Gas-puff Implosions Research Articles

Simulations of nozzle gas flow and gas-puff Z-pinch implosions on the Weizmann Z-pinch

We present simulations of an oxygen gas puff Z-pinch on a University scale generator at the Weizmann Institute of Science. The work accounts for the detailed geometry of the nozzle, the initial neutral gas density distribution, and the subsequent implosion. The modeling results show significant improvement with data for the current at the time of stagnation in comparison with a previous effort [Rosenzweig et al., Phys. Plasmas 27, 022705 (2020)]. As a first step, we performed simulations of the flow of neutral diatomic oxygen from a plenum through a nozzle within a recessed cathode, across a gap, and into the anode with a recessed grounded honeycomb. These simulations show an agreement with the measured initial gas density profiles within the region not blocked by the recesses and accessible to visible measurements. The computed neutral gas flow profile serves as the initial condition for a radiation magnetohydrodynamic simulation of the implosion using the MACH2-TCRE code. By considering the specific details of the nozzle and chamber geometry, we find agreement with the measured current profile, including the inductive notch. The simulations predict that the plasma undergoes a strong pinch within the hidden anode recess. The simulations also predict the strongest radiation pulse occurs within the anode recess and at the time of the observed inductive notch.

Comparison of round and knife-edge-like cathodes on gas-puff implosions.

Preionization is believed to play an important role on the implosion of gas-puff Z pinches. Some experiments have used an external preionization source, e.g., UV light or electron beam. In contrast, other experiments rely completely on over voltage breakdown by the own generator's voltage pulse. However, this approach lacks shot-to-shot reproducibility since self-breakdown is mainly a stochastic process. In this work, we performed a systematic study on self-breakdown using two different cathode geometries: (i) a smooth, round cathode to provide a homogeneous electric field, (ii) a sharp, knife-edge-like geometry to enhance the electric field locally and eventually electron emission. The experiments were carried out on the Llampudken current generator, which provides a current pulse of ∼400kA amplitude and 200ns rise time (10%-90%). We implemented gated XUV imaging, filtered diodes and time-integrated x-ray imaging to obtain information about the implosion as well as the stagnation phase for the two cathode geometries. We found that erosion of the knife-edge cathode might be a serious problem, and we had to replace it every 15 shots. On the other hand, the round cathode lasted for the whole series of experiments. We also measured a more reproducible and larger peak current for the knife cathode. From the photo-conductive detectors we observed that even if the round cathode might present shots with higher x-ray yield compared to the knife cathode, dispersion is almost twice as large. Moreover, after a statistic analysis, it is demonstrated that the dispersion in the yield is due solely to differences imposed by the cathodes and not to variations in the driver, as no correlation was found between them. We found that in order to fit the experimental data with the snowplow model, only ∼60% of the total mass is compressed in the knife cathode while ∼20% for the round one, highlighting the importance of the cathode and preionization. Therefore, we conclude that the use of the knife cathode increases the reproducibility of the experiment in comparison with the round cathode.

Dependence of Plasma-Current Coupling on Current Rise Time in Gas-Puff Z-Pinches

Experiments measuring the azimuthal magnetic field of gas-puff Z-pinch implosions on two drivers of significantly different timescales are presented. Using a Zeeman-based spectroscopic technique, measurements of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$B_{\theta }$ </tex-math></inline-formula> are compared with calculated values using Ampere’s law and the load current measured by inductive probes to define the “plasma-current coupling.” Oxygen gas-puff implosions carried out on the compact experimental system for Z-pinch and ablation research (CESZAR) linear transformer driver (LTD) with a peak current of ~500-kA and ~180-ns rise time exhibited substantially lower plasma-current coupling when compared to implosions on the Weizmann Institute of Science (WIS) current driver (300-kA peak and 1600-ns rise), 48% ± 18% and 95% ± 6%, respectively. Potential causes for the significant differences in plasma-current coupling between the two drivers are examined. Shunted current across instability structures present in the CESZAR implosions is ruled out as a source of current loss. The radial charge state distribution is discussed, and a trailing plasma composed of higher charge states is hypothesized to carry a significant portion of the driver current.

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)].

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.

Azimuthal magnetic field distribution in gas-puff Z-pinch implosions with and without external magnetic stabilization.

