kJ Dense Plasma Focus Research Articles

Evolution of filament-like compact structures in small 3 kJ dense plasma focus discharges

Evolution of filament-like compact structures in small 3 kJ dense plasma focus discharges

Effect of insulator length and fill pressure on filamentation and neutron production in a 4.6 kJ dense plasma focus

Optimization of neutron yields from dense plasma focus devices is a complex multi-faceted challenge that necessitates the prudent selection of mechanical constraints such as the electrode and insulator geometries. Here, the neutron yield is found to significantly depend on the insulator length. As the length of the insulator increases, the exposed anode length traveled by the sheath during the run-down phase decreases. This suggests an increase in the optimal fill pressure with increasing insulator length to maintain the pinch time near peak current. However, in the present study, the opposite trend is observed—the optimal fill pressure for neutron production decreases with increasing insulator length. Optical probing of the sheath from run-down to the pinch reveals significant plasma filamentation with increasing pressure and a dependence of insulator length on filamentation onset. A direct consequence of increased filamentation is a reduction in mass sweeping efficiency, directly quantified as a function of fill pressure for the first time.

Deposition of molybdenum carbide thin films by dense plasma focus device

This work was motivated by the promising physiochemical properties and catalytic activity of molybdenum carbide. Thin films of molybdenum carbide were deposited on the mono-crystalline cubic Si (400) substrate by utilizing a 3.6 kJ dense plasma focus device (Mather-type) with multiple shots. The structure of the films was examined by X-ray diffraction (XRD). XRD results indicated the absence of crystalline film for 5 focus shots, while for increased numbers of focus shots, the emergence of Mo-C peaks in the orientations (120), (014) and (232) confirmed the formation of orthorhombic β-Mo2C polycrystalline films. Moreover, a small variation in the crystallite size was observed by increasing the number of focus shots. The presence of molybdenum and carbon was confirmed by energy-dispersive X-ray (EDX) analysis. Surface morphology was studied under scanning electron microscope (SEM). SEM micrographs revealed non-uniform films with some surface defects such as cracks and craters of micron size, and rough surface. Revealed by the ellipsometry technique, the film thickness increased by increasing the number of focus shots. The electric conductivity, measured by the four-point probe technique, was the highest for 10 focus shots. However, the films have some structural defects, low conductivity and impurities like silicon and silicon carbide, but it was the first ever successful attempt to deposit molybdenum-carbide (Mo2C) thin films using the dense plasma focus device.

Effect of insulator surface conditioning on the pinch dynamics and x-ray production of a Ne-filled dense plasma focus

The dense plasma focus (DPF) can be an intense source of x rays, wherein the insulator sleeve strongly dictates the electrical breakdown, which subsequently affects the formation of a plasma sheath and a collapse phase. Experiments on a 25 kJ DPF (operated at 4.4 kJ) are carried out to demonstrate the influence of insulator surface morphology on the pinch structure, dynamics, and x-ray yield using a Ne fill. Two borosilicate insulators are directly compared, one with a smooth finish and the other machined with four circumferential grooves traversing the perimeter of the exterior insulator surface. Comparisons are made through same-shot imaging diagnostics of the evolving plasma sheath during breakdown, rundown, and at the pinch in addition to the time-resolved measurements of emitted x rays via filtered photodiodes. The presence of structures on the insulator sleeve reduces x-ray production across all fill pressures by a factor of 2.8 ± 2.4 on average and reduces the highest x ray producing shots by a factor of 5.5 ± 1.8. Observations of sheath asymmetry and inhomogeneity at lift-off are observed and correlated with subsequent observations of off-axis radial collapse. Taken together, this suggests that local variations in the insulator surface decrease the spatial uniformity of the sheath, leading to an azimuthally asymmetric focus, reduced electron densities, and, ultimately, degraded x-ray production.

Characteristics of Fast ion beam in Neon and Argon filled plasma focus correlated with Lee Model Code

In this paper, results of time-of-flight measurements of fast ion beam generated and accelerated in the final pinch phase of a 2 kJ dense plasma focus device at 12 kV in Neon and Argon are presented. The measurements were made using a 50 Ω F Cup and these results were correlated with Lee Model Code computations. The results include fast ion beam properties such as ion speed, energy, flux and fluence at 2–4 Torr Neon and 0.5–1.5 Torr Argon. Ion signals with FWHM minimum values of 42 ns and 47 ns respectively for 2.5 Torr Neon and 0.9 Torr Argon were measured. The maximum most common ion energy of 450 ± 30 keV at 2.5 Torr Neon and 1230 ± 90 keV at 0.9 Torr Argon were measured. Ion time-of-flight measurements including ion speed and energy, ion flux, ion fluence by Faraday Cup show very consistent correlation with Lee Model Code results for Neon and Argon. This paper validates the computation of plasma focus ion beam properties by the Lee Code.

