EFFECT OF INSULATOR SLEEVE CONTAMINATION ON THE LOW-ENERGY PLASMA-FOCUS PERFORMANCE

The characteristics of a low energy plasma focus of copper electrodes operated by a single 32 muF, 15 kV (3.6 kJ) capacitor with an insulato r sleeve contamination are studied. When the plasma focus is operated, the insulator sleeve is contaminated due to the deposition of copper evaporated from the electrodes, A slight contamination improves the sy stem performance When the cumulative discharge energy over successive shots across the insulator sleeve exceeds approximately 1 MJ, the copp er deposition on the sleeve surface makes it rough with a grain-type s tructure, with as result that the system becomes less reproducible and shot-to-shot variations in neutron yield are pronounced. In addition, a high voltage probe records multiple foci formation, giving rise to multiple neutron pulses, multiple X-ray pulses as well as multiple ion beams. When the cumulative discharge energy approaches 1.6 MJ, the ne utron yield starts deteriorating, and the resistor divider signal begi ns to indicate less compression. It is suggested that the neutron yiel d degradation occurs due to copper coating with grain structure on the sleeve surface, which decreases the resistance of the sleeve surface and may therefore increase the current partition and eventually lower the snowplow efficiency. The situation may improve if low-sputtering-r ate conductors are employed for the electrodes of the device or the tr uncated end of the anode is lined with low sputtering rate material li ke molybdenum.