An experimental study of the magnetic field distribution in gas-puff Z pinches with and without a preembedded axial magnetic field (B_{z0}) is presented. Spatially resolved, time-gated spectroscopic measurements were made at the Weizmann Institute of Science on a 300 kA, 1.6 μs rise time pulsed-power driver. The radial distribution of the azimuthal magnetic field, B_{θ}, during the implosion, with and without a preembedded axial magnetic field of B_{z0}=0.26T, was measured using Zeeman polarization spectroscopy. The spectroscopic measurements of B_{θ} were consistent with the corresponding values of B_{θ} inferred from current measurements made with a B-dot probe. One-dimensional magnetohydrodynamic simulations, performed with the code trac-ii, showed agreement with the experimentally measured implosion trajectory, and qualitatively reproduced the experimentally measured radial B_{θ} profiles during the implosion when B_{z0}=0.26T was applied. Simulation results of the radial profile of B_{θ} without a preembedded axial magnetic field did not qualitatively match experimental results due to magneto-Rayleigh-Taylor (MRT) instabilities. Our analysis emphasizes the importance of MRT instability mitigation when studying the magnetic field and current distributions in Z pinches. Discrepancies of the simulation results with experiment are discussed.

Implosion mediated gas-puff hybrid X-pinch

Imploding gas-puff z-pinches are efficient sources of intense x-rays. However, the unpredictable size, number, and location of hot spots along the pinch axis make them unsuitable for applications such as point-projection radiography. Crossed wire or hybrid X-pinches, on the other hand, provide small, localized sources but require fast current rise times and must be reloaded after each pulse. In this Letter, we present the first experimental demonstration of an implosion mediated gas-puff hybrid x-pinch in which an annular gas-puff implosion is shaped and focused onto a pair of conical electrodes separated by a small gap. This setup produces an intense x-ray source with controllable timing that can be operated with or without a central wire. Because the gas-puff implosion time determines the formation of the pinch, this configuration can continue to operate efficiently as a point x-ray source for longer current pulses than conventional x-pinches.

Simulations of Recent Argon Gas-Puff Implosions on Z With Xe and Kr Dopants

An intriguing result is that a small fraction of Xe (0.8% by number in the center jet) can have a significant impact on the emitted K-shell radiation in an Ar gas-puff implosion. This result was recently obtained from a pair of experiments on Sandia National Laboratories Z machine. These shots were Z2603 (with Xe) and Z2605 (without Xe). The pair had similar initial density profiles (outer shell, inner shell, and center jet) but one had the small aforementioned percentage of Xe dopant in the center jet. The resultant Ar K-shell yield was considerably reduced from 373 ± 9% to 129 ± 9% kJ without an analogous change in the total emitted power or total radiation yield. Furthermore, the K-shell power pulse for Z2603 reduced to a broad bimodal, 7 terra watts (TW) pulse, much smaller than the 32 TW conventional single peak for Z2605. Radiation-magnetohydrodynamic simulations presented herein qualitatively reproduce major trends from experiments. Detailed analyses of the implosion dynamics, temperature and density profiles, and emitted radiation agree qualitatively with the previous study. The effect of varying the xenon fraction on K-shell radiation suggests that enhanced Xe concentration reduces K-shell emission by diminishing K-Shell radiation from the central jet.

Effect of the gas distribution on implosion dynamics and the K-shell yield of the neon gas-puffs with the outer plasma shell

The experiments have been carried out on the GIT-12 generator to characterize the effect of the anode and cathode meshes installed on the path of the gas flow on the initial gas density distribution, the gas-puff implosion dynamics, and the resulting radiative characteristics of the Ne K-shell plasma radiation source (PRS) operated in the microsecond implosion regime. The experiments showed that the obstacles located directly on the way of the main gas flow have the greatest influence on the gas distribution in the interelectrode gap of the generator. At all other factors being the same, the presence or absence of the anode and cathode meshes changed significantly the implosion dynamics and radiative characteristics of the Ne K-shell PRS. In our experiments, we observed two-fold decrease in the K-shell radiation yield and more than four-fold decrease in the K-shell radiation power. These observations should be taking into account in development of the gas-puff loads for Z-pinch experiments.

Enhancing the x-ray output of a single-wire explosion with a gas-puff based plasma opening switch

We present experiments performed on the 1 MA COBRA generator using a low density, annular, gas-puff z-pinch implosion as an opening switch to rapidly transfer a current pulse into a single metal wire on axis. This gas-puff on axial wire configuration was studied for its promise as an opening switch and as a means of enhancing the x-ray output of the wire. We demonstrate that current can be switched from the gas-puff plasma into the wire, and that the timing of the switch can be controlled by the gas-puff plenum backing pressure. X-ray detector measurements indicate that for low plenum pressure Kr or Xe shots with a copper wire, this configuration can offer a significant enhancement in the peak intensity and temporal distribution of radiation in the 1–10 keV range.

