Time-Resolved Wall Ion Flux Linked to Discharge Current Oscillations in an SPT-100-Type Electric Thruster

An SPT-100-type electric thruster is simulated at nominal operating conditions using a two-dimensional, axisymmetric, fully kinetic particle-in-cell/Monte Carlo collision (PIC/MCC) plasma model. The model reproduces PIVOINE facility measurements, predicting a thrust and discharge current of 77.2 mN and 4.30 A versus 80.0 mN and 4.15 A measured, with thrust efficiency and specific impulse of 42.4% and 1575 s versus 45.0% and 1490 s. The power spectral density of the discharge current waveform reveals a dominant breathing-mode oscillation peak at 19.5 kHz and a secondary feature near 107 kHz. Time-resolved ion flux to the inner and outer radial walls exhibits quasi-periodic modulation and intermittent near-exit enhancements, demonstrating that instantaneous erosion drivers vary substantially over the oscillation cycle. Cross-correlation of the breathing-band (15–25 kHz) components indicates that the wall ion flux lags the discharge current by approximately 63° (≈ 9 µs, ≈ 18% of a breathing period), suggesting a delayed near-wall response associated with motion and reshaping of the ionisation/acceleration region and the near-exit sheath/field structure rather than ballistic ion time-of-flight. These results provide a phase-resolved pathway for connecting breathing-mode oscillations to wall ion bombardment in SPT-class Hall-effect thrusters.