Session NO6: ICF: Z-Pinch, X-Pinch, Exploding Wires, and Dense Plasma Focus

Thomson Scattering for Differentiating Sources of Spatial Velocity Distributions in Gas-Puff Z-Pinches

The conditions and dynamics of neon gas-puff z-pinch plasmas during the implosion phase are studied on the COBRA pulsed power generator (rise time \textasciitilde 240 ns to 0.9 MA peak current). A 526.5 nm, 10 J, 2.2 ns Thomson scattering diagnostic laser enables probing of the plasma conditions with both spatial and temporal resolution. Collective scattering spectral profiles are observed from which electron and ion temperatures and plasma fluid flow velocity can be obtained from the low-frequency ion acoustic spectral feature. Under some plasma conditions electron temperature and density can be obtained from the high-frequency electron plasma wave spectral feature. Scattered laser light from the same scattering volume can be split to two spectrometers of differing bandwidths in order to obtain both the ion acoustic and the electron plasma wave features. The width of the electron plasma wave feature is affected by gradients in the electron density as well as electron temperature, T$_{\mathrm{e}}$. By comparing the T$_{\mathrm{e}}$ derived from the electron plasma wave feature to that derived from the ion acoustic feature, it may be possible to detect the presence of small-scale, local density variations in the plasma. Past experiments show that including a spatial velocity distribution when fitting the ion acoustic spectra in some cases improves the fit quality; with density fluctuations included in the analysis, the presence of non-thermal, small scale hydromotion in the scattering volume may be indicated.   

Hybrid gas-puff z-pinch as a source of protons and deuterons up to 45 MeV

Efficient acceleration of deuterons has been observed in z-pinches and dense plasma foci since the 1950s. In 2013, we tested a hybrid configuration of a deuterium gas-puff z-pinch on the 3 MA GIT-12 generator (IHCE, Tomsk). In this configuration, a hollow cylindrical plasma shell was injected around an inner deuterium gas puff to form a uniformly conducting layer before z-pinch implosion [1]. The stable implosion at the maximum velocity of 650 km/s was important to deliver more current onto the z-pinch axis. After the implosion, one 10-20 ns pulse of multi-MeV photons and neutrons was observed. The average neutron yield was 2x10$^{\mathrm{12}}$. In the best shots, hydrogen ions were accelerated up to 45 MeV which is the highest energy observed in z-pinches and dense plasma foci. Detailed knowledge of the ion emission was used to increase neutron yields above 10$^{\mathrm{13}}$ with a neutron-producing catcher [2,3]. Recently, we have attempted to use accelerated deuterons to study magnetic fields in z-pinches via ion deflectometry. [1] D. Klir, et al., PRL 112, 095001 (2014). [2] D. Klir, et al., NJP 20, 053064 (2018). [3] D. Klir, et al., PPCF 61, 014018 (2019).   

Comprehensive magnetic field characterization of O and Ar gas-puff z-pinches using spectroscopic techniques

Gas-puff z-pinches are a long-studied platform for producing hot dense plasma and are a well-known intense x-ray source. Knowledge of the magnetic field in gas-puff z-pinches is necessary to understand the dynamics and characteristics of the implosion including thermal conductivity, current distribution in the plasma, and instability development and mitigation in the presence of a pre-embedded axial magnetic field. Non-invasive experimental determination of the magnetic field strength in imploding plasmas is limited to a handful of techniques. Here, we present experimental results of a polarized Zeeman-based spectroscopic measurement of the magnetic field in both oxygen and argon gas-puff z-pinches obtained using a pulsed power driver (400 kA, 1.6~$\mu $s rise) at the Weizmann Institute of Science. An array of optical fibers aligned along the pinch axis coupled to a high-resolution, time-gated spectrometer allowed for spatially resolved determination of both the azimuthal and axial magnetic field strength during the implosion. This work, in part, aims to develop the diagnostic capabilities at UC San Diego to include Zeeman-based spectroscopy on gas-puff implosions using an 800 kA (200 ns rise) linear transformer driver.   

