Numerical Study of Two-Frequency Heating in ECR Ion Source

The two-frequency heating (TFH) method is widely used to improve the performance of electron cyclotron resonance (ECR) ion sources, yet its physical mechanisms remain insufficiently understood. In this work, the TFH regime in the DECRIS-PM ECR ion source is studied numerically using the NAM-ECRIS code. Single-frequency heating at 14.51 GHz (500 W) is compared to two-frequency heating at 14.51 GHz and 13 GHz (450 W + 50 W) at equal total power. It is found that while the global plasma parameters remain largely unchanged, the peak density of highly anisotropic electrons (E > 1 keV, Vperp/Vlong > 10) decreases by ~20% and their distribution broadens radially in the TFH regime. This reduction in electron anisotropy is attributed to the randomizing effect of the lower-frequency wave on electron gyromotion, which suppresses plasma micro-instabilities and leads to improved ion production.