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Unexpected Post-Pulse Secondary Ionization in Nanosecond Pulsed Discharges

In many plasma applications it is critical that the plasma electron density is changed as fast as possible. In collision driven ionization this typically means maximizing the applied electric field. We perform PIC simulations1 of Argon plasma near 1 Torr on nanosecond time scales. Unexpected secondary ionization is found to occur in pulsed argon discharges subsequent to voltage pulse termination between planar electrodes. The secondary ionization, which occurs at the falling edge of the voltage pulse, is induced by charge separation in the bulk plasma region. This process occurs on nanosecond time scales and is driven by a reverse in the electric field from the cathode sheath to the formerly driven anode. Under the influence of the reverse electric field, electrons in the bulk plasma and sheath regions are accelerated toward the cathode. This electron movement manifests itself as a strong electron current generating high electron energies with significant electron dissipated power. Accelerated electrons collide with Ar molecules and an increased ionization rate is achieved even though the driving voltage is no longer applied. The secondary ionization phenomena bears some resemblance to secondary discharges observed in dielectric barrier discharge (DBD) experiments2. However, in those cases, the energy reservoir are the polarized electrodes and not the bulk plasma itself. The results suggest that more elaborate pulse designs can be used to condition plasma density and temperature as required for fast discharge applications.