Characterization of Ne gas puff Z-pinch on Linear Transformer Driver using spectroscopy

Characterizing gas puff Z-pinch is of significant importance for the alternate fusion schemes and intense X-ray sources. We propose for the first time to study gas puff on a Linear Transformer Driver (LTD). Such data would help to understand in more detail the plasma behavior and validate numerical results. We performed an experiment on an LTD with \textasciitilde 200 kA peak and 150~ns rise time at UC San Diego. We used fairly high-resolution spectroscopy and 2-D interferometry to characterize the plasma at stagnation at various plenum pressures. An imaging spectrometer equipped with an ICCD allowed detection of Ne spectra from 320 nm to 460 nm, time-integrated over 50 ns. In addition, the measurements were spatially resolved along the z-axis. The experimental data are compared to hydrodynamic simulations and are part of a larger campaign dedicated to collect reliable data on the current distribution in gas puff Z-pinch plasmas.   

Shock heating versus photoionization of a central jet in gas puff z-pinches on COBRA

Gas-puff z-pinch experiments on Cornell University's 1 MA COBRA generator are conducted using a custom triple-nozzle gas-puff valve. For argon center jet densities above 5E16 cm$^{\mathrm{-3}}$, ionization of this region is observed early in the implosion, before the arrival of the magnetic piston. At the start of this process, the electron density is coincident with the initial neutral gas density; later however, the electron density forms an annular shell at the boundary of the center jet that remains approximately stationary until the arrival of the magnetic piston. Early interpretations suggested this feature was indicative of a stagnated shock predicted and observed in some staged z-pinch experiments [1]. A competing explanation postulates photoionization by the radiating magnetic piston followed by ohmic heating. Here we present the results of experiments designed to identify the mechanism(s) behind the observed ionization features. We also present results from a complimentary experiment investigating the photoionization of an annular gas puff by an on-axis wire. [1] F. J. Wessel, et al. AIP Conference Proceedings 1721, 060002 (2016)   

Dynamics and MRT stability of axially pre-magnetized multi-shell, multi-species gas-puff Z-pinches

Gas-puff Z-pinch implosions onto a central column of target plasma are a well-known source of neutrons or X-rays, depending on the target material. [1] However, they are highly susceptible to the magneto-Rayleigh-Taylor instability, necessitating the use of one or more mitigation mechanisms, like axial premagnetization [2] and/or density profile tailoring [3,4]. Previous 2-D HYDRA simulations on a 160-ns, 800-kA LTD [5] demonstrated the stabilization of a 2.5-cm-radius Ne liner onto D target implosion with the addition of a second liner at radius 1.25 cm and B$_{\mathrm{z0}}$ of 0.2 T. Presented here is an extension of that work which considers the effects of radial and axial mass distribution, liner material (Ne, Ar, or Kr), and liner mixing (by prescribing an impurity fraction in the load) on peak target conditions and deuterium-deuterium neutron yield. $^{\mathrm{1}}$ J. Giuliani and R. Commisso, IEEE Trans. Plasma Sci. \textbf{43}, 2385 (2015). $^{\mathrm{2}}$ F. Beg et al, APS DPP 2018, http://meetings.aps.org/link/BAPS.2018.DPP.YO6.1. $^{\mathrm{3}}$ A. L. Velikovich, F. L. Cochran, and J. Davis, Phys. Rev. Lett. \textbf{77}, 853 (1996). $^{\mathrm{4\thinspace }}$H. Sze et al, Phys. Plasmas \textbf{14}, 056307 (2007). $^{\mathrm{5}}$ J. Narkis, PhD thesis (University of California San Diego, 2019).   

