Paul Lee

• PhD
• Professor (Associate) at National Institute of Education

Tungsten (W) is considered as one of the promising candidates for plasma facing components in fusion reactors since it has high threshold energy, high melting point, low threshold shock resistance, and resistance against formation of co-deposits with tritium. Despite these commendable features, helium ions produced during fusion reaction are known...

This paper is a sequel to the 1998 review paper “Scientific status of the Dense Plasma Focus” with 16 authors belonging to 16 nations, whose initiative led to the establishment of the International Center for Dense Magnetized Plasmas (ICDMP) in the year 2000. Its focus is on understanding the principal defining characteristic features of the plasma...

The emerging paradigm of oxide-based devices are reshaping the frontiers of sustainable electronics and enabling new functionalities like ferroelectric-photovoltaics, photo-catalytic activity, ferroelectric logics, and terahertz-scale actuation. The intriguing advantages of BiFeO3 (BFO) based oxide electronics offers the possibility of combining el...

Zinc oxide (ZnO), a wide direct band gap (3.37 eV) II-VI semiconductor, is a fascinating technological material capable of exhibiting both semiconducting and piezoelectric characteristics and distinguished performance in photonics and optoelectronics. We report the synthesis of ZnO thin films composed of randomly oriented, 1-dimensional (1-D) and m...

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 t...

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...

Zirconium and beryllium fast-neutron activation detectors are used to investigate the fusion neutron emission from the NX3 Plasma Focus (PF) device operated in deuterium gas, with 7.2 kJ bank energy. Both Zr and Be activation cross-sections increase with energy, but with different trends, enabling an effective neutron energy Eneff to be inferred fr...

Imaging detectors that use X-ray radiation and pulsed neutron sources have increased in sophistication in recent years due to the use of solid-state detectors. A key method for neutron detection is the nuclear activation of materials by neutrons. Neutron activation can generate radionuclides whose decay produces secondary particle emission that can...

Comparative study of fast focus mode (FFM)and slow focus mode (SFM)of INTI Plasma Focus device at 12 kV, based on Lee Model, are presented in this paper. Results of numerical experiments for D, Ne, and Ar gases at different pressure ranges show that as a rule-of-thumb, diameter-optimized SFM is considered to occur when fast plasma stream speed gene...

This paper shows the correlation of the characteristic Ne soft X-ray (SXR) signal pulse with computed dynamics of the plasma focus using doped neon in Ne (97.5%)/Kr (2.5%) admixtures. The Lee model code is coupled with a specifically designed correlation excel template. The results show that typically, the characteristic Ne SXR pulse signal starts...

In this paper, we examined teachers’ understanding of electrical concepts such as power, current and potential difference based on how these concepts were applied to understand the relative brightness seen in bulbs of different wattage under different connections—series or parallel. From the responses of teachers to a concept question, we identifie...

Intense X-ray pulses in nanoseconds time duration from pinch plasma can give the exposure time of about six orders of magnitude shorter than conventional X-ray sources. The controllable parameters of X-rays emission from plasmas and together with the degree of penetration of the radiation made the pulsed plasma devices an interesting alternative to...

Measurement of X-rays is a non-destructive technique to provide information about density and temperature regions in the plasma. The emission of X-rays from pulsed plasma is characterized by high intensity and wide spectral range. This chapter provides the details of how filtered detectors can be used to find the spectrum of X-rays emission.

In this work, damage results of four 12 mm × 12 mm × 5 mm PLANSEE double forged tungsten target samples exposed in INTI PF device at different distances from anode operating in the fast focus mode at 12 kV, 2.5 Torr deuterium are presented. SEM and EDX images and analysis show the extent of damage on the exposed tungsten surface. Micro-cracks up to...

In this work, the design and construction of a 50 Ω fast Faraday cup and its results in correlation with the Lee Model Code for fast ion beam and ion time of flightmeasurements for a Deuterium filled plasma focusdevice are presented. Fast ion beam properties such as ion flux, fluence, speed, and energy at 2–8 Torr Deuterium are studied. The minimum...

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-Dneutrons in INTI plasma focusdevice with clear and high signal...

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 yiel...

Dilute magnetic semiconductors are attractive due to their potential in spintronic devices. In this work, vanadium doped ZnO system has been studied to see its future as a dilute magnetic semiconductor. Vanadium doped ZnO thin films where vanadium percentage is 2, 3, and 5% are deposited by pulsed laser technique (PLD). The lattice parameter c deri...

