Plasma density measurement in an imperfect microwave cavity

The density of a plasma generated in a TM010/2.45 GHz microwave cavity has been measured from the shift of the TM110 mode excited at a higher frequency. Since the design was aimed at obtaining optimum power coupling, the cavity has several openings and is imperfect from the viewpoint of the density measurement. A technique based on the dipole resonance behavior of cavity modes with nonzero axial mode number has been utilized to calibrate the observed TM110 mode frequency shift, which is found to differ considerably from the ideal cavity formula. A new theoretical formula which accounts for the particular form of cavity imperfection used in the experiment, has been derived and verified. It is also shown that the cavity shift method is conveniently applicable to pulsed microwave discharges.