Stuart Victor Springham

A 3 kJ plasma focus was operated with a 3He-D2 gas mixture, with partial pressures in the ratio of 2:1, corresponding to an atomic number ratio of 1:1 for 3He and D atoms. The fusion reactions D(3He,p)4He and D(d,p)3H were measured simultaneously using CR-39 polymer nuclear track detectors placed inside a pinhole camera positioned on the forward plasma focus axis. A sandwich arrangement of two 1000 μm thick CR-39 detectors enabled the simultaneous registration of two groups of protons with approximate energies of 16 MeV and 3 MeV arising from the D(3He,p)4He and D(d,p)3H reactions, respectively. Radial track density distributions were obtained from each CR-39 detector and per-shot average distributions were calculated for the two groups of protons. It is found that the D(3He,p)4He and D(d,p)3H proton yields are of similar magnitude. Comparing the experimental distributions with results from a Monte Carlo simulation, it was deduced that the D(3He,p)4He fusion is concentrated close to the plasma focus pinch column, while the D(d,p)3H fusion occurs relatively far from the pinch. The relative absence of D(d,p)3H fusion in the pinch is one significant reason for concluding that the D(3He,p)4He fusion occurring in the plasma focus pinch is not thermonuclear in origin. It is argued that the bulk of the D(3He,p)4He fusion is due to energetic 3He2+ ions incident on a deuterium target. Possible explanations for differing spatial distributions of D(3He,p)4He and D(d,p)3H fusion in the plasma focus are discussed.

The NX2 device, a low energy plasma focus, at the Nanyang Technological University in Singapore, was used as a soft X-ray (SXR) source for micromachining. The gas used was neon which produced SXRs in a narrow spectral range of 0.9 - 1.6 keV. The SXR yield from repetitive operation of the NX2 device was monitored and measured using a cost effective multi-channel SXR spectrometric system. The system consists of filtered BPX65 PIN diodes, with the associated electronics --- an integrator, sample and peak holder, analogue switch, an A/D converter and a microcontroller. The system enables easy shot-to-shot statistical analysis under repetitive operation at adjustable preset trigger frequencies. A total of 4000 shots were fired at 0.5 Hz, using the same gas filling. The SXR production was at an average yield of 60 J/shot and a maximum single-shot yield of more than 100 J. The SXRs emitted by the NX2 device was used for contact micromachining, producing structures with an excellent aspect ratio of up to 20:1 on 25 μm SU-8 resist.

A study of the body of experimental results as well as modelling indicates that all plasma focus machines big and small operate with the same energy density both in the axial and radial phases related to an almost constant current per unit radius for all machines. This results in operational speeds and compressed plasma temperatures which are the same for all machines. It is proposed that the existing network of 3 kJ machines may be the most efficient and effective way to produce a comprehensive experimental picture of the plasma focus, apart from 'large machine' effects such as the neutron 'saturation' effect. Operational benchmarks are established for these 3 kJ machines for the purpose of standardisation to increase the effectiveness of comparative studies using different measurements obtained from different, though nearly identical machines.

A modified Problem Based Learning (PBL) approach has been implemented and evaluated for the laboratory sessions of the Thermal Physics first year undergraduate module in academic year 99/00. The principal objective of this PBL approach is to get away from the 'cookbook' approach to teaching laboratories, and instead transform them into micro-research projects which demand higher order cognitive involvement of the students. This modified PBL approach involved conducting two hour laboratory sessions for each experiment: one hour from each of two consecutive weekly classes. The first week's session was rounded off with a question and answer session aimed at promoting higher order cognitive involvement by the students in the experimental work. The students conducted and discussed the experiments in groups of 3 to 4, while I acted as facilitator. One observation is that the weaker students in the class are uncomfortable with the lack of a detailed experimental demonstration. Comparing the student reports for academic year 99/00 (PBL approach) with those from 97/98 (conventional approach), indicates that student's have higher cognitive understanding of the experiments enabling them to answer questions more competently, and their integration and retention of knowledge is enhanced. Moreover the students were more involved and devoted more energy and enthusiasm to the experimental work when the modified PBL approach was employed in comparison to the more conventional approach to laboratory work.

There are several definitions of Problem Based Learning (PBL). For example, those of the Basudur Simplex Model, Kaufman and Swartz. The common features are: 1) Find and define the problem; 2) Examine facts and possibilities; 3) Consider alternative solutions; 4) Implement the best solution and 5) Problems should be related to the “real world”. However, in the natural sciences and mathematics, one often proceeds from “real world” problems to the conceptualisation of the abstract. Conceptualisation of the abstract is one of the tenets of the natural sciences and mathematics. Perhaps it is required less in the biological sciences, but it is increasingly required in physics and almost entirely in mathematics. The usual definitions of PBL have to be adapted to take into account the fact that conceptualisation of the abstract, rather than solving “real world” problems, is the end-product of many problems in the scientific disciplines. We give examples and counter-examples of the applicability of PBL integrated with information technology in our disciplines.

The anisotropies of neutron and high energy deuteron emissions from the NX2 plasma focus device [M. V. Roshan et al., Phys. Lett. A373, 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.

The coded aperture imaging (CAI) technique has been used to investigate the spatial distribution of DD fusion in a 1.6 kJ plasma focus (PF) device operated in, alternatively, pure deuterium or deuterium-krypton admixture. The coded mask pattern is based on a singer cyclic difference set with 25% open fraction and positioned close to 90° to the plasma focus axis, with CR-39 detectors used to register tracks of protons from the D(d, p)T reaction. Comparing the coded aperture imagingprotonimages for pure D2 and D2-Kr admixture operation reveals clear differences in size, density, and shape between the fusion sources for these two cases.

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. A373, 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 C 12 (d,n)N 13 , which has a deuteron threshold energy of 328 keV.

Yttrium is presented as an absolute neutron detector for pulsed neutron sources. It has high sensitivity for detecting fast neutrons. Yttrium has the property of generating a monoenergetic secondary radiation in the form of a 909 keV gamma-ray caused by inelastic neutron interaction. It was calibrated numerically using MCNPX and does not need periodic recalibration. The total yttrium efficiency for detecting 2.45 MeV neutrons was determined to be fn~4.1x10−4 with an uncertainty of about 0.27%. The yttrium detector was employed in the NX2 plasma focus experiments and showed the neutron yield of the order of 108 neutrons per discharge.