Plasma Focus Discharge Research Articles

Puffing Deuterium Compressed by a Neon Plasma Sheath at the Initial Poloidal Magnetic Field in Plasma Focus Discharge

This paper presents the results of the experimental study of the influence of the applied poloidal magnetic field on the transformation of the plasma column formed by the compression of (10 ± 3)- $\mu \text{g}$ /cm deuterium injected by the gas-puff nozzle in front of the anode axis by the neon plasma sheath of (50 ± 10) $\mu \text{g}$ /cm at a current of 2 MA. The permanent magnet with an initial induction of 20–40 mT placed inside the anode: 1) increased the diameter of the pinch and of the stagnation; 2) depressed the evolution of instabilities and formation of the dense structures in the column; 3) delayed the dip of the current derivative in time (and start of the hard X-ray and neutron emission) 200–300 ns after the first peak of the soft X-ray emission (and the pinch with a minimal diameter near the anode); and 4) decreased the total neutron yield to 10–20%. These results were interpreted in terms of the increase in the repulsive pressure of the compressed poloidal magnetic field, which increases the stabilizing helicity of the discharge current. The transformations are independent of the polarity of the magnet.

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.

Investigation of compression of puffing neon by deuterium current and plasma sheath in plasma focus discharge

This paper presents the results of the research of the influence of compressed neon, injected by the gas-puff nozzle in front of the anode axis by the deuterium current and plasma sheath on the evolution of the pinch, and neutron production at the current of 2 MA. The intense soft X-ray emission shows the presence of neon in the central region of the pinch. During the implosion and stopping of the plasma sheath, the deuterium plasma penetrates into the internal neon layer. The total neutron yield of 1010–1011 has a similar level as in the pure deuterium shots. The neutron and hard X-ray pulses from fusion D-D reaction are as well emitted both in the phase of the stopping implosion and during the evolution of instabilities at the transformation of plasmoidal structures and constrictions composed in this configuration from both gases. The fast deuterons can be accelerated at the decay of magnetic field of the current filaments in these structures.

Temporal distribution of linear densities of the plasma column in a plasma focus discharge

Abstract Experiments were carried out on the PF-1000 plasma focus device, with a deuterium filling and with deuterium puffing from a gas-puff nozzle placed on the axis of the anode face. The current was reaching 2 MA. 15 interferometric frames from one shot were recorded with a Nd:YLF laser and a Mach–Zehnder interferometer, with 10–20 ns delay between the frames. As a result, the temporal and spatial distribution of the linear densities and the radial and axial velocities of the moving of plasma in the dense plasma column could be estimated.

Recent measurements of soft X-ray emission from the DPF-1000U facility

Abstract Soft X-ray imaging is a very useful diagnostic technique in plasma-focus (PF) experiments. This paper reports results of four experimental sessions which were carried out at the DPF-1000U plasma-focus facility in 2013 and 2014. Over 200 discharges were performed at various experimental conditions. Measurements were taken using two X-ray pinhole cameras with a line of sight perpendicular to the z-axis, at different azimuthal angles (about 20° and 200°), and looking towards the centre of the PF-pinch column. They were equipped with diaphragms 1000 μm or 200–300 μm in diameter and coated with filters of 500 μm Al foil and 10 μm Be foil, respectively. Data on the neutron emission were collected with silver activation counters. For time-resolved measurements the use was made of four PIN diodes equipped with various filters and oriented towards the centre of the PF-column, in the direction perpendicular to the electrode axis. The recorded X-ray images revealed that when the additional gas-puff system is activated during the discharge, the stability of the discharge is improved. The data collected in these experiments confirmed the appearance of a filamentary fine structure in the PF discharges. In the past years the formation of such filaments was observed in many Z-pinch type experiments. Some of the recorded X-ray images have also revealed the appearance of the so-called hot-spots, i.e. small plasma regions of a very intense X-ray emission. Such a phenomenon was observed before in many PF experiments, e.g. in the MAJA-PF device, but it has not been investigated so far in a large facility such as the DPF-1000U. The time-resolved measurements provided the evidence of a time lapse between the X-ray emission from plasma regions located at different distance from the anode surface. The formation of distinct ‘hot-spots’ in different instants of the DPF-1000U discharge was also observed.

