Ing. Vojtěch Munzar, Ph.D.

Dynamics of the implosion of the dense plasma focus play an essential role in converting electrical energy into the kinetic energy of the current sheath and subsequent production of accelerated electrons, ions, hard X-ray, and neutron emission. This paper presents the analysis of the implosion parameters, such as the implosion velocity and imploding mass, coupled with electrical parameters observed on the PF-1000 facility with a modified electrode system. The first two parameters are based on the 16-frame Mach–Zehnder interferometer, which provides the spatial distribution of electron density in a time sequence. Measurement of the total current, current derivative, and voltage enables us to evaluate the total inductance and kinetic energy driven by the capacitor bank. Then comparing the inductances and kinetic energies evaluated from the interferograms and electrical waveforms can provide more precise information on the current flowing in the imploding sheath. We present a possible way to deal with the fact that only part of the total current flows through the imploding layer. With the supposition that the rest of the current flows close to the insulator, we conclude that roughly 70% of the total current flows through the pinch, which is in good agreement with an input parameter of the Lee model, for example.

This paper concerns the correlation of hard x-ray and neutron signals, which were recorded with scintillation detectors oriented in the axial and radial directions, in a comparison with interferometric and extreme-ultraviolet radiation frames, as recorded within the plasma focus (PF)-1000 facility operated with a deuterium filling. The considered signals showed two different phases. In the initial phase, the fusion neutrons are mainly produced by deuterons moving dominantly downstream during the disruption of a pinch constriction (lasting tens nanoseconds). In the later phase (usually after about 100 ns), the fusion neutron emission reaches its maximum in the radial directions. This emission (lasting 100–200 ns) is caused by the fast deuterons moving in both the downstream and radial directions. It correlates usually with a decay of dense plasma structures in remnants of the expanding pinch column. This can be explained by a decay of internal magnetic fields. The neutron signal is usually composed of several sub-pulses of different energies. It was deduced that the primary deuterons producing the observed fusion neutrons undergo a regular and repeated temporal, directional, and energy evolution.

The paper characterizes sources of the fast deuterons which can produce the D–D fusion neutrons. Two pinhole cameras, the axial one and the slant one (oriented at 0° and 60° in relation to the z-axis), were equipped with solid-state nuclear track detectors and applied to investigate the fast deuterons of energies about 100 keV, which produce small quasi-circular track spots of diameters ranging (1–3) mm. They are often observed in plasma-focus shots with higher neutron yields, when they constitute a part of the recorded ion images in a form of azimuthal arcs and/or radial strips. An analysis of an influence of the global magnetic field, which acts along the fast deuteron trajectories, made it possible to determinate the deuteron sources localization, also outside the dense plasma column. The recorded spatial distribution of the fast deuterons, their temporal correlation with disruptions of the ordered plasma structures inside and outside the pinch column, and a regular evolution of the energy of fast deuterons—indicate their strong interconnection and the link with filamentary structure of the current flow.

Z-pinches have been explored as efficient soft x-ray sources for many years. To optimize x-ray emission, various z-pinch configurations were tested. This paper presents data obtained with a hybrid gas-puff z-pinch imploding onto on-axis wires on a microsecond, multi-megaampere GIT-12 generator. In our previous experiments, the hybrid gas puff, i.e., an inner deuterium gas puff surrounded by an outer hollow cylindrical plasma shell, was used to produce energetic protons, deuterons, and neutrons up to 60MeV [Klir et al., New J. Phys. 22, 103036 (2020)]. The behavior of the hybrid gas-puff z-pinch on GIT-12 was interpreted as a high-density plasma opening switch with a microsecond conduction time, 3 MA conduction current, nanosecond opening, and up to 60MV stand-off voltage. These properties can be employed to transfer the current into an on-axis load with a high rise rate. In the recent experiments on GIT-12, we therefore placed single or multiple aluminum wires on the axis of the hybrid gas-puff z-pinch. Before a current sheath arrived at the axis, a coronal plasma was seen around the wire. A rapid increase in x-ray radiation was observed when the coronal plasma imploded onto the axis. The coronal plasma implosion resulted in a long (2cm), narrow (similar to mm) column radiating in the Al K-shell lines. With the single Al wire of 80 mu m diameter, the K-shell x-ray output reached 5.5 +/- 0.8kJ in a 0.6 +/- 0.1 TW peak power and 7 +/- 1ns pulse. The higher K-shell yield of 12 +/- 2kJ and peak K-shell power of 0.7 +/- 0.1 TW were achieved with four 38 mu m diameter Al wires. (Their cross section formed the corners of a square with 1mm side.) The presence of the wires on the axis significantly suppressed ion acceleration and neutron production. Deuterium-deuterium (DD) neutron yields of about 1.2x10(11) were 20 times smaller than the yields produced in shots without any wire. The DD neutron yield was increased up to 4.5x10(11) when the Al wire was replaced by a fiber fr