Radiative properties of argon gas puff z-pinch implosions on COBRA

Spatially resolved and time-integrated x-ray spectroscopy, combined with modeling of the spectra with detailed radiation kinetics and transport, is a powerful method to study the conditions in a hot moving plasma. K-shell argon spectra were measured from gas puff implosions with different center jet masses on the 1 MA COBRA generator at Cornell University. The outer to inner plenum pressures (1 and 3 psia, respectively) were the same for all shots producing an outer to inner mass ratio of 1:1. This paper uses non-local thermodynamic equilibrium kinetic modeling to infer the ion density, electron temperature, K-shell radiating mass, and K-shell powers from stagnating argon gas puff z-pinch implosion. We find that the implosions with a center jet produced bright spot regions of plasma with higher temperature and density than those without a jet.

Investigating the effect of adding an on-axis jet to Ar gas puff Z pinches on Z

Double-shell Ar gas puff implosions driven by 16.5 ± 0.5 MA on the Z generator at Sandia National Laboratories are very effective emitters of Ar K-shell radiation (photon energy &amp;gt;3 keV), producing yields of 330 ± 9% kJ [B. Jones et al., Phys. Plasmas 22, 020706 (2015)]. Previous simulations and experiments have reported dramatic increases in K-shell yields when adding an on-axis jet to double shell gas puffs for some configurations. We report on a series of experiments on Z testing Ar gas puff configurations with and without an on-axis jet guided by 3D magneto-hydrodynamic (MHD) simulations. Adding an on-axis jet was found to significantly improve the performance of some, but not all, configurations. The maximum observed K-shell yield of 375 ± 9% kJ was produced with a configuration that rapidly imploded onto an on-axis jet. A dramatic difference was observed in the plasma conditions at stagnation when a jet was used, producing a narrower stagnation column in experiments with a higher density but relatively lower electron temperature. The MHD simulations accurately reproduce the experimental measurements. The conversion efficiency for electrical energy delivered to the load to K-shell x-rays is estimated to be ∼12.5% for the best-performing configuration, similar to the best results from experiments at smaller facilities.

The effect of gradients at stagnation on K-shell x-ray line emission in high-current Ar gas-puff implosions

Argon gas puffs have produced 330 kJ ± 9% of x-ray radiation above 3 keV photon energy in fast z-pinch implosions, with remarkably reproducible K-shell spectra and power pulses. This reproducibility in x-ray production is particularly significant in light of the variations in instability evolution observed between experiments. Soft x-ray power measurements and K-shell line ratios from a time-resolved spectrum at peak x-ray power suggest that plasma gradients in these high-mass pinches may limit the K-shell radiating mass, K-shell power, and K-shell yield from high-current gas puffs.

A Renewed Capability for Gas Puff Science on Sandia's Z Machine

A comprehensive gas puff capability is being developed on the Z pulsed power generator. We describe the methodology employed for developing a gas puff load on Z, which combines characterization and modeling of the neutral gas mass flow from a supersonic nozzle, numerical modeling of the implosion of this mass profile, and experimental evaluation of these magnetic implosions on Z. We are beginning a multiyear science program to study gas puff z-pinch physics at high current, starting with an 8-cm diameter double-shell nozzle, which delivers a column of Ar gas that is imploded by the machine's fast current pulse. The initial shots have been designed using numerical simulation with two radiation-magnetohydrodynamic codes. These calculations indicate that 1 mg/cm should provide optimal coupling to the driver and 1.6:1 middle:outer shell mass ratio will best balance the need for high implosion velocity against the need to mitigate the magnetic Rayleigh–Taylor instability. The models suggest 300–500-kJ Ar K-shell yield should be achievable on Z, and we report an initial commissioning shot at lower voltage in which 250 kJ was measured. Future experiments will pursue optimization of Ar and Kr K-shell X-ray sources, study fusion in deuterium gas puffs, and investigate the physics of gas puff implosions including energy coupling, instability growth, and radiation generation.

Study of the stability of Z-pinch implosions with different initial density profiles

Stability of metal-puff Z pinches was studied experimentally. Experiments were carried out on a facility producing a load current up to 450 kA with a rise time of 450 ns. In a metal-puff Z pinch, the plasma shell is produced due to evaporation of the electrode material during the operation of a vacuum arc. In the experiment to be reported, a single-shell and a shell-on-jet pinch load with magnesium electrodes were used. Two-dimensional, 3 ns gated, visible-light images were taken at different times during the implosion. When the shell was formed from a collimated plasma flow with small radial divergence, Rayleigh–Taylor (RT) instability typical of gas-puff implosions was recorded. The RT instability was completely suppressed in a mode where the initial density distribution of the shell approached a tailored density profile [A. L. Velikovich et al., Phys. Rev. Lett. 77, 853 (1996)].

