Deuterium gas-puff z-pinches are researched primarily as efficient sources of DD fusion neutrons. The first experiment with a deuterium gas jet was carried out in 1978 (Shiloh et al 1978 Phys. Rev. Lett. 40 515518). Since then, several D2 gas-puff experiments have been performed on various pulsed-power generators. The highest, so far published, DD neutron yields of 4x1013 were observed on the Z machine at Sandia National Laboratories around 2005 (Coverdale et al 2007 Phys. Plasmas 14 022706). More recently, z-pinch experiments with a plasma-shell on a deuterium gas puff were carried out on the GIT-12 higher-impedance pulsed-power generator at 3 MA currents. On GIT-12, unique results were high neutron and ion energies, which approached 60 MeV. Comparison of deuterium gas-puff experiments on different generators allows the identification of the parameters essential for optimizing neutron production. These parameters include the optimal mass, preionization, short deuterium gas-injection time, and zippering towards a cathode. Neutron yields appear to depend not only on a current, but also on other parameters of a generator, such as an impedance and the energy stored in a capacitor bank. Our conclusions regarding the optimal conditions were tested on the Hawk generator (NRL, Washington, DC). At a current of 0.7 MA, Hawk accelerated deuterons up to 15 MeV producing one neutron pulse with the yield of the order of 1010 and a broad energy spectrum in the axial and radial direction. These results show that ion acceleration mechanisms in deuterium gas-puff z-pinches could be very efficient and attractive, with a variety of potential applications in high-energy-density physics, materials science, and controlled thermonuclear fusion research.

The analysis of Z-pinch implosion dynamics plays one of the most important roles in the study of pulsed power discharges. At the same time, it is difficult to determine the density distribution together with the current density (current coupling to the imploding layer) to provide more detailed information about the dynamics. Numerical simulations can now provide high-resolution results that are almost unattainable in experiments. The challenge, however, is to obtain reliable results that are close enough to the experimental data to describe individual physical phenomena. In this paper, we show that it is possible to use a combination of experimental data and magnetohydrodynamic (MHD) simulations to verify and identify the physical processes during the stagnation of a Z-pinch. We focus on the analysis of the density profile from experimental data of the mega-ampere plasma focus PF-1000 and its reconstruction using an extended MHD code. Thanks to multi-frame interferometry, we recorded a total of 29 interferometric images of two shots, each in a 200 ns time window around the pinch phase. We were then able to obtain density profiles and observe the reflection of the shock wave from the axis. By the appropriate choice of initial conditions and boundary values in the simulation, we were able to obtain reasonable agreement with the experimental values. We also evaluated the possible shortcomings of the 1D simulation, such as mass loss and current flow at the periphery.

This paper presents the filamentary structure of the pinched column in a smaller plasma focus device filled with deuterium. The deflections were observed using schlieren and differential interferometry techniques. The observed filaments have a transverse diameter of 40–200 μm, which could be interpreted based on the electric current hypothesis as local concentrations of electric current. The evolution of filaments was compared with global structures recorded by extra ultraviolet frames. These results provide a basis for considering the possibility of a filamentary composition of the poloidal current in compact structures. The model of filaments with a helical shape of electrical current may be able to explain the central narrow and dense cord in the axis of the column, the different lifetimes of the structures, and the submillimeter sources of fast electrons and ions.

In this contribution, we present deuterium implosion experiments on the PFZ-200 plasma focus. The experiments are performed with the initial deuterium gas pressure in a range of 280–360 Pa and the capacitor battery charging voltage of 18 kV corresponding with 2.6 kJ of electrical energy and a current of about 200 kA at the maximum of plasma compression. Instabilities and subsequent disruptions of the imploded plasma generate electric fields that accelerate deuterons. The beam-target nuclear fusion reactions of the accelerated deuterons lead to neutron emission with a yield of $\sim 10^{8}$ per single shot. The imploded plasma is investigated with the help of a comprehensive diagnostic system including a multiframe fast x-ray camera and laser imaging (interferometry and schlieren). The novel modification of the schlieren diagnostics allows us to detect different laser-path-integrated density gradients. The experimental results are compared with simulated schlieren images of assumed electron density profiles.

Fifteen-frames interferometric diagnostics at the PF-1000 facility was enhanced by adding four frames of the schlieren diagnostics and by splitting of four channels in the optical delay line. This setup enabled the visualization of gradients in the plasma density perpendicular to the direction of the diagnostic laser beam, and their relationship with larger structures visualized by using laser interferometry. The schlieren pictures showed filamentary structures of submillimeter 200–300 μm diameter in shots performed with pure deuterium filling. Filaments were observed in a thin (millimeter-thick) lateral-boundary layer, in lobules, and in internal fast transforming regions of the dense plasma column. Their high-density gradients and location in the regions of recorded (or inferred) currents indicated local concentrations of the magnetic field and current distribution. Millimeter- and submillimeter-size sources of fast charged particles, which were identified in the recorded ion pinhole pictures, have been conjectured to be a manifestation of high local concentrations of the magnetic energy.