B-field measurements are crucial for the study of high-temperature and high-energy-density plasmas. A successful diagnostic method, ion deflectometry (radiography), is commonly employed to measure MGauss magnetic fields in laser-produced plasmas. It is based on the detection of multi-MeV ions, which are deflected in B-fields and measure their path integral. Until now, protons accelerated via laser–target interactions from a point-like source have been utilized for the study of Z-pinch plasmas. In this paper, we present the results of the first Z-pinch-driven ion deflectometry experiments using MeV deuterium beams accelerated within a hybrid gas-puff Z-pinch plasma on the GIT-12 pulse power generator. In our experimental setup, an inserted fiducial deflectometry grid (D-grid) separates the imploding plasma into two regions of the deuteron source and the studied azimuthal B-fields. The D-grid is backlighted by accelerated ions, and its shadow imprinted into the deuteron beams demonstrates ion deflections. In contrast to the employment of the conventional point-like ion source, in our configuration, the ions are emitted from the extensive and divergent source inside the Z-pinch. Instead of having the point ion source, deflected ions are selected via a point projection by a pinhole camera before their detection. Radial distribution of path-integrated B-fields near the axis (within a 15 mm radius) is obtained by analysis of experimental images (deflectograms). Moreover, we present a 2D topological map of local azimuthal B-fields B(r,z) via numerical retrieval of the experimental deflectogram.

This article presents a study of neutron emission on the PFZ-200 plasma focus at the Department of physics on FEE CTU in Prague, Czech Republic. In order to achieve the highest and most stable neutron yields, the deuterium working gas pressure and the anode shape were systematically varied. We observed the plasma time to the pinch and the discharge current by the Rogowski coil and neutron emission by the silver activation detector and scintillation time-of-flight detectors. The imploded plasma was visualized using a fast X-ray pinhole camera with a gated microchannel plate detector. The experiment presents the z-pinch discharges with the current maximum above 200 kA and the average neutron yields of 3×10^8 neutrons/shot. Measured pinch times were in the range from 1.65 to 1.85 μs . The hollow round anode configuration performed the most stable neutron yields with a deviation under 20%.

This paper describes the structure of a higher extreme ultraviolet (XUV) emission and plasma density region which surrounds a pinched dense-plasma column. It is interpreted as a toroidal-like plasma formation, which is flowing by a closed current with poloidal and toroidal components upon its surface. This current produces a local magnetic field, which separates the external discharge current from the surface of the dense pinch column. We estimated the values of closed currents as well as magnetic- and plasma-pressures in this column and its surrounding on the basis of the measured distribution of interferometric fringes and intense XUV emission, recorded during the pinch stagnation phase. The considered layer forms the region in which the magnetic energy can be conserved, and during its decay, the acceleration of fast deuterons can take place.

Z- pinch experiments with a hybrid configuration of a deuterium gas puff have been carried out on the HAWK (NRL, Washington, DC) and GIT-12 (IHCE, Tomsk) pulsed power generators at 0.7 MA and 3 MA currents, respectively. On GIT-12, neutron yields reached an average value of 2 X 10(12) neutrons, and deuterons were accelerated up to an energy of 30 MeV. This was 50 times the ion energy provided by the generator driving voltage of 0.6 MV and the highest energy observed in z-pinches and dense plasma foci. To confirm these unique results independently on another device, we performed several experimental campaigns on the HAWK generator. Comparison of the experiments on GIT-12 and HAWK helped us to understand which parameters are essential for optimized neutron production. Since the HAWK generator is of a similar pulsed power architecture as GIT-12, the experiments on GIT-12 and HAWK are important for the study of how charged-particle acceleration scales with the current. (C) 2020 Author(s).