The deuterium-deuterium (DD) fusion neutron yield and its anisotropy were measured for the NX2 plasma focus (PF) device by using two compact beryllium fast neutron activation detectors (designed and implemented by the authors) at 0° and 90° to the PF axis. The contribution to the fusion yield produced by the accelerated deuteron beam emitted from t...

A sub-kilo-Joule plasma focus device (FMPF-3, 14 kV/235 J) was operated with deuterium–krypton admixtures (of 1, 2 and 5 % Kr by volume) to study the influence of admixture ratio on neutron yield (Y n). Experiments were performed for different insulator sleeve lengths and cathode geometries. The results reveal that for a carefully optimized electro...

Comparative study of Fast Focus Mode (FFM) and Slow Focus Mode (SFM) of INTI Plasma Focus device at 12 kV, based on Lee Model Code, are presented in this paper. Results of numerical experiments for D, Ne, and Ar in the pressure ranges of 1 to 14 Torr for D, 1 to 5.5 Torr for Ne, and 0.2 to 2.4 Torr for Ar show that as a rule-of-thumb, diameter-opti...

Measurements of the spatial and time-resolved characteristics of pulsed neutron sources require large area detection materials and fast circuitry that can process the electronic pulses readout from the active region of the detector. In this paper, we present a solid-state detector based on the nuclear activation of materials by neutrons, and the de...

WN/a-Si3N4 thin films have been synthesized by utilizing the energetic ion/electron species emanated from hot, dense pinched plasma column in UNU/ICTP plasma focus operated with nitrogen gas. Structural, chemical, morphological and mechanical properties of synthesized films are studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopi...

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 t...

Nano-structure thin film of tungsten nitride was deposited onto Si-substrate at room temperature using Mather-type plasma focus (3.3 kJ) machine. Substrate was exposed against 10, 20, 30, and 40 deposition shots and its corresponding effect on structure, morphology, conductivity and nano-hardness has been systematically studied. The X-ray diffracto...

Neutron yield is enhanced by reduction of the short-circuit plasma focus inductance , which in turn increases the pinch current . Numerical experiments are carried out to optimize the neutron production using the Lee model on small plasma focus devices (several kilojoules), such as UNU/ICTP PFF, NX2, and LDiana. The maximum neutron yield is obtaine...

The PLANSEE double forged W samples were irradiated at different distances (5, 10, 15 and 20 cm) from the anode top for fixed 50 focus shot using a single shot 10 kJ NX3 plasma focus (PF) device with deuterium as the filling gas. The NX-3 PF device is dri ven by a 100 μF, 20 kJ modular capacitor bank which is capable of delivering peak current in t...

The spatial distribution of DD fusion in a 1.6 kJ plasma focus (PF) device is investigated using the coded aperture imaging (CAI) technique, enabling single shot fusion images to be obtained. Simultaneously, two beryllium fast-neutron activation detectors are employed to measure neutron yield and anisotropy. The coded mask patterns used are based o...

X-ray lithography (XRL) can be a potential candidate for next generation lithography. The main objective of this study is to demonstrate XRL using fast miniature plasma focus (FMPF) device as the X-ray source. The advantage of using FMPF as X-ray source is the smaller X-ray spot size due to smaller electrode dimensions leading to higher resolution...

The main problem discussed in this paper involves a simple one-dimensional two-body collision, in which the problem can be extended into a chain of one-dimensional many-body collisions. The result is quite interesting, as it provides us with a thorough mathematical understanding that will help in designing a chain system for maximum energy transfer...

The pinch performance of a plasma focus (PF) device is sensitive to the physical conditions of the breakdown phase. It is therefore essential to model and study the initial phase in order to optimize device performance. An external circuit is self consistently coupled to the electromagnetic particle in cell code to model the breakdown and initial l...

A novel trigger generator for operating a pseudospark switch has been developed based on a series connection of several insulated gate bipolar transistors. The trigger generator can be operated in single shot mode up to a repetition rate of 1 kHz. It is characterized by a fast rise time and low jitter between the output trigger pulses of less than...

A range of plasma focus machines from the 200 J FMPF3 through the 2 kJ NX2 to the 20 kJ NX3 are operated at the National Institute of Education in Singapore. The neutron yield Yn versus pressure P curves for these machines have been measured and published. We use the Lee code to compute these Yn versus P curve for each machine after fitting a compu...

