INVESTIGATION AND OPTIMIZATION OF NEON SOFT X-RAY OF THE INTI PLASMA FOCUS AT 12 KV

Abstract

This thesis describes the theoretical and experimental investigations of the xray (specifically neon soft x-ray) properties and the plasma dynamics of the INTI Plasma Focus. The purpose of the research is two-fold, firstly to optimize the neon SXR yield of the INTI Plasma Focus (3 kJ, Mather type) machine; and secondly to investigate the correlation of the characteristic neon SXR signal pulse with the plasma focus dynamics.

The optimization of the SXR yield involved three steps. Firstly the optimization was carried out numerically using the 6 phase Lee Model Code (version RADPFV6.1b). The numerical experiments were accomplished at 12 kV to determine the optimum configuration for the electrodes using the INTI PF machine. The model parameters were determined and retained as fm = 0.05, fc = 0.7, fmr = 0.2 and fcr = 0.8. The pressure (P), anode length (z), and anode radius (a) were parametrically varied while the value of (b) was fixed at 3.2 cm. Secondly, the optimization experiments carried out at Centre for Plasma Research, INTI International University were guided by the numerical experiments with an optimized anode (a =1.2 cm and z =7.9 cm) designed as a screw-on type that retained the original radius until it just emerged out of the insulator sleeve. Thirdly, enhancement of yield was attempted by doping neon with krypton (Ne 97.5% + Kr 2.5%, Ne 95% + Kr 5%, Ne 90% + Kr 10% and Ne 80% + Kr 20%). Series of
experiments using the original anode (a =0.95 cm and z =16 cm) were performed in the laboratory at 12 kV and operating neon gas pressure of 2.0 Torr. These experimental results were verified using the Lee Model code by adjusting the pressure for each admixture. The adjustments considered volumetric proportions of neon taking into account the difference in atomic weights (Ne-20 and Kr-84).

Finally, the correlation of characteristic neon SXR pulse with the plasma focus dynamics was investigated using a) the original anode of 16 cm length and 0.95 cm radius in neon, b) the optimized anode of 7.3 cm length and 1.2 cm radius in neon and c) the different Ne-Kr admixtures. Using the Lee Model code, an excel template was developed to plot the SXR pulse with the fitted measured and computed current traces. The template also shows different time markers such as the radial phase, the reflected shock phase, the start and end of the pinch and the anomalous resistance phase.

Results show that there is good agreement between the measured and computed SXR yield for the original anode and the optimized anode operated in neon. The dimensionally optimized anode has an improved SXR yield by a factor of 1.5 when compared to the original anode.

The correlation study of the characteristic neon SXR pulse enabled the characterization of shots as normal (N-good shots) and not-normal (NN-bad shots). In all experiments (original anode, optimized anode and the Ne-Kr admixtures) the characteristic neon SXR pulse starts between the start of the reflected shock phase and the start of the pinch phase (usually 10 ns before the start of the pinch phase) for all N type or good shots. For all of the NN type or bad shots, the characteristic neon SXR pulse signal occurs either too early (before the start of the reflected shock phase) or too late (after the start of the pinch phase). All of the shots with neon doped with Kr up to 2.5% are good shots of N type. With increased doping, the quality of the shots was found to deteriorate, with most shots being NN type.