Motion of Finite-Sized Low-Density Photoion Bunch in Electrostatic Potential Wells

A finite-sized low-density photoplasma is produced by a two-step resonant photoionization method. Its density is varied in the range of ~(10⁷-10⁹) cm^-3. The motion of photoplasma is studied in a linear electrostatic potential well that is created by plate-grid-plate geometry. To understand its dynamics, a 1-D particle-in-cell model has been developed. For density range ~(1 × 10⁷-5 × 10⁸) cm^-3, an electric field ≤100 V/cm is sufficient to remove all the electrons from the photoplasma within a time of few nanoseconds, leaving behind an ion bunch. When a photoion bunch evolves in a potential well, a damped oscillation is observed on the current signal recorded on a grid electrode. The structure is explained by single-particle behavior of the photoion bunch. For densities>; 3 × 10⁸ cm^-3, the oscillation frequency depends on both externally applied electric field and internal field that is produced by space-charge interactions among charge particles. This is because, at higher densities, collective behavior dominates and the dynamics is governed by space-charge interactions.