Lee, Paul Choon Keat

In this paper, a novel pulsed broad energy spectrum ion-implantation technique, using the dense plasma focus device (DPF), for uniform oxygen-ion doping along the thickness of a ~250 nm thick magnetic CoFeTaZr layer is investigated. A new operational regime of the dense plasma focus – the off-focus mode – is explored to avoid the surface damage of the exposed sample by the high energy plasma streams/jets and instability accelerated ions, typically observed in conventional efficient-focus mode operation. The faraday cup measurements shows the increase in ion fluence from 3.83 × 10¹³ ion/cm² for efficient-focus mode to 8.76 × 10¹³ ion/cm² for off-focused mode operation in the broad-ion-energy range of 1–100 keV. The x-ray photoelectron spectroscopy (XPS) of the unexposed sample shows the presence of Co in Co⁰, Co²⁺ and Co³⁺, Fe in Fe⁰, Fe²⁺ and Fe³⁺, and Ta in Ta⁰ and Ta²⁺ oxidation states while Zr was observed with only metallic Zr binding energy peaks indicating the surface oxidation of the unexposed sample. The exposure to oxygen plasma in DPF device led to the increase in the higher oxidation states of Co, Fe and Ta with reduction in metallic binding energy peak and the deconvolution of oxygen XPS spectrum confirmed the bonding of oxygen to Co, Fe and Ta. The magnetization dynamics of unexposed and oxygen-ion doped samples was studied using magnetoimpedance measurements in the 1–2.5 GHz frequency range. Gilbert’s damping factor, in-plane anisotropy and effective magnetization of the magnetic substrate were calculated and it is found that these properties can be modulated with a lighter ion dosage using this novel pulsed broad-energy-ion implantation technique. It is concluded that the off-focus mode DPF operation can provide the ions of required energy and fluence to implant oxygen ions across the thickness of the CoFeTaZr magnetic thin film to modulate its magnetic properties.

Laser shadowgraphy has been used to investigate the plasma sheath dynamics in a miniature plasma focus device (FMPF-3, 14 kV/235 J). The occurrence of magneto-hydro-dynamics instabilities are compared for pure deuterium versus deuterium–krypton admixture operation, over the range of gas pressures 2–12 mbar. A cathode-less geometry was also tested to study the influence of cathode configuration on current sheath formation and compression. The average neutron yield, measured using 3He proportional counters, is compared for different geometries and gas pressures. The synchronization of the four pseudo-spark-gap switches was found to be a major factor influencing the plasma sheath dynamics and neutron yield. To make a fair comparison of operation with different gas pressures or admixture proportions, the level of switch synchronization must be in the same range. Laser shadowgraphs of early stage dynamics show that poorly synchronized discharges result in asymmetric plasma sheath formation, and asymmetries in the accelerated sheath typically persist till the end of the final compression.

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.

A time resolved imaging study of pulsed laser ablated Fe and Al plasma plumes with specific interest in the splitting of plumes into the slow and fast moving components as they expand through the background argon gas at different pressures is reported. The material ablation was achieved using a Q-switched Nd:YAG yttrium aluminum garnet laser operating at 532 nm with a pulse duration of 8 ns full width at half maximum and a fluence of 30 Jcm−2 at the target surface. Typical time resolved images with low magnification show that the splitting occurs at moderate background gas pressures 0.5 and 1.0 mbar for Fe, and 0.2 mbar for Al plasma plumes. The plume splitting did not occur for higher background gas pressures.

This paper summarizes a Photomultiplier-Scintillator diagnostic system for use in our plasma focus experiments at the Center for Plasma Research INTI IU. The system features an anode-grounded high pulse linearity voltage divider and uses NE102A plastic scintillators. It has detected D-D neutrons in INTI plasma focus device with clear and high signal to noise ratio. Neutron TOF of 120 ns has been measured from the time difference between hard x-ray pulse peak and neutron peak time over a flight path of 2.6±0.01 m; giving energy of 2.5±0.1 MeV for these side-on neutrons.

The Coded Aperture Imaging (CAI) technique has been applied with CR-39 nuclear track detectors to image alpha particle source spatial distributions. The experimental setup comprised: a 226Ra source of alpha particles, a laser-machined CAI mask, and CR-39 detectors, arranged inside a vacuum enclosure. Three different alpha particle source shapes were synthesized by using a linear translator to move the 226Ra source within the vacuum enclosure. The coded mask pattern used is based on a Singer Cyclic Difference Set, with 400 pixels and 57 open square holes (representing ρ = 1/7 = 14.3% open fraction). After etching of the CR-39 detectors, the area, circularity, mean optical density and positions of all candidate tracks were measured by an automated scanning system. Appropriate criteria were used to select alpha particle tracks, and a decoding algorithm applied to the (x, y) data produced the de-coded image of the source. Signal to Noise Ratio (SNR) values obtained for alpha particle CAI images were found to be substantially better than those for corresponding pinhole images, although the CAI-SNR values were below the predictions of theoretical formulae. Monte Carlo simulations of CAI and pinhole imaging were performed in order to validate the theoretical SNR formulae and also our CAI decoding algorithm. There was found to be good agreement between the theoretical formulae and SNR values obtained from simulations. Possible reasons for the lower SNR obtained for the experimental CAI study are discussed.

Tungsten is one of the prime candidates for a first-wall material near the divertor area due to its high temperature strength, high thermal conductivity, low erosion rate and low tritium retention. The erosion resistance of tungsten to the edge plasmas and transient events are carefully investigated in a simulated fusion environment. Here, we use the dense plasma focus (DPF) device operated in a D₂ as a source for pulsed fusion plasma. The tungsten (α-W) substrates with a preferential growth direction along (110) plane were used. These pristine-W samples were nanostructurized using a (i) low-temperature continuous nitrogen RF plasma system and (ii) coated with 60 nm tungsten film, using high-temperature argon plasma in a dense plasma focus (DPF) device. The low- temperature plasma treatment created mesh-like porous nanostructure on the surface of pristine-W with change in crystalline orientation to (200), while the DPF-based deposition resulted in a nanocrystal (30–50 nm) decorated surface with enhanced (200) orientation. The crack propagation and bubble formation during DPF D2 plasma exposure were significantly controlled by the surface modification of tungsten. The mesh-like structure was modified to form loosely bound spherical nanoparticles, while the nanocrystals remained tightly bound and grew in size with D₂ plasma exposure. The better adhesion of the nanocrystals and controlled growth along the (200) direction resulted in least change in hardness measurements for the nanocrystal decorated samples. Thus, nanocrystal decoration of tungsten with a preferential growth direction of (200) can help reduce the fusion-induced damage in first-wall materials.