Plasma sheath dynamics on the axial phase of a plasma focus device

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=(I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> /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

Data from the 2002 Imperial College plasma focus (ICpf) are used to study the dynamics of axial and radial phase as functions of pressure. The speeds are found to deviate from the ideal constant current electromagnetic rule of v $\sim ~P_{0}^{-n} $ , (where $n = 0.5$ ) with the modulus going below 0.5, due to the back-electromotive force varying the peak drive current even at fixed voltage operation. The amount of deviation depends on machine parameters and varies from a modulus of 0.3 for the NX1 axial phase to 0.47 in the INTI Plasma Focus (INTI PF) axial phase. The radial phase of the ICpf is characterized by a modulus of 0.48. The computed results are experimentally verified in the case of the NX1 and INTI PF axial phase. The effect on neutron yield is briefly discussed.

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 lift phase of the United Nations University/International Centre for Theoretical Physics (UNU-ICTP) plasma focus device. Gas breakdown during the breakdown phase is simulated successfully, following a drop in the applied voltage across the device and a concurrent substantial rise in the circuit current. As a result, the plasma becomes magnetized, with the growing value of the magnetic field over time leading to the gradual lift off of the well formed current sheath into the axial acceleration phase. This lifting off, with simultaneous outward sheath motion along the anode and vertical cathode, and the strong magnetic fields in the current sheath region, was demonstrated in this work, and hence validates our method of coupling the external circuit to PF devices. Our method produces voltage waveforms that are qualitatively similar to the observed experimental voltage profiles of the UNU-ICTP device. Values of the mean electron energy before and after voltage breakdown turned out to be different, with the values after breakdown being much lower. In both cases, the electron energy density function turned out to be non-Maxwellian.

A dense plasma focus (DPF) device has been investigated as a source for EUV lithography. Initial characterizations have been made of a prototype DPF employing an all-solid-state pulse power drive. Using the results from a vacuum grating spectrometer combined with measurements with a silicon photo diode, it has been found that substantial amounts of radiation within the reflectance band of Mo/Si mirrors can be generated using the 13.5 nm emission line of doubly ionized Lithium. This prototype DPF converts 25J of stored electrical energy per pulse into approximately 0.76J of in- band 13.5nm radiation emitted into 4(pi) steradians. The pulse repetition rate performance of this device has been investigated up to its DC power supply limit of 200Hz. No significant reduction in EUV output was found up to this repetition rate. At 200Hz, the measured pulse-to-pulse energy stability was (sigma) equals 6 percent and no drop out pulses were observed. The electrical circuit and operation of this prototype DPF device is presented along with a description of several future modifications intended to improve stability and efficiency.

We describe the axial phase of the Mather plasma focus by two coupled equations of motion and circuit. We non-dimensionalised these equations resulting in two coupled equations which are characterised by only three scaling parameters α, β and δ which are ratios of electrical to transit times, inductances and impedances respectively. The normalised current waveform, trajectory and speed profile are unique for each combination of α, β, δ which are the ratios of characteristic times (electrical discharge vs. axial transit), inductances (tube inductance vs. static inductance) and impedances (stray resistance vs. electrical surge impedance). This leads to important information and insight into various aspects of the axial phase. In the present work we show that in a time-matched plasma focus shot we deduce the value of axial phase current fraction fc simply by measuring the calibrated voltage waveform and the uncalibrated current waveform. The scaling parameters β and δ are fixed; and by form-fitting the measured current waveform to the normalised current waveform using the value of α of the shot is determined uniquely; from which the peak current and the ratio of peak to average speed [the speed form factor (SFF)] are obtained. The average transit speed is measured by time-of-flight using the voltage upturn as indicator of end of axial phase. Then the SFF yields the peak speed. The measured voltage (back EMF), peak current and peak axial speed (all at the end of axial phase) allows the unambiguous measurement of fc. The value of the mass swept-up fraction fm is deduced from α which is the ratio of the characteristic discharge and the characteristic transit times, both deduced during the non-dimensionalisation of the equations. Analysis of a time-matched shot in the INTI PF at 15 kV, 3 Torr D2 gave fc = 0.68 and fm = 0.05.

