On abnormal behaviors of ion beam extracted from electron cyclotron resonance ion thruster driven by rod antenna in cross magnetic field

Abstract

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 gas adsorbed on solid surface during gas breakdown in electron cyclotron resonance discharges

“…The ECR breakdown is not initiated from electron-field emission at the antenna's surface but the electron heating in the ECR layer [28,29,33]. The antenna's effective zone previously described should be the region that can be bombarded by energetic electrons coming from the ECR layer.…”

“…In principle, the gas breakdown can take place in the ECR layer zigzagging through the discharge chamber (see figure 2(b)). The preferential ignition was verified to be at the location where the vertical component of the microwave electric field (E ⊥ ), with respect to the static magnetic field, was highest [29,33]. Since the radius of the disc Mo antenna was increased from 11.0 mm for discharges in reference [29] to 19.0 mm in this paper, the distribution of E ⊥ in the ECR layer was recalculated using the method described in reference [29].…”