Maximizing neutron yields by scaling hollow diameter of a dense plasma focus anode

Experiments were performed to maximize the neutron yield from a 2 kJ dense plasma focus (DPF) and characterize the amount of copper sputtered from the surface of an anode by varying the diameter of the anodes’ on-axis hollow. The hollow is a void in the copper material along the longitudinal axis of the anode. All the anodes had an outer diameter of 1.2 in. and the diameter of the hollow varied from 0 in. (no hollow) to 1 in. The anodes with a hollow produced a greater number of neutrons per discharge than the anode without a hollow. Over 40 discharges, the hollow anode that yielded the most neutrons (9.1±0.4×106 neutrons per discharge produced with the 0.75 in. hollow) produced >6 times more neutrons than the anode with no hollow. A qualitative observation of the anodes after 130 discharges showed less surface damage on anodes with a larger hollow. Quantitative sputter measurements were performed by characterizing the amount of copper sputtered onto on-axis quartz targets for three newly machined anodes, each with a particular hollow diameter. The quantitative results matched the qualitative observations: the copper sputter was reduced using larger hollows. The largest hollow sputtered 17±1.0 nm/sr/discharge of copper, a reduction of 69% compared to the anode with the most damage.

Modelling and Electrical Characteristics of the Thailand Plasma Focus-II (TPF-II)

The Thailand Plasma Focus II (TPF-II) is a 3.3 kJ dense plasma focus that was developed at Walailak University, Thailand. The aim of the device is to study the production of ion beams in the keV energy range and their applications for the color modification of gemstones. A high-energy ion beam is produced by heating and acceleration in the pinch phase of the plasma focus. The heating process is determined by the maximum electrical current, which can be optimized by variation of the system’s inductance. Lee model code was implemented to optimize the configuration of the electrodes. The current waveforms for the different initial conditions were used to obtain the system’s inductance, which was verified by a short circuit test. It was found that the inductance and resistance were about 153 nH and 12 mΩ, respectively.

Axial mass fraction measurements in a 300kA dense plasma focus

The dynamics and characteristics of the plasma sheath during the axial phase in a ∼300 kA, ∼2 kJ dense plasma focus using a static gas load of Ne at 1–4 Torr are reported. The sheath, which is driven axially at a constant velocity ∼105 m/s by the j × B force, is observed using optical imaging, to form an acute angle between the electrodes. This angle becomes more acute (more parallel to the axis) along the rundown. The average sheath thickness nearer the anode is 0.69 ± 0.02 mm and nearer the cathode is 0.95 ± 0.02 mm. The sheath total mass increases from 1 ± 0.02 μg to 6 ± 0.02 μg over the pressure range of 1–4 Torr. However, the mass fraction (defined as the sheath mass/total mass of cold gas between the electrodes) decreases from 7% to 5%. In addition, the steeper the plasma sheath, the more mass is lost from the sheath, which is consistent with radial and axial motion. Experimental results are compared to the Lee code when 100% of the current drives the axial and radial phase.

L-shell spectroscopic diagnostics of radiation from krypton HED plasma sources.

X-ray spectroscopy is a useful tool for diagnosing plasma sources due to its non-invasive nature. One such source is the dense plasma focus (DPF). Recent interest has developed to demonstrate its potential application as a soft x-ray source. We present the first spectroscopic studies of krypton high energy density plasmas produced on a 3 kJ DPF device in Singapore. In order to diagnose spectral features, and to obtain a more comprehensive understanding of plasma parameters, a new non-local thermodynamic equilibrium L-shell kinetic model for krypton was developed. It has the capability of incorporating hot electrons, with different electron distribution functions, in order to examine the effects that they have on emission spectra. To further substantiate the validity of this model, it is also benchmarked with data gathered from experiments on the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory, where data were collected using the high resolution EBIT calorimeter spectrometer.