The breakdown phase of the UNU-ICTP plasma focus (PF) device was successfully simulated using the electromagnetic particle in cell method. A clear uplift of the current sheath (CS) layer was observed near the insulator surface, accompanied with an exponential increase in the plasma density. Both phenomena were found to coincide with the surge in th...

Magnetic properties of pulsed laser deposited (PLD) FePt thin films are investigated at three different laser energy fluences of 51, 136 and 182 J/cm2. Deposition at lower laser energy fluence (51 J/cm2) yields softer out-of-plane coercivity (≤0.4 kG), whereas deposition at higher laser energy fluence (136 and 182 J/cm2) results in harder out-of-pl...

Advanced pulsed power technologies provide an efficient conversion of electrical energy into the pinch plasma for higher neutron production with longer life than existing pulsed neutron sources suitable for materials identification and non-destructive testing. A compact nanosecond pulse neutron generator based on the interactions of fast deuterons...

Fast miniature plasma focus (FMPF-3), a low energy (235 J) device, is an attractive option for being a potential source for soft X-ray (SXR) (0.9 – 1.6 keV) lithography due to its smaller X-ray generation spot size and the capability of being a high repetition source. As a continuation of our work on the enhancement of SXR emission from this device...

A model for the theta pinch is presented with three modelled phases of radial inward shock phase, reflected shock phase, and a final pinch phase. The governing equations for the phases are derived incorporating thermodynamics and radiation and radiation-coupled dynamics in the pinch phase. A code is written incorporating correction for the effects...

Fast Miniature Plasma Focus Device (FMPF-3) device used for our experiments is of sub-kilojoule (100 – 240 J) energy capacity. It is an order of magnitude lesser than the other well established plasma focus devices that are generally used for neon soft x-ray emission studies. The prospect of enhancement of soft X-ray (SXR) (900 – 1600 eV) emission...

We have prepared a series of (ZnO)1−x(Fe2O3)x≤0.10 bulk samples with various concentrations of Fe dopant by ball milling and investigated their structural, compositional, optical and magnetic properties by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectrometer and vibrating sample magnetometer (VSM), respective...

Amorphous hydrogenated carbon (a-C:H)/diamond-like carbon (DLC) coatings have been achieved on AISI 304 stainless steel (SS) substrates by employing energetic ions emitted from a repetitive plasma focus operated in CH4 discharge. The Raman spectroscopy of the coatings exhibits the evolution of a-C:H/DLC coatings with clearly observed D and G peaks...

The magnetic field distribution substantially affects mechanisms for the generation of radiation in Z-pinches. Investigation of the axial component of the magnetic field is one of the important problems in plasma focus studies. The designed magnetic probe is intended to use for the study of current sheet in INTI plasma focus device with energy of a...

An investigation on the possibility of enhancement of soft X-ray (SXR) (900–1600 eV) emission from a fast miniature plasma focus (FMPF) device of 235 J (at 14 kV) storage energy through doping of operating gas was performed. Neon (Ne), the operating gaseous medium, was doped with krypton (Kr) in different volumetric ratios at various operating pres...

The shaping and formation of the current sheath takes place in the breakdown phase of a plasma focus device and critically controls the device performance. Electrostatic particle in cell codes, with magnetic effects ignored, have been used to model the breakdown phase. This Letter reports the successful development and implementation of an electrom...

There is an increasing requirement for alternative and improved detection of fast neutrons due to the renewed interest in neutron diagnostics applications. Some applications require heavily shielded neutron sources that emit a substantial proportion of their emission as fast neutrons and so require high performance fast neutron detectors. In some a...

The design and construction of a pulsed plasma focus device to be used as a portable neutron source for material analysis such as explosive detection using gamma spectroscopy is presented. The device is capable of operating at a repetitive rate of a few Hz. When deuterium gas is used, up to 105 neutrons per shot are expected to be produced with a t...

It is known that the radius ratio of a plasma pinch depends on the specific heat ratio γ of the pinch plasma, where radius pinch ratio is defined as: radius of plasma pinch column/radius of anode. The lower the specific heat ratio the lower would be the pinch radius ratio with corresponding increased compression and pinch density. The deuterium pla...

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 thi...

This paper presents the synthesis of iron oxide nanoparticles using the atmospheric microplasma (AMP). The properties of iron oxide nanoparticles synthesized using AMP are compared with particles (i) formed in as-prepared solution and (ii) prepared using thermal decomposition method. Iron oxide nanoparticles prepared by all the 3 treatment methods...