Existence of a return direction for plasma escaping from a pinched column in a plasma focus discharge

The use of multi-frame interferometry used on the PF-1000 device with the deuterium filling showed the existence of a return motion of the top of several lobules of the pinched column formed at the pinched plasma column. This phenomenon was observed in the presence of an over-optimal mass in front of the anode, which depressed the intensity of the implosion and the smooth surface of the pinched plasma column. The observed evolution was explored through the use of closed poloidal currents transmitted outside the pinched plasma. This interpretation complements the scenario of the closed currents flowing within the structures inside the pinched column, which has been published recently on the basis of observations from interferometry, neutron, and magnetic probe diagnostics on this device.

Study of the interrelation between the electrotechnical parameters of the plasma focus discharge circuit and the plasma compression dynamics on the PF-3 and PF-1000 facilities

The main stages of the plasma current sheath (PCS) dynamics on two plasma focus (PF) facilities with different geometries of the electrode system, PF-3 (Filippov type) and PF-1000 (Mather type), were studied by analyzing the results of the current and voltage measurements. Some dynamic characteristics, such as the PCS velocity in the acceleration phase in the Mather-type facility (PF-1000), the moment at which the PCS reaches the anode end, and the plasma velocity in the radial stage of plasma compression in the PF-3 Filippov-type facility, were determined from the time dependence of the inductance of the discharge circuit with a dynamic plasma load. The energy characteristics of the discharge circuit of the compressing PCS were studied for different working gases (deuterium, argon, and neon) at initial pressures of 1.5–3 Torr in discharges with energies of 0.3–0.6 MJ. In experiments with deuterium, correlation between the neutron yield and the electromagnetic energy deposited directly in the compressed PCS was investigated.

Imperative function of electron beams in low-energy plasma focus device

A 2.2 kJ plasma focus device was analysed as an electron beam and an X-ray source that operates with argon gas refilled at a specific pressure. Time-resolved X-ray signals were observed using an array of PIN diode detectors, and the electron beam energy was detected using a scintillator-assisted photomultiplier tube. The resultant X-rays were investigated by plasma focus discharge for pressures ranging from 1.5 mbar to 2.0 mbar. This range corresponded to the significant values of X-ray yields and electron beam energies from the argon plasma. The electron temperature of argon plasma at an optimum pressure range was achieved by an indirect method using five-channel BPX65 PIN diodes of aluminum foils with different thicknesses. X-ray yield, electron beam energy, and electron temperature of argon plasma were achieved at 1.5–2.0 mbar because of the strong bombardment of the energetic electron beam.

Neutron energy distribution and temporal correlations with hard x-ray emission from a hundreds of joules plasma focus device

In this paper, experimental measurements of neutron energy distribution and hard x-ray emission from a 320J deuterium operated plasma focus device are presented. Temporal measurements of neutron and hard x-ray emission are studied using different arrangements of six scintillator-photomultiplier detector systems, located either radially or axially with respect to the focus symmetry axis. The axial-to-radial ratio of both total neutron yield and neutron energies indicates anisotropic emission, which is consistent with a 100 keV kinetic energy of the deuterons in the axial direction. The energy spread among different shots was ~0.5 MeV in the axial direction which is 2.5 times the spread in the radial direction. Furthermore, temporal differences on hard x-rays and neutron production over each direction are found. These differences show correlation with neutron energies. This could be related to the existence of two temporally separated neutron production times corresponding to different moments during the plasma focus discharge.

Development of a plasma focus neutron source powered by an explosive magnetic generator

This paper presents the results of laboratory and explosive experiments with a plasma focus discharge Mather-type chamber at a discharge current amplitude of 1.3–1.4 MA. It has been found that in laboratory experiments, the yield of a deuterium-deuterium neutrons reached 1011, and in an explosive experiment using the chamber filled with a deuterium-tritium gas mixture, the integral yield of a deuterium-tritium neutrons with an energy of 14 MeV was more than 1012 neutrons.