AFM, XRD and Optical Studies of Silver Nanostructures Fabricated under Extreme Plasma Conditions

This paper reports the deposition of silver on silicon and glass substrates using ions generated in a 3.3 KJ dense plasma focus device. The hot and dense argon plasma formed during the focus phase ionizes the silver disc placed above the top of the anode. Glass and silicon substrates are placed at an axial position of 4.0 cm and 5.0 cm from the top of the anode and are exposed by two focus shots. The deposited materials obtained were characterized using X-ray diffraction, Atomic Force Microscopy (AFM) and UV-vis Spectroscopy. The X-ray diffraction shows the [111] and [200] reflections of silver deposited on silicon substrates whereas on glass only [111] plane is observed. The AFM of silver deposited on silicon substrates shows nanostructures which have triangular like shape. The UV spectra for silver nanostructures placed at 4.0 cm and 5.0 cm on glass substrate shows the absorption peak which originates from the surface plasmon absorption of nanosized silver particles. A red shift of ~13 nm is observed for silver deposited on glass substrate placed at the distance of 5.0 cm from the top of anode.

A Comprehensive Time-Resolved Study of X-Ray Signals in a 4 kJ Dense Plasma Focus Facility

In the present research, time-resolved of X-ray signals are investigated in a 4 kJ PF device using five filtered PIN-diodes and a Scintillation detector. At applied voltage 12 kV using nitrogen (N2) gas, the optimum pressures for maximum yield of HXR and SXR emission were 3 torr and 4 torr respectively. At each operating pressure, multiple pulses in SXR signals with respect to amplitude and duration of emission were studied in a time span of 700–800 ns. According to energy response of filters used, the origin of X-ray pulses and also the approximate energy of X-ray photons were analyzed. It was found that at higher pressure 4 torr, the yield of SXR photons in different pulses increases 20–30% in comparison with the results obtained at pressure 3 torr while the yield of HXR decreases. The results confirm that the operating pressure is an effective parameter on the yield of SXR and HXR emitted from this device.

Nitrogen doping in pulsed laser deposited ZnO thin films using dense plasma focus

Pulsed laser deposition synthesized ZnO thin films, grown at 400 °C substrate temperature in different oxygen gas pressures, were irradiated with 6 shots of pulsed nitrogen ions obtained from 2.94 kJ dense plasma focus to achieve the nitrogen doping in ZnO. Structural, compositional and optical properties of as-deposited and nitrogen ion irradiated ZnO thin films were investigated to confirm the successful doping of nitrogen in irradiated samples. Spectral changes have been seen in the nitrogen irradiated ZnO thin film samples from the low temperature PL measurements. Free electron to acceptor emissions can be observed from the irradiated samples, which hints towards the successful nitrogen doping in films. Compositional analysis by X-ray photoelectron spectroscopy and corresponding shifts in binding energy core peaks of oxygen and nitrogen confirmed the successful use of plasma focus device as a novel source for nitrogen ion doping in ZnO thin films.

Austenite modification of AISI 316L SS by pulsed nitrogen ion beams generated in dense plasma focus discharges

Abstract We present the results of a surface modification of AISI 316L stainless steel by surface irradiation with high energy, pulsed nitrogen ion beams generated with 0.8 kJ dense plasma focus. The surface characterization was done using GAXRD, Auger electrons spectroscopy, TEM and optical microscopy. After the irradiation, we found a modification of a 1 μm thick surface layer, on which a gradual lattice expansion of the austenite with the number of irradiation pulses, i.e. with the total nitrogen ion fluence, was observed. In addition, ~ 40 nm close to the surface layer, a disordered lattice structure had been observed through TEM analysis. Those results can be explained in terms of the extreme thermal effect induced on the surface through the fast high energy release during the pulsed ion interaction with the steel surface, followed by an also rapid cooling down process which limits the nitrogen diffusion to the bulk.

Dense plasma focus ion-based titanium nitride coating on titanium

We report the synthesis of titanium nitride coating on a titanium substrate by utilizing energetic nitrogen ions emitted from a 2.3kJ dense plasma focus device for 30 focus shots. The number of nitrogen ions transferred to the sample by a single ion pulse of about 140ns duration in the energy interval (40–600keV) is about 1.09×1012 with a mean energy per ion of 58keV. The corresponding energy flux delivered to the titanium surface is estimated to be 6.17×1014keVcm−3ns−1 leading to a transient temperature rise of the top layer of about 5400K which helps layer growth. The coating is investigated on the basis of its morphological, compositional and hardness properties. X-ray diffraction analysis of the sample reveals the formation of a nanocrystalline titanium nitride coating having (111) and (200) plane reflections with an average crystallite size of 40nm. The compressive residual stresses in the nitride coating have been evaluated to be 2.80GPa and 6.81GPa corresponding to (111) and (200) plane orientations. A complete restructuring of the manually polished titanium substrate has been observed with appearance of nano-sized multidimensional granular surface morphologies. The thickness of the nitride coating is about 1μm, whereas the coating has a nitrogen content of 35.35at.% and 13.78±3.57wt.% and a surface hardness of 8.19GPa.

