Instability of the sheath-plasma resonance

An electrode with a positive dc bias inserted into a Maxwellian plasma is observed to generate coherent high-frequency oscillations at the sheath-plasma antiresonance frequency (ω≲${\omega }_{\mathrm{pe}}$). The phenomenon is explained by a negative resistance associated with the finite electron transit time through the sheath. Harmonics of the fundamental are radiated as electromagnetic waves but are clearly shown not to be caused by electron orbital instabilities.