Imploding Plasma Research Articles

X-ray compressed ultrafast photography under the constraint of time-integrated-image for X-pinch

This paper presents a time-integrated-image-constrained X-ray compressed ultrafast photography system for comprehensive measurement of the aluminum X-pinch evolution process. The system incorporates a pinhole and a scintillator to convert the X-ray video to fluorescence video. The fluorescence video is recorded by a compressed ultrafast photography channel and a time-integrated imaging channel respectively. By recording the fluorescence video instead of the self-emission visible light video, the long tailing effect can be significantly inhibited, which is more conducive to the image reconstruction of the compressed ultrafast photography channel. In addition, by combining the time-integrated imaging channel, the reconstruction quality is significantly improved. Based on the “Qin-I” pulsed power facility, we recorded the continuous two-dimensional X-ray evolution process of aluminum X-pinch for the first time. Our results contribute to deep insights into the temporal-spatial distribution of the X-pinch imploding plasma. It can also offer direction for the assessing of X-ray backlight photography applications at the X-pinch facility.

Snapshot compressive imaging at 855 million frames per second for aluminium planar wire array Z-pinch.

This paper present a novel, integrated compressed ultrafast photography system for comprehensive measurement of the aluminium planar wire array Z-Pinch evolution process. The system incorporates a large array streak camera and embedded encoding to improve the signal-to-noise ratio. Based on the "QiangGuang-I" pulsed power facility, we recorded the complete continuous 2D implosion process of planar wire array Z-Pinch for the first time. Our results contribute valuable understanding of imploding plasma instabilities and offer direction for the optimization of Z-Pinch facilities.

Impact of strong magnetization in cylindrical plasma implosions with applied B-field measured via x-ray emission spectroscopy

Magnetization is a key strategy for enhancing inertial fusion performance, though accurate characterization of magnetized dense plasmas is needed for a better comprehension of the underlying physics. Measured spectra from imploding Ar-doped D2-filled cylinders at the OMEGA laser show distinctive features with and without an imposed magnetic field. A multizone spectroscopic diagnosis leads to quantitative estimates of the plasma conditions, namely revealing a 50% core temperature rise at half mass density when a 30-T seed field is applied. Concurrently, experimental spectra align well with predictions from extended-magnetohydrodynamics simulations, providing strong evidence that the attained core conditions at peak compression are consistent with the impact of a 10-kT compressed field. These results pave the way for the validation of magnetized transport models in dense plasmas and for future magnetized laser implosion experiments at a larger scale. Published by the American Physical Society 2024

Efficiency of Conversion of the Magnetic Energy into Z-Pinch Radiation of Nested Arrays of Mixed Composition at the Angara-5-1 Facility

The results of experiments on the study of the generation of high-power pulsed soft X-ray (SXR) emission with a photon energy higher than 100 eV (in the spectral range with wavelengths λ shorter than 120 Å) during the plasma implosion of nested arrays of mixed composition with different ratios of array radii carried out on a pulse power facility Angara-5-1 with a discharge current level of up to 3.5 MA are presented. The outer array consisted of fibers of a substance with a low atomic number (plastic), and the inner array consisted of a substance with a high atomic number (tungsten, W). In the case of nested arrays of this design, a significant increase in the peak SXR power was obtained compared to single tungsten arrays with the same parameters as for the tungsten array in the inner array. By optimizing the linear mass of the outer array and the ratio of array radii, powerful SXR pulses were prepared with a high pulse power up to 18 TW, pulse energy of ~140 kJ and short pulse duration of ~5 ns. It is shown that by optimizing the linear mass of the outer array (fiber array) it is possible to achieve ~90% conversion of the electromagnetic energy pumped into the vicinity of the array into X-ray emission pinch energy. In this case, the fraction of the kinetic energy of the plasma implosion into the emission energy is not higher than 30%. In shots optimal over the output SXR power, an increase in the fraction of the X-ray emission energy in the spectral range of λ ∈ (30, 40) Å was recorded that is 30–100% than that in single tungsten arrays with similar parameters.

