A method which makes it is possible to measure the electric field strength in microwave discharges is presented. A condition for this method is that the plasma has such a low conductivity that the associated skin depth is larger than the discharge radius. It is found that the field strength in high-pressure sulphur lamps is around 400 V . Furthermore, this method allows the determination of the power absorbed in the resonator's wall and in the plasma and the estimation of the effective electric conductivity in the discharge.
van Dongen, M., Körber, A., Heijden, van der, H. W. P., Jonkers, J., Scholl, R., & Mullen, van der, J. J. A. M. (1998). Field strengths and dissipated powers in microwave-excited high-pressure sulphur discharges. Journal of Physics D: Applied Physics, 31(21), 3095-3101. https://doi.org/10.1088/0022-3727/31/21/015