Cavity perturbation techniques have been around since the birth of microwave engineering. Cavity perturbation refers to the shift in the resonant conditions of a microwave cavity caused by the presence of a load within the cavity. By precisely measuring these shifts, one may determine certain characteristics of the load. Cavity perturbation has become a diagnostic widely applied to plasma discharges, because the technique quickly provides measurements of the electron density and collision frequency of the discharge. However, in its most fundamental form, cavity perturbation is applicable only to a limited range of electron densities (
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