Plasma Focus Operation Research Articles

On filamentation in the dense plasma focus

Striking pictures showing filamentary structures in the plasma focus have intrigued researchers from the early days of plasma focus research. A definitive understanding of their occurrence, origin, structure, and role in plasma focus physics is still not in sight as summarized in a recent comprehensive review. This is because they are often not observed in a “standard mode” of plasma focus operation with pure deuterium, particularly in large installations, but are found in smaller experiments or those with gaseous admixtures. This has led to the suspicion that filaments are not a native feature of the plasma focus phenomenon. Recent success in observation of filaments in PF-1000 in pure deuterium operation by novel modification of the interferometer system that allows simultaneous interferometry and schlieren photography changes this situation. This Letter looks at the implications of this development in the larger context of plasma focus physics. Conceptualization of filamentation as a native feature of the traveling current distribution behind an ionizing strong shock wave is shown to be a feasible paradigm that can be formulated as a computable model for filamentation in the plasma focus.

Stable current sheath dynamics caused by preionization source in a small plasma focus device using He as an operating gas

ABSTRACTPinching of the current sheath in the plasma focus (PF) device is an important phase to transfer the stored energy to the high density short lived plasma for the production of a group of useful radiations. The current sheath formation during the initial breakdown of the gas is a critical process in the PF operation. In this regard, the understanding of the current sheath dynamics is of prime importance. In this paper, we present the effects of alpha particle induced preionizaion by U238 in 3.3 kJ He operated PF device on the dynamics of current sheath, focus amplitude and axial rundown velocity by employing Rogowski coil and High voltage probe as main diagnostic tools. The formation of parabolic axisymmetric sheath is confirmed by Rogowski coil and high voltage probe signals which also reveals the variation in focusing time as well as amplitude during collapse phase by using the preionization source. Time resolved X-ray diagnostics are also carried out using PIN diodes covered with Al and Mg filters of 12.5 and 25 µm which provide an important information about an early onset of X-ray generation. Time integrated X-ray imaging is also carried out to estimate X-ray production zone in the PF device indicating enlarged and symmetric images with preionization.

On the Relationship Between the Electromagnetic Burst and Inductive Sensor Measurement of a Pulsed Plasma Accelerator

A remote and non-invasive diagnostic of the plasma focus using antennas is presented in this work. The main motivation is the application of such diagnostic in a miniaturized plasma accelerator, based on the plasma focus architecture, as a cube satellite thruster. The evaluation of this proposal was carried out measuring a hundred of joules plasma focus operation simultaneously with the inductive measurement and antennas. Three different antennas tuned in the ultra high frequency range were tested: a monopole, Vivaldi and helical. The high frequency transients detected with the antennas were time correlated to the known inductive measurement features. The initial dielectric breakdown and later plasma pinch and subsequent disruption (i.e. the source of the propulsion) were identified to be the principal phenomena to be detected. Signal parameter correlations between the inductive sensor and the antennas showed that the pinch phenomena can be correlated with the antenna signals. Good correlation results were obtained with the monopole antenna when using peak value and signal energy parameter from the antenna transient. An improvement in the correlation results, for the helical and Vivaldi antennas, was obtained when calculating the frequency band energy. In this case, the Vivaldi antenna achieved the best results. The results of the monopole antenna make it an alternative remote sensor for plasma focus, but for the application of a miniaturized plasma focus as pulsed plasma thruster, the Vivaldi antenna is a more feasible design to replace the inductive diagnostic due to its compact design in comparison to the monopole.

Results of Ultracompact Plasma Focus Operating in Repetitive Burst-Mode

The results of a miniature plasma focus are presented in this paper which is operated with energy less than or equal to 150 J. The miniature plasma focus is driven by a small capacitor bank and the peak current delivered in the focus is 75–80 kA. The deuterium gas is filled with a pressure range of 5–7 mbar inside the plasma focus chamber. The quartz glass is used for generating initial surface breakdown at a 4–5-kV discharge, which is a typical value for low-voltage plasma focus discharges. The repetitive operation (record 50 Hz) of the device is achieved by a combination of a simple and high power (5 kW) supply with the synchronized triggering of the capacitor bank at the time of isolation between supply and the capacitor bank. The diameter of cathode is 25 mm and anode diameter is 8–12 mm and both of them are made of stainless steel. The length of anode and gas pressure is adjusted in such a way that the pinching occurs at the time of occurrence of the peak of current. It enhances the neutron emission from the device. The time-of-flight diagnostic is used to distinguish neutron and X-ray emission from the plasma focus. The neutron measurement using a helium-3 detector and also a fast-scintillator-backed photomultiplier tube demonstrates production of neutron pulses in the time separated by nearly 20 ms, which corresponds to 50 Hz in the burst mode. The device can serve the purpose of being a portable and compact repetitive neutron source for various applications as the flux of the radiation is comparable with that of bigger plasma focus operating at the similar current.

