Modeling of the stability of electron cyclotron resonance ion source plasmas

A significant factor limiting electron cyclotron resonance ion source (ECRIS) operation is that the currents of extracted ions do not monotonically increase with the input radiofrequency (rf) power. In fact, the currents tend to saturate at high power levels and in some cases the ECRIS plasma goes unstable if the rf power becomes too high. We have simulated this high rf power instability using FAR-TECHʹs Generalized ECRIS Model (GEM). The simulations reproduce a key feature of the experiments, which is that the threshold rf power for the instability increases with increasing gas pressure. Whereas previous researchers have invoked turbulence, kinetic instabilities, and/or magneto-hydrodynamic instabilities to explain the experimental observations, our simulations indicate that the driver behind the instability is the pitch angle scattering of the electrons by the ECR heating process itself. We present a simple zero-dimensional model that both explains the scaling of ECR ion source plasma parameters with rf power and gas pressure as well as predicts the conditions for the instability to occur.