PARTICLE-IN-CELL SIMULATION OF GRID SHEATH OSCILLATIONS IN A DOUBLE-PLASMA DEVICE

The oscillation process around a strongly negative grid in a double-pl asma device is studied with one-dimensional particle-in-fell simulatio n, The code is an extension of PDP1, and its proper adaption to the pr esent situation is described. For asymmetric operation of the double-p lasma de,ice, the formation of ''virtual anode oscillations'' (VAO's) is observed, The scaling law f x n(1/2)U(g)(-alpha), which formerly wa s derived from the transit time through astatic Child-Langmuir sheath, is now recovered with alpha = 0.33 by self-consistent simulations, A group of ions, that is trapped in the sheath, leads to feedback stabil ization of the VAO. The simulations show that ion bunches that have ga ined energy in the VAO process are periodically injected into the sour ce plasma where they excite damped ion waves.