Structural and Mechanical Properties of Zirconia Film Deposited by Plasma Focus Device

Nano-crystalline ZrO2/ZrSiO4 composite films are deposited on Si by the irradiation of energetic oxygen ions by plasma focus operation. The XRD patterns confirm the development of different phases (ZrO2 and ZrSiO4) which grow along various orientations and their peak intensities strongly depend on increasing focus shots and sample angular positions. The average crystallite size of the films deposited at 0° angular position is increased with increasing focus shots while it is decreased for the films deposited at 10° angular position. The dislocation density in ZrSiO4 (2 0 4) plane is increased from 3.51 × 10−5 to 8.67 × 10−5 (nm)−2 while it is decreased from 8.85 × 10−5 to 0.011 × 10−5 (nm)−2 in m-ZrO2 (1 2 3) plane when the samples are treated at 0° and 10° angular positions for five focus shots respectively. The FESEM analysis confirms the formation of rounded nano-particles, complicated network of nano-rods, formation of compact, uniform and smooth multi-layers. The EDS analysis confirms the presence of Si, Zr and O in the composite films deposited for different focus shots and sample angular positions. The maximum microhardness of the composite film deposited for 25 focus shots is found to be 10.56 ± 0.51 GPa which definitely will improve the life time of the coated material.

Determination of Ion Beam Properties In Nitrogen and Helium Using Mather Type Plasma Focus Device

Some of ion beam properties have been investigated by using Lee model code on plasma focus devices which is operated with nitrogen and helium gases. The operation of plasma focus in different pressure regime gives a consistent ion beam properties which can make the plasma focus a reliable ion beam source .These ion beam properties such as ion beam flux, ion beam fluence, ion beam energy, ion beam current, and beam ion number corresponding to gas pressure have been studied for Mather type plasma focus device. The result shows the differences between helium as lighter gas and nitrogen as heavier gas in term of ion beam properties. The fluence and flux are decrease for nitrogen while increase for helium.

Note: Repetitive operation of the capacitor bank of the low-voltage miniature plasma focus at 50 Hz

We have already reported the low-voltage operation of a plasma focus describing the operation of plasma focus at 4.2 kV which proposes possibility of making a repetitive system using compact driving source. Another recent article describes that the same capacitor-bank can drive the plasma focus for a measured ~5 × 10(4) neutrons per shot at 5 kV and 59 kA current. In the present work, repetitive operation of the capacitor-bank of plasma focus is done and that too is being reported at a very high repetition rate of 50 Hz using very simple scheme of charging and triggering the bank. The bank is continuously discharged to burst duration of 20 s in this configuration admeasuring a thousand shots.

Low voltage operation of plasma focus

Plasma foci of compact sizes and operating with low energies (from tens of joules to few hundred joules) have found application in recent years and have attracted plasma-physics scientists and engineers for research in this direction. We are presenting a low energy and miniature plasma focus which operates from a capacitor bank of 8.4 muF capacity, charged at 4.2-4.3 kV and delivering approximately 52 kA peak current at approximately 60 nH calculated circuit inductance. The total circuit inductance includes the plasma focus inductance. The reported plasma focus operates at the lowest voltage among all reported plasma foci so far. Moreover the cost of capacitor bank used for plasma focus is nearly 20 U.S. dollars making it very cheap. At low voltage operation of plasma focus, the initial breakdown mechanism becomes important for operation of plasma focus. The quartz glass tube is used as insulator and breakdown initiation is done on its surface. The total energy of the plasma focus is approximately 75 J. The plasma focus system is made compact and the switching of capacitor bank energy is done by manual operating switch. The focus is operated with hydrogen and deuterium filled at 1-2 mbar.

