Grid-transmitted multi-spectral microscopic imaging for reconstructing plasma parameters in ECR ion thrusters

With the rapid development of deep-space exploration, gridded ion thrusters are facing increasingly stringent requirements for performance and lifetime. Measuring the plasma-parameter distribution inside the discharge chamber is important for understanding discharge physics and optimizing thruster design. However, conventional intrusive diagnostics can perturb the confined plasma and are difficult to apply in miniature ECR ion thrusters with strong magnetic fields and limited internal space. In this work, we present a non-intrusive diagnostic method based on grid-transmitted multi-spectral microscopic imaging. The grid apertures are used as optical access windows, and the emission signals are extracted from the center region of each aperture to reduce the influence of aperture-wall shadowing and near-wall sheath depletion. A forward fitting model combining a xenon collisional-radiative model with parameterized radial distributions is then used to reconstruct the electron temperature and density. The method is applied to a miniature electron cyclotron resonance ion thruster during microwave-power scanning. The reconstructed plasma distributions show clear changes during discharge mode transition and hysteresis. These results demonstrate the potential of the proposed method for high-resolution, non-intrusive plasma diagnostics in confined electric-propulsion devices.