The effect of power balance on the heating mode transition in micro-dielectric barrier helium glow discharges

A particle-in-cell simulation has been adapted to analyze the helium discharge characteristics of a planar micro-dielectric barrier discharge with a gap distance less than 100 mu m for variation of driving frequency from 10 to 500 MHz. Two scaling laws for the breakdown voltage were obtained with the change in heating mechanism from the gamma-to the alpha-mode with increasing frequency. The optimal condition for the efficient generation of high-density plasmas with minimized power is obtained when the ratio of ion transit time to the RF period is about 1/4, so that the secondary electrons emitted by ion bombardment on the dielectric-gap boundary are accelerated in phase with the sheath potential. Under this condition, the plasma density as well as the ion current is maximum, the displacement current and the electron current are almost the same, and the ratio of the power delivered to the plasma is 50% of the total power. At high frequencies, the collisional electron heating increases by the induced bulk electric field, enhancing excitation and elastic collisions, and thus ionization efficiency decreases while excitation efficiency increases.