Ing. Jakub Malíř

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 mu m, 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.

We report on the results of point-projection ion deflectometry measurements from a mid-size university z-pinch experiment. A 1 MA 8 kJ LTD generator at the University of Michigan (called MAIZE) drove a hybrid x-pinch (HXP) with a deuterated polyethylene fiber load to produce a point-like source of MeV ions for backlighting. In these experiments, 2.7 MeV protons were generated by DD beam-target fusion reactions. Due to the kinematics of beam-target fusion, the proton energies were down-shifted from the more standard 3.02 MeV proton energy that is released from the center-of-mass rest frame of a DD reaction. In addition to the 2.7 MeV protons, strongly anisotropic beams of 3 MeV accelerated deuterons were detected by ion diagnostics placed at a radial distance of 90 mm from the x-pinch. Numerical reconstruction of experimental data generated by deflected hydrogen ion trajectories evaluated the total current in the vacuum load region. Numerical ion-tracking simulations show that accelerated deuteron beams exited the ion source region at large angles with respect to the pinch current direction.

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.

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 mu m. 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.

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 lm 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.

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.

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.