Role of initial conditions in plasma-current coupling of gas-puff Z-pinches

Azimuthal magnetic field measurements obtained during the implosion phase of an oxygen gas-puff Z-pinch on a 500 kA peak current and 180 ns rise time linear transformer driver are presented. While a fraction of the driver current was measured within the imploding plasma, key initial conditions were found to significantly impact the delivery of current to the plasma load. The electrode geometry was modified to assist the initial dielectric breakdown and resulted in improved shot reproducibility. Optimization of the gas injection plenum pressure and timing resulted in an increase in the current coupling parameter, defined as the ratio of the measured value of Bθ to the expected value, from 50% to 75%. The degree of radial expansion of the gas puff in the load region, which is suspected to lead to the observed current loss during the implosion, was reduced by shortening the valve opening duration. Additionally, a pre-embedded axial magnetic field of up to 0.2 T was found to have no significant impact on the plasma-current coupling of the oxygen implosions.

Observation of Fast Current Redistribution in an Imploding Plasma Column.

Spectroscopic measurements of the magnetic field evolution in a Z-pinch throughout stagnation and with particularly high spatial resolution reveal a sudden current redistribution from the stagnating plasma (SP) to a low-density plasma (LDP) at larger radii, while the SP continues to implode. Based on the plasma parameters it is shown that the current is transferred to an increasing-conductance LDP outside the stagnation, a process likely to be induced by the large impedance of the SP. Since an LDP often exists around imploding plasmas and in various pulsed-power systems, such a fast current redistribution may dramatically affect the behavior and achievable parameters in these systems.

Experimental study of the mechanism of prepulse current on Z-pinch plasma using Faraday rotation diagnosis.

The prepulse current is an effective way to optimize the load structure and improve the implosion quality of the Z-pinch plasma. Investigating the strong coupling between the preconditioned plasma and pulsed magnetic field is essential for the design and improvement of prepulse current. In this study, the mechanism of the prepulse current on the Z-pinch plasma was revealed by determining the two-dimensional magnetic field distribution of preconditioned and nonpreconditioned single-wire Z-pinch plasma with a high-sensitivity Faraday rotation diagnosis. When the wire was nonpreconditioned, the current path was consistent with the plasma boundary. When the wire was preconditioned, the distributions of current and mass density presented good imploding axial uniformity, and the imploding speed of the current shell was higher than that of the mass shell. In addition, the mechanism of the prepulse current suppressing the magneto-Rayleigh-Taylor instability was revealed, which formed a sharp density profile of the imploding plasma and slowed the shock wave driven by the magnetic pressure. This discovery is essential and instructive for the design of preconditioned wire-array Z-pinch experiments.

Improvement of Faraday Rotation and Its Application in Preconditioned Single-Wire Z-Pinch Plasma

Magnetic field measurement in Z-pinch plasma has drawn considerable interest because it is a key problem in the research of dynamic behaviors and the coupling of magnetic field and plasma. In this study, the measurement sensitivity of the rotation angle in Z-pinch plasma was improved by considering the second polarization effect caused by optical elements. The magnetic field distribution of preconditioned single-wire Z-pinch plasma was determined by this method. The Faraday rotation of the 1064-nm probe laser was measured by the beam-splitting polarization method when the plasma acted as a Faraday medium. A second polarization effect on the rotation angle was realized by taking the different transmittances and reflectivities of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$p$</tex-math> </inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$s$</tex-math> </inline-formula> components of the beam splitters into account. This configuration doubled the sensitivity of the Faraday rotation measurement. The diagnostic errors produced by the techniques and data processing were quantified. The early stage of the preconditioned imploding plasma was monitored, and the distributions of areal electron density, Faraday rotation, and magnetic field were measured. The experimental results demonstrated that the current distribution can be divided into three parts by two dense shells. The majority of the current was distributed outside the dense shell which existed inside the coronal plasma. In addition, only a small amount of current was distributed in the wire core.

X-ray imaging and radiation transport effects on cylindrical implosions.