FLASH radiotherapy, characterized by ultra-high dose rates (> 40 Gy/s), potentially spares normal tissues while maintaining antitumor efficacy (the FLASH effect). While electron and proton FLASH are explored, pulsed X-ray sources like plasma focus devices offer unique possibilities. Previous work has reported hyper-radiosensitivity in colorectal cancer cells exposed to ultra-high-dose-rate pulsed X-rays from a kilojoule plasma focus (PF) device, without significant effects on non-cancerous cells. This study further investigates the biological effects of ultra-high-dose-rate (~ 10⁷ Gy/min), low-total-dose pulsed X-rays generated by a PF-2 kJ device on colorectal cancer cell lines, focusing on DNA damage, cell cycle progression, and gene expression. Low-total-dose (~ 0.25 Gy), ultra-high-dose-rate pulsed X-rays (0.025 Gy/pulse, a total of 10 pulses, pulses temporally separated by 15–20 s) generated by a PF-2 kJ device induced a significant increase in the SubG1 population in HCT116 and DLD1 cells over 72 h, an effect indicative of apoptosis, which was not observed with conventional X-rays at similar total doses. In addition, pulsed X-rays induced apoptosis in radioresistant MCF-7 breast cancer cells. Whereas conventional X-rays did not cause a significant increase in double-strand breaks (DSBs), surrogate marker γ-H2AX and phosphor-P53(Ser15) signal were detected 30 min following pulsed X-ray exposure and persisted for up to 24 h, and no evidence of G2/M cell cycle arrest was detected in exposed cells. Gene expression analysis and preliminary transcriptomic data further suggest a DNA damage response leading to cell death and global change in general biological processes related to regulation of gene expression. Low-total-dose (~ 0.25 Gy), ultra-high-dose-rate pulsed X-rays generated by a PF-2 kJ device induce significant and sustained DNA damage (DSBs) leading to increased apoptosis in colorectal (HCT-116, DLD-1) and breast (MCF-7) cancer cells, compared to conventional X-rays. These effects, coupled with distinct changes in gene expression, suggest that ultra-high-dose-rate pulsed X-rays may overcomeradio-resistancee without eliciting a conventional DNA damage repair or cell cycle checkpoint response. These findings support the potential of PF-generated pulsed X-rays as a novel sourcof e radiotherapy modality and warrant further investigation, particularly in in vivo models, to assess clinical applicability and safety.

Dense Plasma Focus (DPF) devices are pulse plasma devices which are capable to generate few keV to MeV ions. The burst of energetic ions presents in plasma shots generated by DPF devices, can be used for impanation in the desired materials for various application from electronics to spintronics. Here, we present an advance way of gradient implantation using DPF in the heavy metal without much effecting the surface properties of the thin film. In order to implant desired ions such as oxygen, carbon, nitrogen, argon etc. new electrodes were designed, providing gentler plasma pinch phase, with increased plasma volume. To study the effect of the new electrode design electrical characteristics were performed in detail and with the help Lee code the plasma parameter like plasma temperature, plasma duration etc. was estimated. Optical imaging diagnostic (using gated ICCD camera), faraday cup measurements and x-ray pin-diode spectroscopy, was used to investigate the plasma dynamics, plasma density and plasma temperature. The designed electrode assembly can be used to generate defects and modify the material properties required for electronics, photonics and spintronics applications.

Effect of preionization on the axial run-down velocity, focus amplitude and current sheath formation in 3.3 kJ small He plasma

We consider the snowplow model to describe the axial phase of a plasma focus device. We show that, within this framework, the dynamics of a given system can be recovered at different energetic and geometric scales. Then, we consider two different criteria that would plausibly optimize the plasma focus performance. By solving the dynamics equations for the axial phase, we estimate the values of the parameters that do indeed satisfy the aforementioned criteria and compare those values with the corresponding ones attached to few practical devices.

