Use of the ТМ110 (E110) Mode of a Cylindrical Microwave Resonator for Estimation of Plasma Density in a Gas Discharge Tube

Traditional microwave resonator methods for plasma diagnostics face fundamental limitations at high plasma densities (ne > 1011 cm–3) due to the response nonlinearity and resonant peak broadening. The search for alternative approaches that provide accurate measurements over a wide range remains a topical issue. In this work, the feasibility of using the TM110 (E110) mode of a cylindrical resonator (radius R = 45 mm, length H = 30 mm) to measure the electron density in a gas-discharge plasma excited by a surface electromagnetic wave is studied experimentally and numerically. It is found that the coupling coefficient Cv for the TM110 mode remains constant in the range ne = 1011–1012 cm–3, ensuring a linear dependence of the resonant frequency shift on the plasma density. The TM110 mode demonstrates resistance to the resonant peak degradation at high plasma densities ne, in contrast to the TM010 mode, where the signal suppression was observed already at ne ≈ 3 × 1011 cm–3. The obtained measurement results are in good agreement with the results of plasma density measurements using the transmitted wave method and the integrated plasma luminosity, as well as with literature data. The proposed modification of the method is suitable for noninvasive diagnostics of the longitudinal electron density distribution of gas discharge plasma, including plasma antennas.