Electron Density Measurement in High-Pressure Plasma with a Microwave Cutoff Probe

The microwave cutoff probe is a well-known diagnostic tool for determining absolute electron density in plasma. However, under high-pressure conditions where the electron–neutral collision frequency is higher than the plasma frequency, the cutoff frequency becomes indistinguishable, which limits the usable pressure range of this method. To address this issue, we present an alternative diagnostic method that estimates electron density using a microwave cutoff probe by analyzing the correlation between the resonance and cutoff frequencies. Electromagnetic simulations and experiments were conducted to verify the validity of this method. Simulation results showed that the measurement discrepancy remained within 1.47 percent at a gas pressure of 10 mTorr. In contrast, under high-pressure plasma conditions at 2500 mTorr, the maximum discrepancy increased to 13.3 percent. However, this discrepancy was significantly reduced to within 1.92 percent by selecting resonance frequencies higher than the electron–neutral collision frequency. The feasibility of this method was confirmed under high-pressure plasma conditions where the cutoff frequency could not be identified by experimental results. We believe that our method for diagnosing high-pressure plasma can significantly enhance the measurement and interpretation of plasma parameters across a variety of industrial applications.