Influence of a negatively biased grid on the plasma in a single-ended Q-machine

An experiment on a ion space charge instability is performed using a single-ended Q-machine. A negatively biased grid is inserted between the hot plate and the cold end-plate, and an ion space charge is observed between the grid and the cold plate when the latter is biased in order to reflect ions. The ions are reflected by the cold plate or the ion space charge and partly penetrate the grid back into the plasma. Under some boundary conditions specified by the grid, the cold plate potential and the plasma density, the potential distribution between the grid and the cold plate becomes unstable and the ion space charge instability is observed. The oscillation frequency is given by the ion transit time through the grid from the plasma region into the space charge region.