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

Electron Cyclotron Resonance (ECR) plasmas are generated by the resonant in teraction of a high-frequency (hf) wave with the electrons of the plasma cl ose to the electron Larmor frequency. At high hf power the collisional rela xation is not sufficient to thermalize the electrons and a complex Electron Distribution Function (EDF) is generated. In ECR Ion Sources (ECRIS), dedi cated to the production of Highly Charged Ions (HCI), such a hot electron c omponent is required to ionize the atoms to very high charged states; in th is paper experiments on the electron population are presented. It is shown that ECRIS plasmas are very far from thermal equilibrium; which is necessar y for the production of HCI; the observed saturation of the performances (i n electron energy and density, and ion charge state) can be explained by th e wave-plasma interaction itself: low-energy electrons interact primarily w ith the electric field of the wave, whereas the high-energy component exper iences mainly pitch-angle scattering due to the interaction with the magnet ic field of the wave, which becomes dominant. A simple model, consistent wi th the results obtained in the domain of strong hf power, is developed. Thi s model explains the saturation phenomena encountered. (C) 1999 American In stitute of Physics. [S1070-664X(99)02308-3].