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

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
_min
/B
_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
_min
rather than (B
_min
/B
_ECR
) and the microwave frequency ω. Decoupling T
_s
from (B
_min
/B
_ECR
), mainly B
_ECR
, implies that T
_s
does not depend on the magnitude of the heating frequency ω. While it certainly appears to be true that plasma density n
_e
(∝ ω
²
∝ 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 ω, 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.