Description of a surface-wave-produced microwave discharge in helium at low pressure in the presence of a gas flow

The characterisation of surface-wave-produced discharges is extended to the case where there is a neutral density gradient along the discharge. Experimental results for a low-pressure helium discharge at 2.45 GHz with a gas flow are presented. In order to model the electron density along the discharge, a law giving the variation of the mean power theta needed to maintain an electron with the neutral density is experimentally established. A good agreement is obtained between the measured and calculated electron densities. Furthermore the variations of theta are explained by a simple energy balance model.