RESONANT COUPLING BETWEEN ION BOUNCE IN A POTENTIAL WELL AND THE POTENTIAL RELAXATION INSTABILITY

When in a double plasma machine (DP-machine) plasma is produced solely in the source chamber, not only ions but also electrons can leak thro ugh the separating grid into the target chamber, so that a low-density plasma forms there. The electrons are trapped by the traveling ion sp ace charge and can thereby overcome the strongly negative grid bias. T he investigations presented here show that a positive space-charge for ms behind the grid in the target chamber and a deep potential well is formed around the grid. When the anode of the target chamber is biased positively, under certain conditions a low-frequency instability is o bserved in the target chamber, the properties of which indicate a pote ntial relaxation oscillation, somewhat similar to the potential relaxa tion instability in a quiescent plasma machine (Q machine). The freque ncy of the instability is determined by the ion transit time through a thin layer of the target chamber plasma. In addition, resonant coupli ng occurs between this frequency and the bounce frequency of ions in t he potential well around the grid.