T.L. Tan

Soft X-ray optimization studies on a dense plasma focus device operated in neon and argon in repetitive mode

This paper investigates the emission characteristics of a high-performance low-energy (3-kJ) repetitive dense plasma focus device, NX2, operated at up to 1-Hz repetition rate to develop it as an intense source of soft X-rays (SXR) for microlithography and micromachining. Various SXR yield optimization studies with argon and neon as filling gases were performed under different operating conditions (charging voltage, filling pressure, anode length, and insulator sleeve length). The SXR yield was computed using signals obtained from a PIN diode SXR spectrometer with appropriate filters. When operated in neon, the average optimum SXR (/spl lambda//spl sim/1 nm) yield in 4/spl pi/ steradians was found to be up to 140 J/shot, which corresponded to a wall plug efficiency of 5.6%. Operation in argon showed that optimized SXR (/spl lambda//spl sim/0.4 nm) yield was up to 1.3 J/shot. While operating with neon under optimized conditions with a water-cooled anode in repetitive mode, the NX2 device was used as a SXR source to imprint a test lithograph on a highly sensitive chemically-amplified resist SU-8. Test structures showing the effect of a stepper with aspect ratio 3:1 on 10-/spl mu/m-thick SU-8 resist film were obtained.

Optimization of the high pressure operation regime for enhanced neutron yield in a plasma focus device

The average total neutron yield is measured, using an indium foil activation detector, at various combinations of filling gas pressures (including the higher pressure operation regime) of deuterium, capacitor bank charging voltages, anode lengths and insulator sleeve lengths to optimize the neutron yield from the NX2 Plasma Focus device. A remarkable six-fold increase in the average maximum total neutron yield, to a record value of (7 ± 1) × 108 neutrons per shot, compared to the similar energy UNU-ICTP Plasma Focus device is achieved for deuterium at a relatively much higher filling gas pressure of 20 mbar. The average peak neutron energy for the axial direction (0°), radial direction (90°) and backward direction (180°) is estimated to be 2.89 ± 0.25 MeV, 2.49 ± 0.20 MeV and 2.11 ± 0.12 MeV, respectively. The average forward to radial neutron yield anisotropy is found to be 1.46 ± 0.28. The neutron energy and anisotropy measurements suggest that the neutron production mechanism may be predominantly beam target.

An improved radiative plasma focus model calibrated for neon-filled NX2 using a tapered anode

This paper presents a 4-phase radiative plasma focus model, where the dynamics of the current sheath is represented using Lees model. The model is based on the snowplow model in the axial phase and the slug model in the radial phase, complemented with sensible estimations made for the plasma parameters. The x-ray emission characteristics are investigated using a corona plasma equilibrium model. A refinement to the code was made, firstly by taking into account the tapering of the anode in the axial phase and secondly by including the energy loss due to recombination radiation in the slow compression (radiative) phase. Our improved code was calibrated for the NX2, a 3 kJ plasma focus device, operated in neon at a pressure range of 4–7 mbar with a tapered copper anode. An additional macro was programmed to the code in order to automate the curve fitting of the simulated current traces with those obtained experimentally. The resulting theoretical x-ray yield predictions are compared against experimental data, showing good agreement in terms of pressure dependence trends. The model, however, appears to consistently underestimate the absolute x-ray yield when compared with the experimentally obtained values.

Order of magnitude enhancement in neutron emission with deuterium-krypton admixture operation in miniature plasma focus device

The effect of varied concentrations of deuterium-krypton (D2–Kr) admixture on the neutron emission of a fast miniature plasma focus device was investigated. It was found that a judicious concentration of Kr in D2 can significantly enhance the neutron yield. The maximum average neutron yield of (1±0.27)×104 n/shot for pure D2 filling at 3 mbars was enhanced to (3.14±0.4)×105 n/shot with D2+2% Kr admixture operation, which represents a >30-fold increase. More than an order of magnitude enhancement in the average neutron yield was observed over the broader operating range of 1–4 mbars for D2+2% Kr and D2+5% Kr admixtures.

FePt nanoparticle formation with lower phase transition temperature by single shot plasma focus ion irradiation

Uniform FePt nanoparticles were synthesized through nanostructuring of pulsed laser deposited FePt thin films by single shot H+ ion irradiation using a plasma focus device. The annealing temperature required for phase transition from low Ku face-centred cubic to high Ku face-centred tetragonal, for ion irradiated samples, is simultaneously lowered down to 400 °C. The energetic H+ ion irradiation significantly reduces the activation energy for atomic ordering by increasing the number of vacancies. The advantage of using a plasma focus device is that it can achieve nanostructuring in much shorter time, in single shot ion exposure with pulse duration of typically about a few hundreds of nanoseconds, as compared with much longer duration required by continuous ion sources.

