Experimental and numerical investigation of nonlinear effects in a dissipative alkali plasma diode

A strong electron current in a dissipative bounded plasma system is able to induce instabilities such as the potential relaxation instability (PRI). The dependence of the PRI on the ratio of the ion to the electron density is investigated experimentally and numerically. The experiments were performed at a single-ended Q-machine, and the numerical investigations were carried out with the one-dimensional PIC code XPDP1 and the two-dimensional PIC code XOOPIC. In the one-dimensional case we have investigated the dependence of the PRI on the so-called neutralization parameter. We have observed that electron-rich emission conditions at the HP, i.e. small neutralization parameter, causes higher harmonic oscillations of the external current. The one-dimensional particle simulation shows also a period doubling of the external current. The simulation results make known that a critical depth of a space charge sheath in front of the HP is responsible for these effects. In the two-dimensional case we have investigated the influence of a constant radial electric field to a plasma column, by biasing a cylindrical wall which surrounds the plasma. Electron-rich conditions, i.e. a negatively biased cylinder, induce higher harmonic oscillations of the external current. The oscillations are caused at critical values of the cylinder bias. Again a critical depth of a space charge sheath is important for the induced oscillations. The two-dimensional simulation clarifies the principle influence of a radial electric field in a cylinder-symmetrical plasma diode. (author)