Limitations to the plasma energy and density in electron cyclotron resonance ion sources

Electron Cyclotron Resonance (ECR) plasmas are generated by the resonant interaction of a high-frequency (hf) wave with the electrons of the plasma close to the electron Larmor frequency. At high hf power the collisional relaxation is not sufficient to thermalize the electrons and a complex Electron Distribution Function (EDF) is generated. In ECR Ion Sources (ECRIS), dedicated to the production of Highly Charged Ions (HCI), such a hot electron component is required to ionize the atoms to very high charged states; in this paper experiments on the electron population are presented. It is shown that ECRIS plasmas are very far from thermal equilibrium; which is necessary for the production of HCI; the observed saturation of the performances (in electron energy and density, and ion charge state) can be explained by the wave–plasma interaction itself: low-energy electrons interact primarily with the electric field of the wave, whereas the high-energy component experiences mainly pitch-angle scattering due to the interaction with the magnetic field of the wave, which becomes dominant. A simple model, consistent with the results obtained in the domain of strong hf power, is developed. This model explains the saturation phenomena encountered.