Optimization of a plasma focus device as an electron beam source for thin film deposition

Electron beam emission characteristics from neon, argon, hydrogen and helium in an NX2 dense plasma focus (DPF) device were investigated in order to optimize the plasma focus device for deposition of thin films using energetic electron beams. A Rogowski coil and CCD based magnetic spectrometer were used to obtain temporal characteristics, total electron charge and energy distributions of electron emission from the NX2 DPF device. It is found that hydrogen should be the first choice for thin film deposition as it produces the highest electron beam charge and higher energy (from 50 to 200 keV) electrons. Neon is the next best choice as it gives the next highest electron beam charge with mid-energy (from 30 to 70 keV) electrons. The operation of NX2 with helium at voltages above 12 kV produces a mid-energy (from 30 to 70 keV) electron beam with low-electron beam charge, however, argon is not a good electron beam source for our NX2 DPF device. Preliminary results of the first ever thin film deposition using plasma focus assisted pulsed electron deposition using a hydrogen operated NX2 plasma focus device are presented.

Pinching evidences in a miniature plasma focus with fast pseudospark switch

We report the observations of pinching in a miniature plasma focus (PF) (58?160?J) operated in repetitive mode using fast pseudospark switch (PSS). The size of the device, which includes the capacitor bank, PSS and the focus chamber, is of the order of 22?cm ? 22?cm ? 38?cm. Several diagnostic tools, the gated imager, streak camera, current and voltage probe, are employed simultaneously to confirm the occurrence of pinching in this fast miniature PF device. The device is optimized for operation in neon and hydrogen as the working gas. The best focus formation was obtained at pressures between 0.5 to 8.0?mbar for neon and between 7.0 to 15.0?mbar for hydrogen. When the system was operated at 100?J with hydrogen as the filling gas, the typical dip in the current derivative signal and the typical peak in the voltage signal associated with pinch compression, are observed to be most intense indicating efficient pinching in the miniature PF device.

Drive Parameter as a Design Consideration for Mather and Filippov Types of Plasma Focus

Experiments show that the performance of the plasma focus (PF) depends on several macroscopic parameters like the energy of the capacitor bank, current, voltage, electrodes dimension, and curvature of current sheath in the axial phase. Recent work (IEEE Trans. Plasma Sci., vol. 24, no. 3, pp. 1101-1105, 1996) shows that overriding the dependence of performance on the individual parameters listed above is the dependence on a combined parameter called the drive or speed parameter S=(Ip/a)/radicrho. This parameter S appears as most fundamental in the process of nondimensionalization of the magnetohydrodynamic equations coupling the highly supersonic motion of the plasma layer with the electromagnetic circuit equation. The drive parameter S is found to directly control the speed of the plasma layer in both axial and radial phases of the PF. A literature survey (IEEE Trans. Plasma Sci., vol. 24, no. 3, pp. 1101-1105, 1996) first pointed out that neutron-optimized Mather-type PF devices with a range of energies from a few kilojoules to hundreds of kilojoules all operate with a remarkably constant drive parameter. This constancy of S has been extended more recently (Plasma Phys. Control. Fusion, vol. 47, pp. A361-A381, 2005) to PF devices over eight orders of magnitude of storage energies, from a fractional of a joule to megajoules. In this paper, experiments on 2-3-kJ dense PF with modified anodes have been conducted to show that this drive parameter remains fairly constant for the different ratios of the anode length to anode radius. It is also suggested that there may be significant differences in the values of drive parameters for Filippov-type focus devices, Mather-type focus devices, and also hybrid-type devices

Realization of enhancement in time averaged neutron yield by using repetitive miniature plasma focus device as pulsed neutron source

In this paper, we report the experimental demonstration of enhancement in time averaged neutron yield by an order of magnitude, using the fast miniature plasma focus FMPF-2 in repetitive mode. The newly developed FMPF-2 device (2.4 µF, 56 nH, 13.8 kV, T/4 ~ 575 ns) operates up to 10 Hz in the quasi-continuous mode. Using pure deuterium as the fuelling gas, the time averaged neutron output of (6.2 ± 4) × 105 neutrons s−1 at 1 Hz operation was enhanced to (6.5 ± 0.6) × 106 neutrons s−1 at 10 Hz operation for the burst length of 30 consecutive shots. The stability in neutron emission in repetitive mode of operation from 1 to 10 Hz has also been investigated and the evidenced consequential issues of critical concern that are responsible for degradation in neutron yield during the repetitive mode of operation have been highlighted.

Order of magnitude enhancement in x-ray yield at low pressure deuterium-krypton admixture operation in miniature plasma focus device

In a 200J fast miniature plasma focus device about 17- and 10-fold increase in x-ray yield in spectral ranges of 0.9–1.6keV and 3.2–7.7keV, respectively, have been obtained with deuterium-krypton (D2–Kr) admixture at operating pressures of ⩽0.4mbar. In the pressure range of >0.4–1.4mbar, about twofold magnification in average x-ray yield along with broadening of optimum pressure range in both spectral ranges were obtained for D2–Kr admixtures. An order of magnitude enhancement in x-ray yields at low pressures for admixture operation will help in achieving high performance device efficiency for lithography and micromachining applications.