This paper summarizes PF-160 Dual Plasma Focus (DuPF) numerical experiments using the Lee Model code and preliminary 3D design drawings using SolidWorks software. This DuPF consists of two interchangeable electrodes enabling it to be optimized for both Slow Pinch Mode (SFM) and Fast Pinch Mode (FFM); the latter using a speed factor (SF) of 90 kA cm...

The shaping and formation of the current sheath takes place in the breakdown phase of a plasma focus device. Achieving a clear understanding of the current sheath formation process is important because the plasma focus device performance depends on the quality of this sheath. In this paper, we created and successfully run an electromagnetic particl...

The six-phase Lee model code was developed to compute the anomalous resistance phase (RAN) following the pinch phase in a plasma focus (PF) discharge. One important method to check such modeling is to look at the soft X-ray (SXR) emission time profile and to correlate this to the PF dynamics. A two-channel filtered SXR spectrometer coupled with an...

The usage of chemically-amplified, negative tone SU-8 photoresist is numerous, spanning industrial, scientific and medical fields. Hence, in this study, some preliminary studies were conducted to understand the dosage and heat treatment requirements of the SU8 photoresist essential for pattern generation using X-ray lithography. In this work, using...

The formation of impurity phases in FePt thin films severely degrades its magnetic properties. The X-ray diffraction patterns of FePt thin films, synthesized using pulsed laser deposition (PLD), showed peaks corresponding to impurity phases, resulting in softer magnetic properties. A systematic investigation was carried to determine the factors tha...

(ZnO)1−x (Co3O4)x≤0.05 thin-film series were deposited in vacuum Ar–O2 and N2 environment using the pulsed-laser deposition (PLD) technique. Bulk pellets were prepared by the solid-state reaction technique by mixing ZnO and Co3O4 powder and further utilized for deposition of thin films. XRD patterns of pellets exhibit homogeneous ZnCoO phase format...

The six-phase Lee model code was developed to compute the anomalous resistance phase (RAN) following the pinch phase in a plasma focus (PF) discharge. One important method to check such modeling is to look at the soft X-ray (SXR) emission time profile and to correlate this to the PF dynamics. A two-channel filtered SXR spectrometer coupled with an...

The neon soft X-ray (SXR) emission characteristics of a Fast Miniature Plasma Focus (FMPF-3) device have been investigated. The FMPF-3 device used for our experiment is of sub-kilojoule energy capacity, which is an order of magnitude lesser than the other well established plasma focus devices. The influence of different geometrical parameters of th...

The present study is focused on the copper-doped ZnO system. Bulk copper-doped ZnO pellets were synthesized by a solid-state reaction technique and used as target material in pulsed laser deposition. Thin films were grown for different Cu doped pellets on sapphire substrates in vacuum (5×10−5 mbar). Thin films having (002) plane of ZnO showed diffe...

form only given. The optical investigation of the plasma dynamics in breakdown, axial and radial phases of fast miniature plasma focus (FMPF-3) device under repetitive opeartion is being performed by means of laser shadowgraphy. Uniform parallel light beams are provided by using a monochrome (532 nm) Nd:YAG short pulsed laser (120 ps). A Phantom V2...

form only given. The neon soft X-ray (SXR) emission characteristics of a Fast Miniature Plasma Focus (FMPF-3) device have been investigated. The FMPF-3 device used for our experiment is of sub-kilojoule (100-240 J) energy capacity, which is an order of magnitude lesser than the other well established plasma focus devices on which most of the neon x...

The presence of structural defects degrade the crystalline quality of ZnO:Mn thin films and affects the magneto-optical properties of ZnO:Mn thin films. The donor defects in ZnO, which are known to be the source of n-type conductivity in ZnO host matrix, play an important role in limiting the ferromagnetism to lower temperatures. A systematic study...

An activation detector for fast-neutrons has a yttrium target exposed to a neutron source. Fast-neutrons which have energy in excess of 1 MeV (above a threshold energy level) generate gamma rays from a nuclear reaction with the yttrium, the gamma rays having an energy level of 908.96 keV, and the resultant gamma rays are coupled to a scintillator w...