Deuterium gas-puff z-pinches are very efficient laboratory sources of neutron pulses. Using a novel hybrid gas-puff load on the GIT-12 generator, a significant increase in the neutron yields up to

Acceleration of ions to multi-MeV energies is investigated in various plasma devices to better understand processes in astrophysical plasmas and to develop efficient accelerators for a variety of applications. This paper reports the production of proton, deuteron, and electron beams in a z-pinch-a cylindrically symmetric plasma column that is compressed by its own magnetic field. For this work, the GIT-12 pulsed-power generator was used to drive a novel configuration of z-pinch that dramatically enhanced ion acceleration associated with disruption of the current by instabilities in the compressed plasma. During the disruption of 3 MA current, hydrogen ions were accelerated up to at least 50 MeV, which is almost a hundred-times the ion energy provided by the generator driving voltage of 0.6 MV. Under optimal conditions, the total numbers of hydrogen ions with energies above 20 and 50 MeV were 4 x 10(13)and 10(11), respectively. Accelerated deuterons produced one 20 ns (full width at half maximum) pulse of fast neutrons via D(d, n)He-3 and other nuclear reactions. A maximum neutron output of (1.0 +/- 0.2) x 10(12)neutrons/sr was observed downstream, i.e., in the anode to cathode direction. In this direction, the maximum neutron energy reached 58 +/- 7 MeV. Both ion and neutron beams in our experiment reached an end-point energy of about 60 MeV, which is the highest value observed in pulsed-power devices. A localized peak voltage of greater than or similar to 60 MV was driven by the inductive energy that was stored around the plasma column and that was extracted during a sub-nanosecond current drop. Considering the natural occurrence of current-carrying columns in laboratory and space plasmas, the current interruption observed in z-pinches could represent a more general physical process that contributes to the efficient conversion of magnetic energy into the energy of particle beams in various plasmas.

The paper discusses a possible energy transformation that leads to the acceleration of fast ions and electrons. In plasma-focus discharges that occur during deuteriumfilling, which have a maximum current of about 1MA, the accelerated deuterons producefastfusion neutrons andfast electrons hard X-ray emissions. Their total energy, which is of the order of several kilojoules, can be delivered by the discharge through a magnetic dynamo and selforganization to the ordered plasma structures that are formed in a pinch during the several hundreds of nanoseconds of the pinch implosion, stagnation, and evolution of instabilities. This energy is finally released during the decay of the ordered plasma structures in the volume between the anode face and the umbrella front of the plasma and current sheath in the form of induced electric fields that accelerate fast electrons and ions.

Mega-ampere dense plasma foci and deuterium gas-puff z-pinches can accelerate deuterons to multi-MeV energies. Diagnostic measurements of the properties of these ions provide information about ion acceleration in z-pinch plasmas. In particular, the results from ion pinhole cameras seem to be useful for the discussion of ion acceleration mechanisms. Recently, we have used various configurations of ion pinhole cameras in deuterium gas-puff experiments on the GIT-12 generator at the Institute of High Current Electronics in Tomsk and on the HAWK generator at the US Naval Research Laboratory in Washington. The stack of radiochromic films and CR-39 solid-state nuclear track detectors recorded deuterons with energies up to 30 MeV. From our ion diagnostics, we obtained the spatial distribution of the ion source and the ion-beam divergence during the ion emission. This ion-beam divergence was found to decrease with increasing deuteron energy. At 20 MeV, the divergence of each of the individual micro-beams that composed the ion source was on the order of 10 mrad. The deflection of each micro-beam due to the azimuthal magnetic and/or radial electric fields resulted in radial stripes observed by the beam-profile detectors. By analyzing the ion pinhole images, we found that the deuterons were emitted both from a central spot and from a ring-shaped region with a rather large diameter, on the order of 1 cm. The origin and particular diameter of this ring is attributed to the geometry of the electrodes and to the distribution of the current density before the disruption.