Research on anisotropy of fusion-produced protons and neutrons emission from high-current plasma-focus discharges.

The paper concerns fast protons and neutrons from D-D fusion reactions in a Plasma-Focus-1000U facility. Measurements were performed with nuclear-track detectors arranged in "sandwiches" of an Al-foil and two PM-355 detectors separated by a polyethylene-plate. The Al-foil eliminated all primary deuterons, but was penetrable for fast fusion protons. The foil and first PM-355 detector were penetrable for fast neutrons, which were converted into recoil-protons in the polyethylene and recorded in the second PM-355 detector. The "sandwiches" were irradiated by discharges of comparable neutron-yields. Analyses of etched tracks and computer simulations of the fusion-products behavior in the detectors were performed.

Filamentary structure of plasma produced by compression of puffing deuterium by deuterium or neon plasma sheath on plasma-focus discharge

The present experiments were performed on the PF-1000 plasma focus device at a current of 2 MA with the deuterium injected from the gas-puff placed in the axis of the anode face. The XUV frames showed, in contrast with the interferograms, the fine structure: filaments and spots up to 1 mm diameter. In the deuterium filling, the short filaments are registered mainly in the region of the internal plasmoidal structures and their number correlates with the intensity of neutron production. The longer filamentary structure was recorded close to the anode after the constriction decay. The long curve-like filaments with spots were registered in the big bubble formed after the pinch phase in the head of the umbrella shape of the plasma sheath. Filaments can indicate the filamentary structure of the current in the pinch. Together with the filaments, small compact balls a few mm in diameter were registered by both interferometry and XUV frame pictures. They emerge out of the dense column and their life-time can be greater than hundreds of ns.

Magnetic field distribution in the plasma flow generated by a plasma focus discharge

The magnetic field in the plasma jet propagating from the plasma pinch region along the axis of the chamber in a megajoule PF-3 plasma focus facility is studied. The dynamics of plasma with a trapped magnetic flow is analyzed. The spatial sizes of the plasma jet region in which the magnetic field concentrates are determined in the radial and axial directions. The magnetic field configuration in the plasma jet is investigated: the radial distribution of the azimuthal component of the magnetic field inside the jet is determined. It is shown that the magnetic induction vector at a given point in space can change its direction during the plasma flight. Conclusions regarding the symmetry of the plasma flow propagation relative to the chamber axis are drawn.

On the current layer in the run-down phase of the plasma focus discharge

In the experiments with the 3 kJ plasma focus device in the Sofia University are measured the basic characteristics, namely the discharge current, the current derivative, the soft X-ray and the hard X-ray emission from the plasma. By a set of magnetic probes the velocity of the current sheath during the run-down axial phase is determined. In the last two thirds of the axial phase the current layer velocity is a constant.

Measurements of ions emission using ToF method and CR39 SSNTD in a Small Plasma Focus device of Hundreds of Joules

Ion beam emission in plasma focus (PF) discharges was originally investigated to explain the strong forward anisotropy observed in the neutron emission when D2 is used as filling gas in the PF devices. Several properties of emitted deuteron beam, including its angular distribution and energy spectra in PF devices operating at energies from 1kJ to 1MJ, have been measured. At present there is a growing interest in the development of very small PF devices operating under 1kJ. As part of the very low energy (< 1kJ) PF devices physics characterization program carried out at the Chilean Nuclear Energy Commission, the charged particle emission is being studied when hydrogen (H2) and mixture (H2+ %Ar) is used as filling gas in the PF device. The experiments have been performed in a plasma focus device of 400 joules (PF-400J). In order to estimate the ion beam energy spectrum and its ionization degree, by means of the time of flight method, a graphite collector system operating in the bias ion collector mode was constructed. On the other hand, measurements of the energy and flux of the ions have been corroborated using CR39 SSNTD. Preliminary results mainly show the presence of hydrogen ions with an average energy of 40keV. Also, in some cases, copper (from anode) and aluminum (from insulator) ions with energies lower than 1keV have been observed.