The Magnetic Reynolds Number (MRN) in neon is computed as a function of Neon shock speed. The magnetic field profiles at various positions in the axial run down phase of the INTI Plasma Focus device are measured over a range of pressures from 2 to 20 Torr. These profiles are assessed for good electromagnetic coupling including measuring the current...

This paper reports the results of characterization and optimization experiments carried out on a newly developed NX-3 dense plasma focus device (20 kJ at 20 kV, a quarter time period of ~ 3 μs, and 10 kJ/600 kA at 14 kV) at the Plasma Radiation Source Laboratory, NIE, Nanyang Technological University, Singapore. Initial experiments were conducted w...

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 th...

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 wer...

Current sheath dynamics generated in INTI plasma focus device operated with neon gas has been studied. A 3-turn Rogowski coil design has been used to measure derivative current. A new magnetic probe was designed and used to study of current sheath arrival time, current profile and velocity variation in the axial phase at different experimental cond...

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 Degree-Sign...

form only given. A comparative study of DD fusion in a 1.6 kJ plasma focus (PF) device operated with pure deuterium and a deuterium-krypton (D-Kr) admixture has been performed. Fast-neutron beryllium activation detectors positioned at 0° and 90° to the PF axis measured the time-integrated neutron yield and anisotropy. Typical fusion yields were in...

The coded aperture imaging (CAI) technique has been applied to investigate the spatial distribution of DD fusion in a deuterium filled 1.6 kJ plasma focus (PF) device operated in its neutron-optimized regime. The coded mask has been fabricated in Havar alloy with 20 × 20 pixels and 57 square holes (14% open area) using a Singer-set Cyclic Differenc...

A compilation of articles presented at IPFS Conference in Bangkok 30 March to 1 April 2012. This conference was commissioned by EXECO Consulting of Luxembourg. Titles include: From Electric Birth to Streaming Death of the Plasma Focus- Knowledge and Applications; Commercial Applications; Research Basis & Results at IPFS; Plasma Focus Research at IN...

The un-doped and Mn doped ZnO thin films, with oxygen rich stoichiometry, were deposited onto Si (1 0 0) substrate using spin coating technique. The structural analysis revealed the hexagonal wurtzite structure without any impurity phase formation. A consistent increase in cell volume with the increase in Mn doping concentration confirmed the succe...

This paper investigates the ferromagnetism in ZnO:Mn powders and presents our findings about the role played by the doping concentration and the structural defects towards the ferromagnetic signal. The narrow-size-distributed ZnO:Mn nanoparticles based powders with oxygen rich stoichiometery were synthesized by wet chemical method using zinc acetat...

A high performance high repetition rate plasma focus device with significant time averaged neutron yield and greater shot to shot reproducibility could be a highly valuable neutron source for Homeland Security applications. The single module fast miniature plasma focus ‘FMPF-2’ (2.4 μF, 56 ± 3 nH, 89 kA @ 14.0 kV, T/4~575 ns) was upgraded to four m...

A compact fast neutron detector based on beryllium activation has been developed to perform accurate neutron fluence measurements on pulsed DD fusion sources. It is especially well suited to moderate repetition-rate (3×104cm−2, the statistical uncertainty in the fluence measurement is better than 1%. A small plasma focus device has been employed as...

This paper reports the temperature-dependent tailoring of acceptor defects in oxygen rich ZnO thin films, for enhanced p-type conductivity. The oxygen rich p-type ZnO thin films were successfully grown by pulsed laser deposition on silicon substrate at different postdeposition annealing temperatures (500–800 °C). The oxygen rich ZnO powder was synt...

Current sheath dynamics is an important parameter for good focusing or pinching in a dense plasma focus device. In this paper, we present pinching evidence and details of the filamentary structure of the current sheath in a miniature plasma focus device using a time-resolved laser shadowgraphic technique and time-integrated optical imaging.

The six-phase Lee model code is used to fit the com- puted current waveform to the measured current waveform of INTI plasma focus (PF; 2.2 kJ at 12 kV), a T2 PF device, operated as a source of neon soft X-ray (SXR) with optimum yield around 2 torr of neon. The characteristic He-like and H-like neon line SXR pulse is measured using a pair of SXR det...

The structural and photoluminescence analyses were performed on un-doped and Mn doped ZnO thin films grown on Si (1 0 0) substrate by pulsed laser deposition (PLD) and annealed at different post-deposition temperatures (500–800 °C). X-ray diffraction (XRD), employed to study the structural properties, showed an improved crystallinity at elevated te...