The neutron and x-ray production is investigated in various pulse-power devices for a deeper understanding of the ion and electron acceleration mechanisms and the application of pulsed neutron sources. We present the extensive results from an anode shape experiment carried out on the PFZ-200 plasma focus device. The various shapes of anodes were tested, including cylinders, tapers, or rounded tips. The experimental shots with a peak current above 200 kA were performed in pure deuterium working gas at 280–600 Pa pressure to obtain maximal neutron yield for each anode shape. The average neutron yields are in the range of (1–2) ×108 neutrons/shot. Outstanding findings about x-ray emission were obtained with the group of tapered anode tips. Using the scintillation detectors shielded by 5 cm thick lead bricks, we obtained the hard x-ray signals with photons exceeding 600 keV energy. Such relatively high x-ray energy indicates the optimized conditions for electron and ion acceleration. At the same time, the individual shots have been well reproducible. Therefore, we were able to study plasma dynamics with the schlieren images taken at different times at different shots.

Ion deflectometry is a diagnostic method for measuring electric and magnetic fields. It is used in high energy density plasmas but can also probe vacuum environments. Studied fields deflect trajectories of probing ion beams, and the measured ion displacements correspond to the line integrals of the deflecting fields. Ion deflectometry/radiography is widely used in laser-produced plasmas, but its development in z-pinches was difficult due to an unavailable source of MeV ion beams. We developed a unique way of utilizing a multi-MeV deuteron source for z-pinch-driven ion deflectometry. Current and B-field measurements were performed successfully on a 3-MA z-pinch. In order to extend the applicability of the ion deflectometry, MeV ion sources were investigated on other sub-MA devices in the configurations of the plasma focus and hybrid x-pinch.

The two conical electrodes formed a hybrid X-pinch configuration for the first time in a small dense plasma focus PFZ-200 with a peak current exceeding 200 kA and a rise time of around 1.8 μs. The experimental shots were performed in deuterium gas to achieve D-D fusion reactions with the primary goal of characterizing and localizing the proton source in the plasma between electrodes. For this purpose, we used a set of nuclear detectors CR-39 placed on the z-axis and in the radial direction. The detectors were shielded by aluminum foil with a thickness of 70 μm, corresponding to the energy of detected protons above 2.75 MeV. Furthermore, we evaluated the neutron yield by the silver activation counter and the neutron production time by the scintillation time-of-flight detectors. The discharges were visualized using the fast XUV camera (MCP) and the laser schlieren technique.

Filament-like structures were observed during discharges in a small 3-kJ plasma focus device operated with pure deuterium. These structures were recorded by means of two different laser diagnostic techniques: a schlieren system and a differential laser interferometry. They present the novel fine-scale (submillimeter) plasma structures recorded during the radial implosion, at the pinch stagnation, at the development of instabilities, and during a decay of the dense plasma column, when hard x-rays and fusion-produced neutrons were generated. The temporal uncertainty of these observations was about 2 ns, and the spatial one amounted to 40 μm. The filamentation seems be a natural and spontaneous process which occurs in high-current, hot, and dense plasmas produced in plasma focus devices. The observed filaments have usually longitudinal and/or azimuthal orientations. Their higher plasma density and appearance in regions of the measured and assumed current flows can be interpreted as the formation of plasma-current filaments with concentrated magnetic energy. These filamentary effects should be studied due to their possible role during the evolution of instabilities and the formation of small sources emitting fast electrons and ions.

Neutron diagnostics, not only in z-pinch fusion research, are used mainly as indirect diagnostics of ions. The energy spectrum of produced neutrons corresponds to the velocity distribution of fusing ions and can be reconstructed by a backward technique using fast scintillation detectors that record neutron time-of-flight (nTOF) signals. However, the backward technique has some limitations. Only the neutron spectrum, not the ion distribution, can be reconstructed (basic TOF method). In the case of using several nTOF signals in two opposite directions (extended TOF method [1]), one can obtain the time-resolved spectrum of fusion neutrons or one component of the colliding ions $(\mathrm{E}_{\mathrm{d}}\cos^{2}(\theta))$ in maximum. It is the limit of the backward technique; the ion distribution cannot be revealed.

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.

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.

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.

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 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.

The paper discusses a possible energy transformation that leads to the acceleration of fast ions and electrons. In plasma-focus discharges that occur during deuterium filling, which have a maximum current of about 1 MA, the accelerated deuterons produce fast fusion neutrons and fast 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 self-organization 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.

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 × 1012 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.

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.

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 × 1013 and 1011, respectively. Accelerated deuterons produced one 20 ns (full width at half maximum) pulse of fast neutrons via D(d, n)3He and other nuclear reactions. A maximum neutron output of (1.0 ± 0.2) × 1012 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 ≳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.