechanism of the plasma electrons is warranted. The disassociation of $T_{s}$ from the heating frequency $\omega$ , while an interesting discovery, implies that we must change the way we understand how ECRISs operate. This paper presents new bremsstrahlung measurements, analyses, and discussions of the results.