In a 2.45 GHz electron cyclotron resonance (ECR) ion thruster powered with rod antenna under a cross magnetic field, abnormal behaviours such as sudden drop of ion beam current (I b) and larger increasing-rate of I b in the high microwave power (P w) discharges at high gas flow rates were observed. A differential method was proposed to reveal the changes in the radial profiles of gray values extracted from the end-view discharge images. The increasing-rate of I b with respect to P w was used to evaluate efficiencies of ion production and transport. Analyses indicate that discharges are dominantly sustained by ordinary wave via electron heating in the electron plasma resonance layer that can shift along the rod-antenna, and extraordinary wave can only ignite a discharge in the ECR layer in the low gas flow rate regime. In terms of the confinement region defined by the magnetic field lines intercepting with the screen grid, the confinement region of the optimized 2.45 GHz cross magnetic field takes the shape of hourglass, enabling the high increasing-rate of I b with respect to P w in high power discharges at high gas flow rates. Correlated with the accompanied bright boundary layer appearing in the differentiated image, the sudden drop of I b in the low gas flow rate regime is attributed to the discharge ignited by the enhanced extraordinary wave in the ECR layer neighbouring the narrowest confinement region, where the produced ions can promptly enter the loss region.

Effects of ion extraction on gridded ion source discharge or the coupling between ion extraction and gridded ion source discharge are investigated for the first time by correlating the ratio of the ion impingement current (Ia) to ion beam current (Ib) with the total gray values extracted from images of discharge glows with and without ion extraction. The results indicate that, under equal power and gas pressure, discharges with high and low Ia/Ib values in the presence of ion extraction are, respectively, more intensive and weaker than those without ion extraction. The competing factors behind the coupling between ion extraction and ion source discharge are ion loss and energetic secondary γ-electron injection from the accelerator grid, which weakens and enhances ion source discharge, respectively.