Bremsstrahlung Emission of an X-Band Permanent Magnet Minimum-B Quadrupole Electron Cyclotron Resonance Ion Source

We have carried out bremsstrahlung measurements on a permanent magnet minimum-B quadrupole electron cyclotron resonance (ECR) ion source operating at 10.3–11.5 GHz microwave frequencies. The bremsstrahlung spectral temperature is found at 30–35 keV in a wide ranges of microwave power, frequency, and neutral gas feed rate. The result implies that the Bmin-scaling of the spectral temperature, observed for conventional ECR ion sources, is a fundamental property of the electron heating in minimum-B devices, not specific to the field topology. Similar to conventional ECR ion sources, the bremsstrahlung count rate increases linearly with the microwave power but does not depend on the neutral gas pressure. It is demonstrated that dual frequency heating can improve the beam currents of high charge state argon ion beams and reduce the bremsstrahlung emission. Plasma breakdown and decay transient measurements of the bremsstrahlung power flux indicate that the electron confinement in the quadrupole field is weaker than in conventional ECR ion sources using a superposition of solenoid and sextupole fields.