The study of plasma sheath structure and its dynamics in the presence of external axial magnetic field (B/sub z/) in dense plasma focus has been made with the help of N/sub 2/-laser shadowgraphic technique. The shadowgraphs of axial phase show that in the presence of external B/sub z/ the current sheath gets diffused near the end attached to the central electrode when it reaches to the top of the central electrode. The shadowgraphs of radial collapse phase show clearly distinct hump formation above central electrode under the influence of B/sub z/. Moreover, during the radial collapse phase a part of the current sheath lying close to central electrode gets highly diffused in comparison to the rest of the current sheath. Diffused bubble formation on the ionization wave front and dominantly visible remnant of plasma sheath above central electrode have also been observed during the quiescent phase (the expansion phase after the peak compression).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of preionization on the dynamics of current sheath in a small plasma focus device

A new application of the Sohrabi albedo neutron dosimeters is reported for the first time for determination of very low-level neutron ambient dose equivalents on and around a 3.5 kJ plasma focus device (PFD). The Sohrabi dosimeters basically use a polycarbonate track detector as bare and/or in contact with B convertor(s) under special cadmium cover arrangements. Its sensitivity was improved by using enriched B under new cadmium arrangements in order to detect epithermal neutrons in addition to fast and thermal neutrons. Results of 12 dosimeters installed externally around the PFD at different azimuthal (φ) and polar (θ) angles showed that azimuthal (φ) fast, epithermal, thermal, and total neutron ambient dose equivalents were symmetric and isotropic, respectively, with values 55.15 ± 8.36, 1.36 ± 02, 0.53 ± 03, and 57.04 ± 8.62 μSv/shot at ~25 cm from anode top. Polar (θ) neutron ambient dose equivalent values on z-axis relative to 90 angle were relatively higher. Results of 38 dosimeters placed on PFD facility walls for workplace monitoring and on a BOMAB phantom at operator's location for personal dose equivalent determination showed values below minimum detection limits after exposure to 130 PFD shots. However, an operator's personal dose equivalents at ~1.0 and ~3.0 m from the anode top were estimated to be, respectively, ~13.7 and ~1.52 mSv y using azimuthal angle (φ) values if the PFD operates, for example, up to 20 shots per day for 200 d y. Even under such an extreme assumption, annual personal dose equivalent is still much below 20 mSv, the annual ICRP dose limit for workers.

To overcome the discontinuity between the axial phase and the radial phase in the plasma focus, a new model (the Masoud model) has been developed. It is assumed that, according to the snowplough model, the radial phase moves in the angular direction with continuity between the axial and the radial phases. This model is based on the introduction of an angle of motion in all equations of motion and circuit. Then, the plasma sheath position and velocity were calculated numerically from the previous equations. A slug model has been used to calculate the shock wave velocity, and hence the electron temperature. Values of the discharge current, the axial speed, the axial position, the spike voltage, the radial piston speed, the radial piston position, the plasma column length and the plasma temperature have been obtained using the new model. The values calculated by using this model show good agreement with the published experimental results. Keywords: Plasma focusLee modelSnowplough modelSlug model

Fabrication of nanoparticles and nanostructured materials is at the heart of modern nanoscience. In the present work, we are reporting the fabrication of Iron nanoparticles making use of high temperature, high density, and extremely non equilibrium pulsed plasma produced in 3.3 KJ Mather type Dense Plasma Focus (DPF) device. The Iron nanoparticles were deposited on quartz substrate at a distance of 5 cm above the ion source. The Iron nanoparticles are characterized structurally using X-ray diffractometer and topographically using Atomic force microscopy (AFM). The magnetic property was investigated using Magnetic Force Microscopy (MFM). It is found that these nanoparticles have a dimension in the range of 20-60 nm size and are magnetic in nature.

Hard TiCx/SiC/a-C:H nanocomposite thin films using pulsed high energy density plasma focus device