Expansion to Higher Mode in Electron Density Measurement by Microwave Resonator Probe

Microwave resonator probe enables electron density measurement from the observed shift in resonance frequency of U-shape antenna of length $L$ in plasma. The fundamental resonance ($m=1$) occurs at the frequency where $L$ coincides with a quarter of the wavelength. In this paper, operation of microwave resonator probe is expanded from the fundamental mode resonance to the higher mode resonances. For the given mode $m$, the electron density can be determined simply by the frequency difference between the resonance frequency $f_{pm}$ in plasma and that $f_{vm}$ in vacuum. The measurable highest density is experimentally found to be given by the plasma cutoff density corresponding to fvm. This is because the u-shape antenna is not efficiently exited in overdense plasma. On the other hand, the lowest electron density depends on the resonance broadening mainly caused by electron- neutral collisions. Use of the higher mode resonance expands a range of measureable electron density by an order of magnitude or more. Examples of the electron density measurement of surface wave plasma with use of the higher mode are presented.