The first and the final fifty nanoseconds of a fast focus discharge

Conventional plasma focus devices of different capacitor bank energy (kilojoules to megajoules) show very similar discharge behavior with respect to breakdown, compression, and pinch data. The use of SPEED 1—a plasma focus driven by a fast high impedance bank (voltage 200 kV, initial current rise >5×1012 A/sec, bank impedance 160 mΩ),—allowed the investigation of a 400 nsec discharge in a parameter range not previously accessible. Considerable improvement of current and neutron efficiency (30 A/J; 106/J) has been achieved as compared with values (≤20 A/J; 2×105/J) of conventional slow devices of equal energy (20 kJ). Exceptional compression dynamics (current derivative spike >1013 A/sec FWHM 10 nsec) is observed from electric signals. This is due to the current stabilizing effect of the high bank impedance and especially to improved current sheath structure (reduced thickness, enhanced conductivity). Current density and conductivity of the sheath depend on ignition parameters at breakdown (voltage, current rise), the low values of which are possibly responsible for the similarity of the discharge behavior of conventional devices. Conclusions on further improvements are drawn.