Lee, Paul Choon Keat

A phenomenographic study of Singapore students' understanding of physics was undertaken to compare results to a previous study by Australian researchers. The focus was on simple mechanics concepts related to displacement, velocity and frames of reference study. The research found many similarities but some differences between the two groups of students. There are several implications for successful physics learning and teaching in Singapore.

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.

"The main purpose of this project is to develop, implement and evaluate a guided-inquiry curriculum using the Physics by Inquiry (PbI) instructional approach. We wanted to investigate the effect of the inquiry-based curriculum and instruction on students' conceptual development and attitude towards the learning of Science. The research questions guiding the project were: 1) What are the favourable modifications in adapting the PbI instructional approach for improving the learning of lower secondary Science in Singapore? 2) What are the differences in the learning outcomes when students learn Physics through the inquiry approach as compared to the standard classroom practices? 3) What are the perceptions of teachers and students on teaching and learning Physics by inquiry in the classroom?" -- p. 2.

This study reports the enhanced ferromagnetic ordering in ZnO:Mn nanoparticle thin films, grown at different substrate temperatures using pulsed laser deposition. The optimum growth conditions were deduced from X-ray, photoemission and magnetic measurements. The X-ray measurements reveal that there was an optimum substrate temperature where the thin films showed relatively stronger texture, better crystallinity and lower strain. Substrate temperature tuned the deep level recombination centers in ZnO:Mn, which changed the optical quality by altering the electronic structure. The M-H curves, in the present study, revealed superior ferromagnetic response of 20-nm sized particles in ZnO:Mn thin film grown at a substrate temperature of 450 °C. Ferromagnetic ordering becomes weaker at higher/lower substrate temperatures due to the activation of native defects in ZnO host matrix.

The main aim of this 3-year project was to develop, implement and evaluate a guided-inquiry curriculum in a local secondary school using the Physics by Inquiry (PbI) “research–development–instruction” iterative approach. Our study showed that students' interest in Science is a key predictor of future aspiration to learn the subject and reinforces the need to shape positive attitudes towards Science, especially at the lower secondary level. Overall, students and teachers held favourable perceptions regarding the inquiry-based curricula and instruction, citing hands-on activities, self-directed learning, learning from peers, and opportunity for consolidation as important features in engaging them in learning. Students' conceptual understanding and reasoning abilities were also enhanced after the intervention. These findings suggest that the curriculum materials have been effective in providing structured guidance to students to promote their learning of Science through evidence-based reasoning, problem solving and argumentation.

The dense plasma focus (DPF) device is a coaxial plasma gun that uses a large electric current to heat and compress a gas to high temperatures (1-2 keV), densities (1025-26 m-3) and pressures (thousands of atmospheres). Under such extreme conditions, the gas radiates copious ultraviolet, X-rays and particle beams such as relativistic electrons and ion beams. At Plasma Radiation Sources Laboratory (PRSL), NIE, Singapore, our group has six plasma focus devices and our research efforts encompass a very wide range of topics covering various fundamental aspects of plasmas to the application of this device to lithography, soft and hard x-ray imaging, material modification and thin film deposition. This review paper reports the use of single shot and “repetitive” PF device for processing and deposition of thin films using plasma focus devices. To synthesize the magnetic thin films, the conventional hollow copper anode was substituted with an anode fitted with suitable material tip (FeCo or CoPt). Si wafer and copper mesh were placed axial down the anode axis at a suitable distance of about 25 cm above the anode top to improve the uniformity of deposited samples over bigger substrate size. The plasma focus device is operated at 1 Hz repetition rate at various combinations of charging voltage and filling pressure of hydrogen gas for different number of focus deposition shots. For the processing of thin films, the magnetic thin films of FePt were initially deposited using pulsed laser deposition and later exposed to energetic ions from hydrogen operated plasma focus device. The morphology, structure and magnetic properties of the synthesized and processed thin films are investigated using TEM, SEM, XRD and VSM, respectively. The paper will also discuss the fundamental of thin film deposition and irradiation mechanisms in plasma focus devices.

The deuterium-deuterium (DD) fusion neutron yield and anisotropy were measured on a shot-to-shot basis for the NX2 plasma focus (PF) device using two beryllium fast-neutron activation detectors at 0° and 90° to the PF axis. Measurements were performed for deuterium gas pressures in the range 6–16 mbar, and positive correlations between neutron yield and anisotropy were observed at all pressures. Subsequently, at one deuterium gas pressure (13 mbar), the contribution to the fusion yield produced by the forwardly-directed D+ ion beam, emitted from the plasma pinch, was investigated by using a circular Pyrex plate to obstruct the beam and suppress its fusion contribution. Neutron measurements were performed with the obstacle positioned at two distances from the anode tip, and also without the obstacle. It was found that ~ 80% of the neutron yield originates in the plasma pinch column and just above that. In addition, proton pinhole imaging was performed from the 0° and 90° directions to the pinch. The obtained proton images are consistent with the conclusion that DD fusion is concentrated (~ 80%) in the pinch column region.

The effect of laser energy fluence on the onset and growth of Rayleigh–Taylor (RT) instabilities in laser induced Fe plasma is investigated using time-resolved fast gated imaging. The snow plow and shock wave models are fitted to the experimental results and used to estimate the ablation parameters and the density of gas atoms that interact with the ablated species. It is observed that RT instability develops during the interface deceleration stage and grows for a considerable time for higher laser energy fluence. The effects of RT instabilities formation on the surface topography of the Fe thin films grown in pulsed laser deposition system are investigated (i) using different laser energy fluences for the same wavelength of laser radiation and (ii) using different laser wavelengths keeping the energy fluence fixed. It is concluded that the deposition achieved under turbulent condition leads to less smooth deposition surfaces with bigger sized particle agglomerates or network.