The acceleration of hydrogen ions up to 35 MeV is observed in the z-pinch experiments on the GIT-12 generator at a 3 MA current and 0.6 MV driving voltage. High ion energies are obtained with a novel configuration of a deuterium gas-puff z-pinch. In this configuration, a hollow cylindrical plasma shell is injected around an inner deuterium gas puff to form a homogeneous, uniformly conducting layer between electrodes at the initial phase of z-pinch implosion. The stable implosion at the velocity up to 650 km s(-1) is important to deliver more current onto the z-pinch axis. Magnetohydrodynamic instabilities become apparent first at stagnation. After the disruptive development of m = 0 instabilities, similar to 20 ns pulses of high-energy photons, neutrons, electrons, and ions are observed. The average neutron yield is 2 x 10(12). The ion emission is characterized by various diagnostic techniques including those based on the usage of neutron-producing samples. When a large neutron-producing sample is placed onto the axis below a cathode mesh, the neutron yield is increased up to (1.1 +/- 0.3) x 10(13). Considering a similar to 130 kJ energy input into z-pinch plasmas and magnetic field, this implies the neutron production efficiency of similar to 10(8) neutrons per one Joule of the z-pinch energy.

Plasma in a pinch column, as produced by a plasma-focus discharge at the deuterium filling and the current intensity reaching 1 MA, was investigated at the total neutron yield reaching about 1010 per discharge. The use was made of neutron diagnostics, laser interferometry, soft X-ray measurements, optical emission spectroscopy, magnetic probes, as well as electron and ion measurements with the temporal, spatial, and energetic resolutions. The detailed studies showed the ordered toroidal, helical, and plasmoidal structures which could contain currents with poloidal and toroidal components and their associated magnetic fields. Their spontaneous transformations were explained by changes in a topology of magnetic field lines due to magnetic reconnections. A nonthermal acceleration of fast electrons and ions (producing hard X-rays and fusion neutrons, respectively) corresponded to: 1) the formation of plasmoids in the pinch column and 2) a decay of pinch constrictions and secondary plasmoids during the evolution of instabilities. A filamentary structure of the current flow could explain the high energy density and fast transformations of the magnetic energy into kinetic energy of electron and ion beams (reaching energy of hundreds of kiloelectronvolt). This paper summarizes the results obtained with the PF-1000 facility in 2009–2017, and describes the internal transformations in a dense plasma column during the evolution of MHD instabilities.

The paper summarizes important results of the recent experimental studies performed on the plasma-focus PF-1000 facility operated in Warsaw, Poland, mainly with the pure deuterium filling. Attention is focused on the evolution of toroidal and plasmoidal self-organized structures formed by internal closed currents inside the dense plasma column. The production of hard x-rays and neutrons corresponds with the formation and decay of plasmoids, in which charged particles can be accelerated effectively to high energies, during a release of the magnetic energy from current filaments of high energy density. It is noticed that the studies of laboratory fusion and cosmic plasmas deal with similar problems, e.g., the fast release of the magnetic energy in a form of high-energy charged particle beams.

This paper presents the discussion concerning the characteristics of the fast deuterons which have energy above 30 keV and are recorded during high-current plasma-focus (PF) discharges, by means of PM-355 plastic track-detectors placed inside ion pinhole cameras. The fast deuterons evoke D-D fusion reactions, mainly by a beam-target mechanism. The distribution of the magnetic field, which influences the trajectories of the recorded deuterons, is discussed. It is found that the fast deuterons are produced in various local sources and their motion is strongly influenced by a circular symmetry of the local magnetic field, which increases their radial shift with a decrease in their energy. The sources of these deuterons are probably located inside the plasmoids and in some local regions of the ring-shaped plasma structures. These ring-structures can be formed outside the dense pinch column, up to a radius of 5 cm. Global magnetic fields, associated with the total current flow in the PF discharge, have a weaker influence. The observed radial shift of the recorded fast deuterons is interpreted as a result of their deflection by magnetic fields which have opposite orientations of the azimuthal components, associated with the currents flowing in directions towards and from the applied ion detectors. The local sources of the recorded fast deuterons correspond to filamentary structures, in which the stored magnetic energy (having the local high density) can be released in induced electric fields accelerating the deuterons during the magnetic reconnections.

