Internal imaging of an extremely small-scale electron cyclotron resonance source revealing the formation of discharge mode transitions and hysteresis

Electron cyclotron resonance ion thruster can realize efficient ionization at low gas pressure and is expected to be applied to gravitational wave detection missions. However, discharge mode transitions and hysteresis were found in an extremely small size (<1 cm) electron cyclotron resonance ion thruster, limiting continuous thrust adjustment. Specifically, excessive microwave power leads to mode transition, sudden drop in screen grid current, and ionization intensity. Invasive diagnostics are difficult to achieve due to size limitations. Therefore, in this paper, an optical emission spectroscopy method including ionic and atomic lines is adopted to reveal the formation of discharge mode transition and hysteresis. Transition from a high-current mode to a low-current mode, a new electron heating region is created, while competing with the existing one. The typical electron temperature of the existing electron heating region increases from 6.6-7.2 eV to 7.2–9.4 eV. And that of the new electron heating region increases from 5.7-6.4 eV to 6.9–9.3 eV. However, the typical ionization rate decreases from 0.19 to 0.22 to 0.15–0.18. The shrinkage of the ionization region is the main reason for the reduction in ionization intensity. Analysis reveals a strong correlation between complex electron cyclotron resonance surface and formation of two competing electron heating regions. Optimizing the magnetic field configuration is the primary means of avoiding discharge mode transitions and can improve thruster applicability in space missions.