QUENCHING OF THE POTENTIAL RELAXATION INSTABILITY IN A SINGLE-ENDED Q-MACHINE PLASMA WITH NEGATIVE C60 IONS

A number of low frequency instabilities are driven by electron currents. Due to their low mass and therefore high mobility, electrons react about two orders of magnitude faster on potential changes than ions. A strong electron current along the magnetic field of a Q-machine gives rise to large amplitude relaxation oscillations of the space potential and the current. These oscillations are associated with a moving double layer and are usually referred to as the potential relaxation instability (PRI) [1]. Although this kind of instability was observed in a Qmachine for the first time, PRI-like oscillations can be observed in low density current carrying bounded plasma systems. Here we present the results of an investigation on the influence of heavy − 60 C ions on the PRI. The fullerene C60 has a large cross section for electron attachment at thermal energies [2]. The electron temperature in a Q-machine is about 0.2 eV. Thus, by introducing C60 vapour into the plasma column a significant part of the plasma electrons can be replaced by − 60 C ions [3]. Since an essential feature of the mechanism that drives the PRI is the loss of negative charge carriers by the positive voltage of the CP and the inertia of the positive charge carriers, the instability will be strongly influenced by the − 60 C ions. The loss of negative charge carriers due to the positive voltage of the cold plate (CP) will decrease because of the low mobility of the − 60 C ions, although the quasineutrality of the unperturbed C60 plasma is still valid and reads: n+ = n– + ne, where n+ is the density of the potassium ions and n– is the density of the − 60 C ions.