Glow discharge around piezoelectric transformer operated under higher-order vibration modes

We have been developing compact plasma reactors using a piezoelectric transformer (PT). The plasma reactors were operated by three fundamental vibration modes, (1/2)λ,λ, and (3/2)λ-modes according to the general usage of the PT. However, through our experiments, we found that glow discharge appears while driving the PT at higher frequencies than the fundamental vibration modes, which are abnormal for the operations of PT. In such higher-order operations, the glow discharge behaves in diverse and unique profiles. This paper presents some images of low-pressure glow discharge produced by the PT driven in higher-order vibration modes.