Neon soft x-ray emission studies from the UNU-ICTP plasma focus operated with longer than optimal anode length

The UNU-ICTP plasma focus with a significantly longer than conventional anode can still be a reasonably good neon soft x-ray (SXR) source. The highest average neon SXR yield of 3.3 J was achieved at 3 mbar. The time difference between the two first peaks of the voltage probe signal at the radial collapse phase was found to be inversely related to the SXR yield, i.e. the smaller the time difference, the higher the yield and vice versa. The estimation of average current sheath speeds using the shadowgraphic method coupled with laser and focus peak timing signals showed that the average axial rundown speed is similar to the one obtained for the optimal anode length but the average radial compression speed is decreased significantly. The range of pressure for a good neon SXR yield, however, has become much narrower, making efficient plasma focus operation a very sensitive function of the filling gas pressure for longer than the optimal anode length.

Investigation of Plasma Facing Components in Plasma Focus Operation

Both aspects of the plasma–wall interactions, counter effect of plasma and materials, have been considered in our experiments. The AEOI plasma focus, Dena, has Filippov-type electrodes. The experimental results verify that neutron production increases using tungsten as an anode insert material, compared to the copper one. The experiments show decrement of the hardness of Aluminum targets outward the sides, from 135 to 78 in Vickers scale. The sputtering yield is about 0.0065 for deuteron energy of 50 keV.

Depleted uranium (U23892) induced preionization for enhanced and reproducible x-ray emission from plasma focus

The effect of preionization induced by depleted uranium (U23892) around the insulator sleeve on the x-ray emission of (2.3–3.9kJ) plasma focus device is investigated by employing Quantrad Si p-i-n diodes and a multipinhole camera. X-ray emission in 4π geometry is measured as a function of charging voltage with and without preionization. It is found that the preionization enhances CuKα and total x-ray yield about 100%, broadens the x-ray emission pressure range and x-ray pulse width, and improves shot to shot reproducibility of plasma focus operation. The pinhole images of x-ray emitting zones indicate that dominant x-ray emission is from the anode tip.

Enhanced and reproducible neutron emission from a plasma focus with pre-ionization induced by depleted uranium (U238)

The effect of pre-ionization induced by depleted uranium (92U238) around the insulator sleeve on the neutron emission of (1.8–3.3 kJ) plasma focus device is investigated by employing time resolved and time integrated neutron detectors. The maximum average neutron yield of 2.5 × 108 is recorded at 3.5 mbar without pre-ionization, which increases up to 3.85 × 108 with pre-ionization. It is found that the pre-ionization enhances neutron yield, about (50 ± 5)%, broadens the neutron emission pressure range and improves shot-to-shot reproducibility of plasma focus operation for neutron emission. The pinhole images of charged particles emitting zones indicate that the pre-ionization increases the high temperature plasma volume.

Fast valve and nozzle for gas-puff operation of dense plasma focus

A simple and reliable valve and nozzle system for a very fast injection of gas has been designed and constructed for its use in gas-puff mode of dense plasma focus experiments. It delivers a very quick rise time: 55μs. The pressure measured in our setup at a distance of 15 mm from the nozzle output is about 0.285 mbar, with a plenum pressure of 3 bars (absolute). The time between the valve aperture and pressure front arrival is 360μs. This result comes up as an average of about a hundred measurements. The energy input is 95 J (270 V on a 3000μF capacitor bank). The typical dimensions of the valve are 52 mm in diameter and 80 mm in length. The entire volume of the valve is, then, very small. The relative low pressure and voltage operation are significant advantages of this development. The performance of the valve satisfactorily fulfills the objectives of gas-puff plasma focus operation.

Operation of nx2 dense plasma focus device with argon filling as a possible radiation source for micro-machining

Dense plasma focus (DPF) can be a powerful source of X-rays at the wavelengths useful for microlithography and micromachining depending on its working gas (Ne or Ar correspondingly) and operating parameters of the device. Experimental investigations were carried out with /spl sim/0.4-nm wavelength radiation from a specially designed medium-power soft X-ray tube with a water-cooled silver anode and highly sensitive chemically amplified resist SU-8. They have shown us that it is possible to produce 10-/spl mu/m structures replicated in 100-/spl mu/m resist layer and 4-/spl mu/m structures produced in a 35-/spl mu/m resist layer for 30 min. To decrease the time, a DPF device must be implemented for the task. Using pure argon and mixtures of argon with deuterium or krypton, we have found regimes (hot spots, plasma pinching, and plasma compression by a heavy shell) with appreciable soft X-ray yield. Influence on the results of chamber-circuit matching, pinch dynamics, and electron runaway processes are also discussed.