Magnetization of inertial confinement implosions is a promising means of improving their performance, owing to the potential reduction of energy losses within the target and mitigation of hydrodynamic instabilities. In particular, cylindrical implosions are useful for studying the influence of a magnetic field, thanks to their axial symmetry. Here, we present experimental results from cylindrical implosions on the OMEGA-60 laser using a 40-beam, 14.5 kJ, 1.5ns drive and an initial seed magnetic field of B0 = 30T along the axes of the targets, compared with reference results without an imposed B-field. Implosions were characterized using time-resolved x-ray imaging from two orthogonal lines of sight. We found that the data agree well with magnetohydrodynamic simulations, once radiation transport within the imploding plasma is considered. We show that for a correct interpretation of the data in these types of experiments, explicit radiation transport must be taken into account.

Applying Thomson scattering to diagnosing turbulent density and velocity fluctuations in a gas-puff z-pinch

The electron plasma wave feature (EPW) in the time-resolved Thomson scattering spectrum is used to obtain the local electron density in imploding high energy density gas-puff z-pinch plasmas. The optical setup was optimized to allow the relatively weak EPW feature obtained from 1 MA imploding neon gas-puff z-pinches to be seen above the continuum emission as well as the brighter ion acoustic wave (IAW) feature. Using a frequency-doubled Nd:YLF laser (E = 10 J, λ = 526.5 nm, Δt= 2.3 ns, spot size ∼ 250 μm) and two visible light streak cameras, we determined the average electron density in the imploding plasma sheath 40 ns prior to stagnation to be ne=2.5×1018/cm3. At pinch time, it reached ne=1.7×1019/cm3. The electron temperature during implosion measured via the IAW (approximately 50 eV) was four times lower than the implosion electron temperature measured via the EPW (approximately 200 eV), assuming that neither feature is affected by turbulent fluctuations in the plasma. In order for the electron temperatures inferred from the EPW and IAW spectral features to be self-consistent, we find that it is necessary to include velocity fluctuations in the analysis of the IAW feature peaks and corresponding density fluctuation in the peak widths of the EPW feature.

Neutron yield as a measure of achievement nuclear fusion using a mixture of deuterium and tritium isotopes

A mixture of deuterium (D) and tritium (T) is the most likely fuel for fusion reactors and hence the D(d, n)3He and T(d, n)4He fusion reactions are the ones that will fire fusion reactors in the future. Both of the fusion reactions produce neutrons which escape form the reactor core and can be measured directly outside the core. As the neutrons have large mean free path and neutral charge, they readily carry information about the burning fusion plasma from inside to outside the reactor core without being affected by electric ormagnetic fields. From the produced neutrons of the D(d, n)3He and T(d, n)4He fusion reactions, the neutron yield of each reaction and the neutron yield ratio of the two reactions are calculated. This ratio is of critical importance for controlling the fusion fuel burning which is a high priority issue for fusion reactor performance. Because it is very difficult to measure this ratio experimentally, accurate theoretical calculations of the neutron yield ratio besides the related deuterium and tritium energy spectra in the fusion plasma are needed. In the present work, neutron yields of the D(d, n)3He and T(d, n)4He fusion reactions have been calculated using the MCUNED, the ENEA-JSI, the DDT codes and the Geant4 toolkit. The related deuterium and tritium energy spectra have been calculated by the MCUNED code. The relation between the ion temperature and the neutron yield in the imploded fusion plasma is discussed. Calculations are compared to the available experimental data. Comparing to the other codes, the spectrum of the fusion neutrons simulated by the MCUNED is the only one that fit the experimental data.