Co-deposition of titanium and iron nitrides on SS-321 by using plasma focus

This article reports the co-deposition process of TiN0.9 and (Fe,Cr)2N compounds on SS-321 substrate using a 2.3 kJ dense plasma focus device operated with N2 discharges. X-ray diffraction analysis is performed to investigate the ion-induced changes in the near surface structure of the SS-321. Scanning electron microscopy with the energy dispersive X-ray spectroscopy is carried out to analyse the surface morphology and the elemental composition of the nitrided samples. The results reveal that at the low fluence of ion bombardment, a non-stoichiometric tertiary phase (Fe,Cr) x N is developed, which transforms into a stable stoichiometric compound (Fe,Cr)2N by increasing the ion flux. Some CrN precipitates are also observed because of the thermal effect produced by the bombardment of energetic ion beam. Vickers micro-hardness values are increased more than twice for typical ion nitrided samples.

Monte Carlo simulation of neutron backscattering from concrete walls in the dense plasma focus laboratory of Bologna University

Between 2001 and 2003 a 3.2 kJ dense plasma focus (DPF) device has been built at the Montecuccolino Laboratory of the Department of Energy, Nuclear and Environmental Control Engineering (DIENCA) of the University of Bologna. A DPF is a pulsed device in which deuterium nuclear fusion reactions can be obtained through the pinching effects of electromagnetic fields upon a dense plasma. The empirical scale law that governs the total D-D neutron yield from a single pulse of a DPF predicts for this machine a figure of approximately 10(7) fast neutrons per shot. The aim of the present work is to evaluate the role of backscattering of neutrons from the concrete walls surrounding the Montecuccolino DPF in total neutron yield measurements. The evaluation is performed by MCNP-5 simulations that are aimed at estimating the neutron spectra at a few points of interest in the laboratory, where neutron detectors will be placed during the experimental campaigns. Spectral information from the simulations is essential because the response of detectors is influenced by neutron energy. Comparisons are made with the simple r(-2) law, which holds for a DPF in infinite vacuum. The results from the simulations will ultimately be used both in the design and optimisation of the neutron detectors and in their final calibration and placement inside the laboratory.

Study of X-ray emission of dense plasma focus device in the presence of external magnetic field

The X-ray emission from a 3.3 kJ dense plasma focus device filled with argon in the presence of an external axial magnetic field has been quantitatively measured for the first time with the help of a diode X-ray spectrometer. The X-ray energy and electron temperature estimated from signals are found to be lower with application of the external axial magnetic field. The variation of the electron temperature with pressure shows a maximum at 80 Pa and a decrease both with increase or decrease of the filling gas pressure.

Particle beams generated by a 6–12.5 kJ dense plasma focus

Particle beam diagnostic tools involving an ion beam Faraday cup, an electron beam Faraday cup, an electron magnetic spectrometer and solid-state nuclear track detectors have been used to measure the parameters of the particle beams generated by a Mather-type dense plasma focus. At a capacitor bank energy of 12.5 kJ, the energy spectra of the electron and ion beams are found to obey the same power laws: dN/dE ∝ E−x where x ≅ 3.5 ± 0.5. Primary electron beam current and energy spectra were measured as a function of main bank current at pinch time, IMB. The primary electron beam current was found to scale as , reaching a magnitude of 17 kA for a device energy of 12.5 kJ. The exponent x of the electron energy spectra was found to scale as . These results are incorporated into an axial beam target model for neutron production, and it is found that this model could account for the magnitude and scaling of the observed neutron yield with IMB.

An x-ray spectral measurement system for nanosecond plasmas

A 15-channel Ross filter/silicon diode detection system has been developed for measuring nanosecond pulsed x-ray spectra in the energy range from 4.5 to 116 keV. The theory and practical aspects of Ross filters and silicon diode detectors are discussed as applied to this system. The data read-out system consists of a current integrator array, a simultaneous multichannel digital voltmeter, and oscilloscopes. The system is designed for compatibility with a small computer to allow real time spectral measurements. The x-ray spectrum of a 24 kJ dense plasma focus has been obtained with this system and a theoretical thick target electron beam x-ray spectrum fit to the experimental data. The best fit is obtained with an E−2 power law electron beam spectrum extending from 5 to 125 keV.