Effect of Electron-Cyclotron Resonance Conditions in a Microwave Discharge of an Ion Source on the Extracted Current

A model of a microwave discharge in the electron-cyclotron resonance mode based on the direct solution of the Boltzmann kinetic equation, without using the Maxwellian electron distribution function approximation, is considered. A significantly different dependence of the discharge development dynamics on the parameters of the electric and magnetic fields is shown than for the Maxwell distribution function, which consists of a sharp transition of the plasma electrons to the runaway mode when a certain field value is exceeded. The simulation results explain the experimental dependences of the ion current density extracted from the microwave ion source with electron-cyclotron resonance on the magnetic field distribution. A more general principle is formulated for choosing the distribution of the magnetic field for such a source, which ensures the maximum ion current.