This paper concerns the evolution of internal structures and the neutron production in plasma-focus discharges performed in the presence of a permanent magnet (placed inside the anode front) and within a residual magnetic field (after the removal of this magnet). The initial magnetic field generated by this magnet prevented: (i) the effective compression of a dense pinch column, (ii) the formation of plasma organized structures, and (iii) the evolution of plasma instabilities. The experimental results have shown an increase in the initial magnetic field due to a magnetic dynamo effect in the presence of the permanent magnet, as well as in a series of shots performed after its removal. It was observed that the appearance of plasmoidal structures is necessary for the emission of fusion neutrons. A characteristic quasicylindrical plasma layer of the radius corresponding to the plasma lobule tops, which might be identified with a ring region of the acceleration of fast deuterons, was also observed.

The paper describes the behaviour of plasma within a MA plasma-focus with a novel electrode configuration, in which the anode and anti-anode were both equipped with conical tips. This configuration was applied to test the possibility of reducing the pinch axial dimensions during the radial compression of a current sheath. It made it possible to strengthen a dense plasma jet near the anode end, which ejected plasma into a bigger plasmoidal structure formed in the central pinch region. It did not allow forming an opposite anti-anode jet and stopped the axial motion of this structure. In plasma focus discharges with the deuterium filling, the decay of the anode jet and the corresponding plasmoid evolution were accompanied by the fusion-neutron production. Some results obtained with this configuration have also supported the hypothesis of the acceleration of fast electrons and ions at a release of the magnetic energy during magnetic reconnections in the organized dense plasma structures.

This paper considers regions of a fast deuteron production in a correlation with an evolution of ordered structures inside a pinch column of a mega-ampere plasma focus discharge. Ion pinhole cameras equipped with plastic PM-355 track-detectors recorded fast deuterons escaping in the downstream and other directions (up to 60 to the z-axis). Time-integrated ion images made it possible to estimate sources of the deuteron acceleration at the known magnetic field and deuteron energy values. The images of the fast deuterons emitted in the solid angle ranging from 0 to 4 showed two forms: central spots and circular images. The spots of 1–2 cm in diameter were produced by deuterons from the central pinch regions. The circular-shaped images of a radius above 3 cm (or their parts) were formed by deuterons from the region surrounding the dense pinch column. The ion pinhole cameras placed at angles above 20 to the z-axis recorded the ion spots only, and the ring-images were missing. The central region of the deuteron acceleration could be associated mainly with plasmoids, and the circular images could be connected with ring-shaped regions of the radius corresponding to tops of the plasma lobules outside the dense pinch column. The deuteron tracks forming ring-shaped images of a smaller (0.5–1) cm radius could be produced by deflections of the fast deuterons, which were caused by a magnetic field inside the dense pinch column.

Proton deflectometry is a promising way for mapping electric and magnetic fields in high-density and high-temperature plasmas, where an application of the classical methods (B-dot probes, Faraday rotation, and Zeeman splitting) is limited. It is based on the detection of a multi-MeV proton beam deflected in examined B-fields. In the past years, it has been successfully utilized in laser-generated plasmas for E-field and B-field measurements. Using our numerical code, we investigate the capabilities of proton deflectometry as a diagnostic method of MA Z-pinches. We simulate proton trajectories propagating through typical Z-pinch B-fields in two fundamental experimental setups (radial and axial) in order to study synthetic images (deflectograms). We demonstrate where proton deflectometry might be beneficial for the Z-pinch research. We explain a formation of the key features of deflectograms, which give us information about a profile and strength of the Z-pinch B-fields. We introduce a BL parameter, denoting an effective B-field averaged along the deflected proton orbit and show its importance for the proton deflectometry.

Acceleration of high energy ions was observed in z-pinches and dense plasma foci as early as the 1950s. Even though many theories have been suggested, the ion acceleration mechanism remains a source of controversy. Recently, the experiments on the GIT-12 generator demonstrated acceleration of ions up to 30 MeV from a deuterium gas-puff z-pinch. High deuteron energies enable us to obtain unique information about spatial, spectral and temporal properties of accelerated ions. In particular, the off-axis ion emission from concentric circles of a similar to 1 cm diameter and the radial lines in an ion beam profile are germane for the discussion of acceleration mechanisms. The acceleration of 30 MeV deuterons can be explained by the fast increase of an impedance with a sub-nanosecond e-folding time. The high (> 10 Omega) impedance is attributed to a space-charge limited flow after the effective ejection of plasmas from m = 0 constrictions. Detailed knowledge of the ion acceleration mechanism is used with a neutron-producing catcher to increase neutron yields above 10^13 at a current. of. 2.7 MA.