Cylindrical capacitor and its application to measurements on cylindrical gas discharges

The measurement of high electron densities within a plasma is often conveniently performed using microwave techniques, requiring a probing-signal frequency of the same order as the electron-plasma frequency. If the electron density is high, considerable cost and expertise are associated with this form of measurement. The device described is able to evaluate the electron density at a probing frequency much lower than the plasma frequency, and thus eases the measurement considerably. For example, a tube of 10 cm bore, whose wall is 2 mm thick and is composed of material whose relative permittivity is 6, will enable electron densities at plasma frequencies of 12.5 GHz to be checked using a probing signal of only 1 GHz.