Initial stages in hundreds of Joules plasma focus operating in deuterium – argon mixtures: Preliminary results

In plasma focus research, the mixture of deuterium with noble gases has been one of the attempts for both improving the radiation yield and for understanding of the origin of the fusion reactions. However, differences on the neutron emission from the plasma focus operating with gas mixtures according to the device energy have been reported. In an attempt to improve the understanding of such experimental observations, experiments in a very low energy plasma focus device (400J) working with gas mixtures are presented. Different stages prior to the final compression of the plasma focus discharges were studied; such as the breakdown phase and the plasma sheath movement. The electrical signals analysis shows differences on the breakdown voltage but similar axial sheath velocities. Nonetheless, these preliminary observations must be corroborated with additional experimental data.

A model of hard X-rays emission from free expanding Plasma-Focus discharges

A planar-piston model of the hard x-ray production in Plasma-Focus devices is presented. The model applies Von Karman approximations to represent the inner structure of the pinch. The hard x-ray emission is calculated assuming Bremsstrahlung from the collision on the anode base of an electron current running away from the pinch. The model was applied to analyse the experimental data of a small Plasma Focus without surrounding cathode, founding good agreement.

Progress in plasma focus research at the Kurchatov Institute

This paper presents new results from studies of plasma focus (PF) systems at the Kurchatov Institute (Moscow, Russia). High efficiency of the discharge current transport into the axial region of the system was demonstrated at the PF-3 PF facility, where the experiments on implosion of wire arrays by the current carrying plasma sheath of the PF discharge were performed. The efficiency of the sheath current switching over to the wire array and penetration of the plasma with the magnetic field inside the wire array was studied. The emission parameters of the new small-sized PF facility with Wmax = 600 J were studied. When operating at frequencies of up to 10 Hz, neutron fluxes of about ∼ 108 neutrons s−1 were obtained. The first results in the new field of research associated with the use of the PF facilities in the laboratory simulations of astrophysical processes, such as astrophysics jets, are presented.

Experimental Verification of Modified Lee’s Model Using a Newly Designed and Constructed Filippov-Type Plasma Focus Device

A new 20 kJ Filippov-type plasma focus device has been designed and constructed in Isfahan University. The paper reports on the design and construction of the Iranian Filippov-type plasma focus device (UIPFF1) using modified Lee’s model. A Rogowski coil has been used to measure the experimental discharge current. Equivalent electric circuit of the device is RLC circuit; therefore the discharge current has a sinusoidal shape which its amplitude decreases exponentially during the time. The current signal contains a set of data from physical processes in the device as well as discharge current characteristics. In a typical discharge experiment these values were obtained: the discharge current was 181 kA, period of current signal 7.9 µs, the total inductance of the device 132 nH and electrical resistance of the circuit 77 mΩ. By averaging from data obtained with a set of five experiments the calibration factor was obtained 121 kA/V. Temporal changes in plasma focus discharge current, confirmed the occurrence of pinch at a specific pressure of argon, neon and nitrogen gases. UIPFF1 has been tested between 15 and 25 kV and wide range of pressure for various gases. Experiments at various pressures and voltages have also confirmed reproducibility and stability of the plasma focus device.

Development and study of a portable plasma focus neutron source

The work is devoted to designing a compact pulsed neutron source on the basis of a plasma focus (PF) discharge. The main task was to study the physical processes accompanying a sub-kilojoule repetitive PF discharge. A device with a power supply energy of up to 600 J and pulse repetition rate of up to 10 Hz has been developed and put into operation. The dependence of the neutron yield as a function of the pulse repetition rate has been studied experimentally. A neutron flux of ∼108 neutrons/s has been obtained in the 3-s-long packet mode with a repetition rate of 10 Hz and discharge current of 80–90 kA.