Microwave Cavity Resonance Spectroscopy

The two most important parameters in low-temperature plasmas are the electron density and the electron temperature (or energy distribution). However, both are often difficult to measure, especially in weakly ionized plasmas with electron densities below 1016 m-3. In recent years we have started to use a technique that is able to measure such densities. This method is Microwave Cavity Resonance Spectroscopy, MCRS.

MCRS uses a cookie-box shaped microwave cavity which encloses the plasma under investigation. This cavity contains two antennas. On one of the antennas a microwave signal is applied which frequency is swept over a large range. The second antenna is used to sample the microwave signal. The received intensity will now peak at the resonance frequencies of the cavity. These resonance frequencies depend on the geometry of the cavity and on the dielectric constant of its contents. When we now apply a plasma inside this cavity, the free electrons will modify this dielectric constant and thereby shift the resonance frequencies. This allows us to easily determine the space-averaged electron density in the cavity. With some extra tricks we can also estimate the electron temperature in the cavity.

One plasma that we use this technique on is the EUV-induced plasma discussed here.