Study of X-rays/ Neutrons/ Ion beam emitted from Mather-type Plasma Focus

Abstract

The low energy (2.3kJ) plasma focus energized by a single 32 µF capacitor charged at 12 kV with filling gases hydrogen, neon and argon is investigated as an X-ray source. Experiments are conducted with copper and aluminium anode. Specifically, the attention is paid to tailor the radiation in different windows, 1.2-1.3 keV, 1.3-1.5 keV, 2.5-5 keV and Cu-Ka line radiation. The highest X-ray emission is observed with neon filling and copper anode in the 1.2-1.3 keV window, and speculated to be generated due to recombination of hydrogen like neon ions with few eV to few tens of eV electrons. The wall plug efficiency of the device is found to be 4%. The other significant emission occurs with hydrogen filling that exhibits wall plug efficiency of 1.7% for overall X-ray emission, and 0.35% for Cu- Kα line. The emission is dominated by the interaction of electrons in the current sheath with the anode tip. The emission with aluminum anode and hydrogen filling is up to 10 J that corresponds to wall plug efficiency of 0.4%. The X-ray emission with argon filling is less significant. These radiations are found suitable for backlighting in Al (1-1.56 keV) and Ti (2.9-4.96 keV) energy transmission bands. Correlation of neutron emission with plasma electron temperature in a low energy (2.3 kJ) plasma focus is investigated. To determine the plasma temperature by continuum X-ray analysis, cobalt is selected as the filter, which discriminates the line radiation from the background impurities like carbon, nitrogen and oxygen, or the copper of which plasma focus electrodes are made. For pressure range of high neutron emission (1-4 mbar), the neutron yield is found correlated with the plasma temperature. The highest temperature recorded is 5 keV at 2.5 mbar, the filling pressure 3for highest neutron emission in this device. The Ni-Co filter combination helps to estimate the Cu-Kα line radiation from the device, which is the highest at 0.5 mbar. The emitted energy is estimated to be 1.2 J in the 4π geometry, which corresponds to system efficiency of about 0.045%. The Cu- Kα is 90-95% of the total X-ray emission. A simple technique to record the fluence anisotropy of X-rays emitted from a source is presented. The simplicity of the technique and response curves of the photographic film along with corresponding filters enables one to readily use the same for diagnostic purposes in different sources like plasma focus, vacuum spark, Z-pinch and laser produced plasmas. As an application example, the technique is employed to measure fluence anisotropy of X-ray emission in a low energy plasma focus operated with hydrogen. With increase in filling pressure, the anisotropy is found to increase, although the total X-ray emission is lowered. It is therefore concluded that at a lower filling pressure of 0.75 mbar, the X-ray emission is dominantly due to interaction of energetic electrons in the current sheath, whereas at a higher filling pressure of 2.5 mbar, the contribution of energetic electron beam is much higher. A study of soft X-ray emission in the 1.0 -1.5keV energy range from a low energy (1.15kJ) plasma focus has been performed. X-rays are detected with the combination of Quantrad Si PIN-diodes masked with Al (50µm), Mg (100µm) and Ni (17.5µm) filters and with a pinhole camera. The X-ray flux is found measurable within the pressure range of 0.1-1.0 mbar nitrogen. In the 1.0-1.3keV and 1.0-1.5keV windows, the X-ray yield in 4n-geometry is 1.03J and 14.0J respectively at a filling pressure of 0.25 mbar and the corresponding efficiencies are 0.04% and 1.22%. The total X-ray emission in 4π-geometry is 21.8J, which corresponds to the system efficiency of about 1.9%. The X-ray emission is found dominantly as a result of interaction of energetic electrons in the current sheath with the anode tip. Images recorded by the pinhole camera confirm the emission of X-rays from the tip of the anode. K-series line radiation emission of Mo and Cu is investigated also. Quantrad Si pin diodes are employed as time resolved X-ray detectors, where as a pinhole camera is used for time integrated analysis. The measurable X-ray flux for hydrogen is observed in the pressure range of 0.5- 3.5 mbar. Mo and Cu K-line radiation emission in this geometry has the highest values of about 0.05 J/sr and 0.17 J/sr respectively at a filling pressure of 2.0 mbar. The corresponding efficiencies are 0.03% and 0.09% respectively. Total X-ray emission and efficiency in 4π-geometry is also obtained with values 4.12 J and 0.18% at 2.0 mbar. The Mo K-line radiation emission may result due to interaction of energetic electron beam emitted from the focus region with the molybdenum insert