Studies of electron heating on a 6.4 GHz ECR ion source through measurement of diamagnetic current and plasma bremsstrahlung

Diamagnetic current and low energy (2-70 keV) x-ray bremsstrahlung measurements taken on a 6.4 GHz electron cyclotron resonance ion source (ECRIS) are presented as a function of microwave power, neutral gas pressure and magnetic field configuration. X-ray flux from confined electrons and plasma energy density depend logarithmically on microwave power. This result differs from previous studies performed on ECRISs that operate at higher microwave frequencies, in which the x-ray power increases in an essentially linear fashion with the microwave power. X-ray power and plasma energy density both saturate as the neutral pressure is increased beyond a certain value. The gradient of the magnetic field is shown to have a large effect on both x-ray power and plasma energy density. Lastly, it is observed that the peak in x-ray power efficiency (x-ray power per unit of absorbed microwave power) and the peak in extracted ion current efficiency (recorded Faraday cup current per unit of absorbed microwave power) occur at different absorbed microwave powers.