Measurements of the imploding plasma sheath in triple-nozzle gas-puff z pinches

Gas-puff z-pinch implosions are characterized by the formation of a dense annular plasma shell, the sheath, that is driven to the axis by magnetic forces and therefore subject to the magneto-Rayleigh–Taylor instability. Here, the conditions within these sheaths are measured on the 1-MA COBRA generator at Cornell University [Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)] for various gas species and initial fill densities. The gas-puff loads are initialized by a 7 cm diameter triple-nozzle gas valve assembly with concentric outer and inner annular nozzles and a central gas jet. Thomson scattering and laser interferometry provide spatially resolved flow, temperature, and electron density profiles midway through the implosion, while extreme ultraviolet pinhole cameras record the evolution of the plasma column and photoconducting diodes measure x-ray emission. Analysis of the scattering spectra includes a means of discriminating between thermal and non-thermal broadening to test for the presence of hydrodynamic turbulence. Two types of sheath profiles are observed, those with sharp discontinuities at the leading edge and those with smooth gradients. In both cases, non-thermal broadening is generally peaked at the front of the sheath and exhibits a characteristic decay length that roughly scales with the sheath ion mean free path. We demonstrate that this non-thermal broadening term is inconsistent with laminar velocity gradients and is more consistent with dissipative turbulence driven by unstable plasma waves in a collisionless shock. The resulting differences in sheath profile are then set by the sheath ion collisionality in a manner consistent with recent 1D kinetic simulations [Angus et al., Phys. Plasmas 28, 010701 (2021)].

Propagation of dipole structure in an inhomogeneous-density plasma using two-dimensional particle-in-cell simulation

We have investigated the propagation of a magnetic dipole assuming a simple model of forward and return flow of fast electrons under a condition of plasma-density inhomogeneity by a particle-in-cell simulation. An exact propagating depiction of the dipolar structure is given under the framework of a simplified ‘electron magnetohydrodynamic’ fluid model (Yadav et al 2008 Phys. Plasmas 15 062308; Yadav et al 2009 Phys. Plasmas 16 040701; Yadav and Das 2010 Phys. Plasmas 17 052306) in a dense plasma. We reproduce this structure in our kinetic calculations. The results indicate that, with a steep plasma density gradient, the structure evolves rapidly toward plasma in a process involving shock formation and rapid dissipation of beam energy, which is consistent with the fluid simulations. In addition, new features are also reported, such as the pinching of the two dipole lobes to form a very strong shear layer, which develops into a Kelvin–Helmholtz instability. The magnetic energy is rapidly converted to kinetic energy of electrons leading to additional plasma heating in inhomogeneous regions, such as the core region in an imploded plasma.

Mapping of azimuthal B-fields in Z-pinch plasmas using Z-pinch-driven ion deflectometry

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.

K-shell radiation and neutron emission from z-pinch plasmas generated by hybrid gas-puff implosions onto on-axis wires

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 60 MeV [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 60 MV 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 (2 cm), narrow (∼mm) column radiating in the Al K-shell lines. With the single Al wire of 80 μm diameter, the K-shell x-ray output reached 5.5 ± 0.8 kJ in a 0.6 ± 0.1 TW peak power and 7 ± 1 ns pulse. The higher K-shell yield of 12 ± 2 kJ and peak K-shell power of 0.7 ± 0.1 TW were achieved with four 38 μm diameter Al wires. (Their cross section formed the corners of a square with 1 mm side.) The presence of the wires on the axis significantly suppressed ion acceleration and neutron production. Deuterium-deuterium (DD) neutron yields of about 1.2 × 1011 were 20 times smaller than the yields produced in shots without any wire. The DD neutron yield was increased up to 4.5 × 1011 when the Al wire was replaced by a fiber from deuterated polyethylene. A characteristic feature of the experiments with the (CD2)n fiber was a rapid expansion with the velocity approaching 900 km/s.

