Lee, Paul Choon Keat

The total current I_total waveform in a plasma focus discharge is the most commonly measured quantity, contrasting with the difficult measurement of I_pinch. However, yield laws should be scaled to focus pinch current I_pinch rather than the peak I_total. This paper describes how I_pinch may be computed from the I_total trace by fitting a computed current trace to the measured current trace using the Lee model. The method is applied to an experiment in which both the I_total trace and the plasma sheath current trace were measured. The result shows good agreement between the values of computed and measured I_pinch.

High energy neutrons, more than 2.45 MeV from deuteron-deuteron fusion reaction, have been measured in backward direction of plasma focus devices in many laboratories. However the experimental evidence for high energy deuterons responsible for such neutrons has not been reported so far. In this brief communication, backward high energy deuteron beam from NX2 plasma focus [M. V. Roshan et al., Phys. Lett. A 373, 851 (2009)] is reported, which was measured with a direct and unambiguous technique of nuclear activation. The relevant nuclear reaction for the target activation is ¹²C (d, n)¹³N , which has a deuteron threshold energy of 328 keV.

We investigated the emf (V) induced with time (t) in a search coil due to an oscillating bar magnet along the axis of the coil using the IOLab device. Experimental data was simultaneously collected with the force sensor which tracked the oscillation of the magnet and the voltage sensor which gave real-time readings of the induced emf in the coil. For small amplitudes of oscillation, where the predominant interaction involved the top end of the magnet cutting through the plane of the coil, the V–t graph appeared sinusoidal. With increasing amplitudes of oscillation, the V–t graph started to lose its symmetry with an initial kink appearing within one half cycle and which eventually became an increasing hump as the amplitude of oscillation increased further due to the influence from the bottom end of the bar magnet which affected the overall rate of change of magnetic flux linkage through the coil. The data collected from the experiment was visualised using a novel parametric series of plots of induced emf (V) with force (F). We discussed various cases of the V–F parametric plot. Using a suitable Φ–h function and taking into consideration the rate of change of magnetic flux linkage with respect to displacement along the coil, we discussed how the emf induced changes with time with increasing amplitudes of oscillation.

The anisotropies of neutron and high energy deuteron emissions from the NX2 plasma focus device [M. V. Roshan et al., Phys. Lett. A 373, 851 (2009)] are studied. The nuclear activation of graphite targets is used to measure the fluences of high energy deuterons in the axial and radial directions. Two bismuth germanate scintillation detectors connected to multichannel analyzer systems are used for the detection of 511 keV gamma rays resulting from positron annihilation in the two targets. In addition, fast neutron activation detectors are employed to measure the axial and radial fluences of fusion neutrons. These detection systems are calibrated using the simulation code _MCNPX [L. S. Waters et al., AIP Conf. Proc. 896, 81 (2007)]. Two distinct regimes of neutron and deuteron anisotropies are observed for the NX2 device. For deuterium gas pressures below 10 mbar, the neutronanisotropy increases with increasing pressure, while the overall neutron yield remains low. For gas pressures of 10–14 mbar, the neutronanisotropy is essentially constant, while, with increasing pressure, the neutron yield rises rapidly and the deuteron anisotropy falls.

This work aims to test the concept of an activation detector for pulsed DD fusion neutron sources, based on the production of metastable ^79m within a LaBr₃(Ce) scintillator crystal via (n, n') inelastic scattering. The pulsed neutron source employed is the NX3 Plasma Focus (PF) device operated in deuterium gas, which yields about 10⁹ neutrons per shot. A range of D₂ gas pressures, from 1 to 13 mbar are used to vary the test conditions. For the sake of comparison, a beryllium fast-neutron activation detector is used simultaneously with the LaBr₃(Ce), and for each NX3 PF shot we derive neutron yield values from both Be and LaBr₃(Ce) detectors, denoted Y_n^Be and Y_n^LaBr. The two detectors are positioned in the equatorial plane (θ=90˚) of the NX3 to expose them to bursts of neutrons with energies close to 2.5 MeV, to simulate a thermonuclear DD fusion source. Overall, the shot-to-shot values of Y_n^Be and Y_n^LaBr obtained compare reasonably well. At each D₂ gas pressures the 10-shot averaged values〈Y_n^Be〉 and 〈Y_n^LaBr〉are mostly within 10% of one another; for the worst case (10 mbar)〈Y_n^LaBr〉is 25% higher than〈Y_n^Be〉. Overall, it is concluded that LaBr₃(Ce) scintillation detectors can function as a capable and readily obtainable fast-neutron activation detector for measuring neutron yields from pulsed DD fusion sources.