Influence of magnetic probe presence on current sheath dynamics in plasma focus operation

The effects of the magnetic probe presence near the focus region on the current sheath dynamics are studied. The system consists of a single 32μF, 15 kV (3.6 kJ) capacitor. It is found that the current sheath during the radial collapse phase strongly interacts with the magnetic probe jacket (Pyrex glass tube of diameter 5.7 mm) and climbs over it up to several centimeters. X-ray images recorded for argon pressure range of 0.25 – 3.25 mbar clearly indicate that the size as well as the intensity of the X-ray source is significantly enhanced compared to the case ehen no probe is inserted in the system. Generally, the hot spots are formed up to about 30 mm above the anode tip and are only rarely observed 75 mm above the anode tip, which is probably due to the refocusing of the current sheath at that height. These preliminary results suggest that using some suitable material, the path of the current sheath can be diverted to any desired direction and then guided to converge at a preselected point.

Current Sheath Studies in a Small Plasma Focus Operating in Hydrogen–argon Mixtures

We present preliminary time and space resolved studies of current sheath formation in Plasma Focus discharges, using a novel array of non-invasive magnetic field probes. The experiments are performed in a Mather type plasma focus, operating at 2 kV. The discharge is formed between a hollowed anode and six symmetrically arranged cathode rods. The array of small magnetic probes is located along the cathode rods. The probes are of millimeter size. They are shielded behind the rods, as to minimize capacitive coupling to the anode electrode, and allow non-perturbing measurements to be made. A simple analytical model of current sheath evolution is used to analyze the probe signals. The experiments have been performed in pure Hydrogen and Hydrogen with Argon mixture, at pressures from below 0.2 Torr upwards. The effect of the Argon mixture on the current sheath structure is investigated with the probe array. It is found that at constant mass density operation, the increase in the percentage of Argon results in a thinner sheath, with steeper current profile.

Effects of anode shape on plasma focus operation with argon

X-ray and ion beam emission from a low-energy Mather-type plasma focus energized by a single , 12 kV (2.3 kJ) capacitor is investigated employing three different anode shapes and using argon as filling gas. The radiation yield and the filling gas pressure for good focus are found to be strongly dependent on the anode shape. An appropriate tapering of the anode end enhances the emission threefold for both x-rays and ions. The intensities of the signals are also found to be correlated mutually as well as with the high-voltage probe signal intensity. Furthermore, time-integrated pinhole images reveal that the x-rays originate predominantly from the anode end surface. In other words, an appropriate shaping of the anode end switches the device to a high-emission mode for both x-rays and a charged particle beam.

Investigations on the transition between column and micropinch mode of plasma focus operation

X-ray emission from pinch plasma devices with pinch currents ranging from 200 to 400 kA operated with pure high-Z gases are investigated with temporal spatial and spectral resolution. If operated using elements Z<18 (e.g., nitrogen or neon) K-shell emission is observed from column-like volumes several 100 μm in diameter and several mm in length (bulk or column mode). For Z≳18 (e.g., krypton or xenon) emission with hν≳1 keV is only observed from micropinches. For argon (Z=18) both modes of operation can be observed. The occurrence of a specific mode depends on the initial gas pressure. In this paper the transition regime between column and micropinches is investigated with particular regard to argon. A criterion is proposed to decide whether column mode or micropinch mode is expected.

TEMPORAL CORRELATION OF NEUTRONS, ION BEAM, AND HIGH VOLTAGE PROBE SIGNALS IN A LOW ENERGY PLASMA FOCUS

A simple, low inductance (~50 nH) capacitor bank is developed for plasma focus operation. The bank comprises three modules, each consisting of two 2µF, 40 kV capacitors along with a field distortion type pressurized sparkgap. The system is employed to investigate the temporal correlation of neutrons, ion beam, high voltage probe, and Rogowski coil signals. From a careful analysis of the data, the high voltage probe signal is found to coincide well with the Rogowski coil signal, whereas the ion beam and the neutron pulses are delayed by 20–30 nsec. These observations suggest that in low energy plasma foci, ion beam generation and neutron emission occur after the maximum compression has taken place and the pinch filament has started to break, most probably due to the onset of sausage type instability.

Pressure range broadening for a plasma focus operation

The effects of insulator sleeve length and anode length variation on the pressure range of neutron emission in a low energy plasma focus are investigated. It is found that the proper choice of the two parameters broadens the pressure range of the high neutron yield and hence improves the shot to shot reproducibility of the system. The experiment also demonstrates that the bulk of the neutrons are emitted from a region within 2 cm above the anode top and that, in turn, reflects on the nature of the dominant production mechanism.