Implosion dynamics of triple-nozzle gas-puff z pinches on COBRA

Experiments on the 1-MA, 220-ns COBRA generator at Cornell University [J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)] were conducted to provide detailed measurements of structured cylindrical gas-puff z pinches. In the experiments, a 7 cm diameter triple-nozzle gas valve assembly with concentric outer and inner annular nozzles and a central gas jet initialize the z-pinch load with various working gases, radial density profiles, and externally applied axial magnetic fields. Planar laser-induced fluorescence provides a measure of the initial neutral gas density of the load, while three-frame laser shearing interferometry and multi-frame extreme ultraviolet (XUV) cameras reveal the formation and propagation of a magneto-Rayleigh–Taylor (MRT) unstable shock layer. Implosion trajectories are compared to simple, experimentally informed models and found to be in good agreement. Differences in the structure of the accelerating plasma sheath and evolution of the MRT instability are observed for different gas species and axial magnetic field strengths, correlating with differences in pinch uniformity and x-ray emission. The average instability growth is compared to linear MRT theory predictions using the instantaneous acceleration of the best-fit implosion models and characteristic instability wavelength, with the effective Atwood number and seed perturbation size as fit parameters. For high density argon center jets, ionization prior to the arrival of the imploding plasma sheath suggests a heating mechanism consistent with photoionization by XUV self-emission.

Developing a novel point X-ray source by regulating the formation of a micropinch with an imploding gas-puff plasma

Implosion mediated gas-puff hybrid X-pinch approach can control the timing, size and location of x-ray sources and can be operated repeatedly without manual reloading

Optimizing of Experimental Load of PFZ-200 Plasma Focus

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 ${3 \times 10^{8}}$ neutrons/shot. Measured pinch times were in the range from 1.65 to 1.85 $\mu \text{s}$ . The hollow round anode configuration performed the most stable neutron yields with a deviation under 20%.

Fast electron transport dynamics and energy deposition in magnetized, imploded cylindrical plasma.

Inertial confinement fusion approaches involve the creation of high-energy-density states through compression. High gain scenarios may be enabled by the beneficial heating from fast electrons produced with an intense laser and by energy containment with a high-strength magnetic field. Here, we report experimental measurements from a configuration integrating a magnetized, imploded cylindrical plasma and intense laser-driven electrons as well as multi-stage simulations that show fast electrons transport pathways at different times during the implosion and quantify their energy deposition contribution. The experiment consisted of a CH foam cylinder, inside an external coaxial magnetic field of 5 T, that was imploded using 36 OMEGA laser beams. Two-dimensional (2D) hydrodynamic modelling predicts the CH density reaches , the temperature reaches 920 eV and the external B-field is amplified at maximum compression to 580 T. At pre-determined times during the compression, the intense OMEGA EP laser irradiated one end of the cylinder to accelerate relativistic electrons into the dense imploded plasma providing additional heating. The relativistic electron beam generation was simulated using a 2D particle-in-cell (PIC) code. Finally, three-dimensional hybrid-PIC simulations calculated the electron propagation and energy deposition inside the target and revealed the roles the compressed and self-generated B-fields play in transport. During a time window before the maximum compression time, the self-generated B-field on the compression front confines the injected electrons inside the target, increasing the temperature through Joule heating. For a stronger B-field seed of 20 T, the electrons are predicted to be guided into the compressed target and provide additional collisional heating.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 2)’.

Progress and opportunities for inertial fusion energy in Europe.

In this paper, I consider the motivations, recent results and perspectives for the inertial confinement fusion (ICF) studies in Europe. The European approach is based on the direct drive scheme with a preference for the central ignition boosted by a strong shock. Compared to other schemes, shock ignition offers a higher gain needed for the design of a future commercial reactor and relatively simple and technological targets, but implies a more complicated physics of laser–target interaction, energy transport and ignition. European scientists are studying physics issues of shock ignition schemes related to the target design, laser plasma interaction and implosion by the code developments and conducting experiments in collaboration with US and Japanese physicists, providing access to their installations Omega and Gekko XII. The ICF research in Europe can be further developed only if European scientists acquire their own academic laser research facility specifically dedicated to controlled fusion energy and going beyond ignition to the physical, technical, technological and operational problems related to the future fusion power plant. Recent results show significant progress in our understanding and simulation capabilities of the laser plasma interaction and implosion physics and in our understanding of material behaviour under strong mechanical, thermal and radiation loads. In addition, growing awareness of environmental issues has attracted more public attention to this problem and commissioning at ELI Beamlines the first high-energy laser facility with a high repetition rate opens the opportunity for qualitatively innovative experiments. These achievements are building elements for a new international project for inertial fusion energy in Europe.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.