Initial magnetic field compression studies using gas-puff Z-pinches and thin liners on COBRA

This magnetic compression of cylindrical liners filled with DT gas has promise as an efficient way to achieve fusion burn using pulsed-power machines. However, to avoid rapid cooling of the fuel by transfer of heat to the liner an axial magnetic field is required. This field has to be compressed during the implosion since the thermal insulation is more demanding as the compressed DT plasma becomes hotter and its volume smaller. This compression of the magnetic field is driven both by the imploding liner and plasma. To highlight how this magnetic field compression by the plasma and liner evolves we have separately studied Z-pinch implosions generated by gas puff and liner loads. The masses of the gas puff and liner loads were adjusted to match COBRA's current rise times. Our results have shown that Ne gas-puff implosions are well described by a snowplow model where electrical currents are predominately localized to the outer surface of the imploding plasma and the magnetic field is external to the imploding plasma. Liner implosions are dominated by the plasma ablation process on the inside surface of the liner and the electrical currents and magnetic fields are advected into the inner plasma volume; the sharp radial gradient associated with the snowplow process is not present.

Gas-puff liner implosion in the configuration with helical current return rods

Results of experiments with double-shell gas-puff liners carried out on a high-current MIG generator (2 MA, 80 ns) are presented. To stabilize the process of liner implosion and increase the efficiency of energy transfer from the generator to the liner plasma, a current return in the form of a multifilar helix was used. The effect of the configuration of the current return on the parameters of the generated pulses of argon and neon K-shell radiation (with photon energies of 3–5 and 0.9–1.5 keV, respectively) and the neutron yield from a deuterium liner were studied.

Efficient production of 100 keV deuterons in deuterium gas puff Z-pinches at 2 MA current

Deuterium gas puff experiments were carried out on the S-300 Z-pinch at the Kurchatov Institute in Moscow. Gas puffs imploded onto the axis before a current peak at about 100 ns. Fusion neutrons were generated after the gas puff implosion during global expansion of a plasma column. Neutron emission lasted on average 35 ± 5 ns (full width half maximum, FWHM). In the downstream direction (on the Z-pinch axis behind the cathode), a mean neutron energy was 2.6 ± 0.1 MeV. Side-on neutron energy spectra peaked at 2.40 ± 0.05 MeV with about 600 ± 150 keV FWHM. A broad width of side-on neutron spectra implied a high radial component of deuteron velocities. An average kinetic energy of fast deuterons, which produced fusion neutrons, was 150 keV. A peak neutron yield reached a value of 6 × 1010 on a current level of 1.5 MA. It was by one order higher in comparison with other deuterated loads used on the same current generator. On the basis of experimental observations, we concluded that a total energy of deuterons accelerated to fusion energies was above 1.5 kJ. It is more than 15% of the energy input into a plasma. Therefore gas puff Z-pinches seem to be not only powerful sources of x-ray radiation but also efficient sources of 100 keV deuterons. Such a result is consistent with high neutron yields observed on the Angara Z-pinch and plasma foci with similar currents.

Study of the current-sheath formation during the implosion of multishell gas puffs

Results are presented from experimental studies of the dynamics of large-diameter multishell gas puffs imploded by microsecond megampere current pulses. The experiments were conducted on the GIT-12 generator in the regime of microsecond implosion (timp = 1.1–1.2 μs, I0 = 3.4–3.7 MA). The influence of the load configuration on the dynamics of current losses and gas-puff radiative characteristics was studied. The correlation between the radial compression ratio (the ratio between the initial and final Z-pinch radii) and the magnitude of the current flowing at the plasma periphery was investigated. The experiments show that, in a multishell gas puff, large-scale instabilities insignificantly affect the gas-puff implosion even over microsecond time intervals and that a compact dense pinch with a relatively high average electron temperature (400–600 eV) forms at the Z-pinch axis. The diameter of the plasma column radiating in the K-shell lines of neon is about 3–4 mm, the K-shell radiation yield being 5–11 kJ/cm. In the final stage of implosion, only a small portion of the current flows through the high-temperature central region of the pinch plasma, whereas the major part of the generator current flows through the residual peripheral plasma.

Deuterium gas-puff Z-pinch implosions on the Z accelerator

Experiments on the Z accelerator with deuterium gas-puff implosions have produced up to 3.7×1013 (±20%) neutrons at 2.34MeV (±0.10MeV). Although the mechanism for generating these neutrons was not definitively identified, this neutron output is 100 times more than previously observed from neutron-producing experiments at Z. Dopant gases in the deuterium (argon and chlorine) were used to study implosion characteristics and stagnated plasma conditions through x-ray yield measurements and spectroscopy. Magnetohydrodynamic (MHD) calculations have suggested that the dopants improved the neutron output through better plasma compression, which has been studied in experiments increasing the dopant fraction. Scaling these experiments, and additional MHD calculations, suggest that ∼5×1014 deuterium-deuterium (DD) neutrons could be generated at the 26-MA refurbished Z facility.