Ion source discharge performance and stability

Very efficient ion sources which utilize electron discharges to produce the plasma have been observed in certain cases to be susceptible to discharge impedance shifts (mode shifts). This results in unreliable plasma generation and unstable operation of the ion source. Studies have shown that the mode shifts are related to a decrease in the plasma potential to values very negative with respect to the anode potential. A general model of the ion source plasma is presented and the equilibrium plasma potential is calculated as a function of the plasma parameters and average loss rates. Parameters which contribute to negative plasma potentials are then identified. The experimentally observed constraints on gas pressure range and minimum discharge voltage at which the source will operate are predicted theoretically and the dependence demonstrated in an ion source. The negative plasma potentials are shown to be generated by enhanced thermalization of the primary electrons in the plasmas for ion sources with good electron confinement. The analysis reveals that plasma generator electrical efficiency is ultimately determined by the requirements on the minimum anode area necessary for stability of the discharge.