MHD modeling of gas-puff Staged Z-pinch implosions on university-scale and multi-MA drivers

The staged Z-pinch (SZP) is a magneto-inertial fusion concept in which a high-Z gas-puff or solid annular liner implodes onto a deuterium or deuterium-tritium target plasma. The target is heated to fusion conditions by shock heating and subsequent adiabatic compression. Successful SZP experiments using Ar/D and Kr/D on the 1-MA Zebra driver at the Nevada Terawatt Facility [1] have motivated an expansion of the study to DT fuel and multi-MA drivers. Presented here is a computational study using the MHD code MACH2, which addresses the effects of grid motion (Lagrangian, Eulerian, or Arbitrary Lagrangian-Eulerian) and liner material (Ar, Kr, or Xe) on neutron yield from drivers with peak currents ranging from 1 MA to 20 MA. $^{\mathrm{1}}$H. U. Rahman, E. Ruskov, P. Ney, F. Conti, J. C. Valenzuela, N. Aybar, J. Narkis, F. N. Beg, E. Dutra, and A. Covington, Phys. Plasmas \textbf{26}, 052706 (2019).   

The effect of breakdown phase on the reproducibility of a 10 kJ dense plasma focus

The dense plasma focus (DPF) is capable of producing intense bursts of X-rays and neutrons. However, reproducibility in DPF's is one of the challenging issues. To address this issue, a 10 kJ DPF with 300 kA peak current and 2 $\mu $s rise time has recently been constructed at the University of California, San Diego to study the effect of plasma breakdown phase on reproducibility. A variety of insulator materials have been employed to investigate the effect on breakdown reproducibility and subsequently its effect on the pinch phase. Results from spectroscopic and optical probing techniques will be presented.   

Validation of PERSEUS and Implementing Ionization Energy Models in PERSEUS

Ultrathin foil liners, with thicknesses of 400 nm, are used in university-scale Z-pinch experiments (\textasciitilde 1 MA) to study physics relevant to inertial confinement fusion efforts on larger-scale facilities (e.g. the MagLIF efforts on the 25 MA Z facility at Sandia National Laboratories). We demonstrate the ability of the 3D MHD simulation code PERSEUS [1] to accurately model the implosions of ultrathin liners by comparing general implosion trends and detailed plasma structures in simulation and experiment. In university-scale experiments [2], ultrathin foils have used a central support rod to maintain structural integrity prior to implosion, and we have now used PERSEUS to study these experiments in detail. The results suggest that it is the support rod which enables the helical structures to persist beyond stagnation. In addition, we report on new efforts to include more robust material ionization models in PERSEUS to enhance the code's simulation capabilities. [1] Seyler, C. E., {\&} Martin, M. R. (2011). Relaxation model for extended magnetohydrodynamics: Comparison to magnetohydrodynamics for dense Z-pinches. \textit{Physics of Plasmas}, \textit{18} [2] Yager-Elorriaga, D. A., et al. (2016). Discrete helical modes in imploding and exploding cylindrical magnetized liners. \textit{Physics of Plasmas}, \textit{23}(12)   

Hard X-ray Line Radiation from Tungsten Wire Array Z-pinches.

During the last few years, there is a renewed interest to study hard x-ray non-thermal inner-shell emission from Z-pinch plasmas of high-atomic-number materials on Sandia's Z and NRL Gamble II generators. For example, the cold (non-thermal) and thermal K-alpha emission from Nested Mo Cylindrical Wire Arrays (CWA) were investigated on Sandia's Z accelerator and it was shown that dominant characteristic cold K-alpha lines are most likely to be produced by hot electrons (Hansen et al, PoP, 2014). To better understand mechanisms of production of non-thermal plasmas in Z-pinches, it is essential to study time evolution of hard x-ray characteristic cold lines. Here we present and interpret the time history of relative intensities of cold L lines (alpha, beta, and gamma) from W wire arrays (in the energy range between 8 and 12 keV) produced on the UNR Zebra generator. The line intensity ratios and x-ray diode signals from Double Planar Wire Arrays and Nested CWAs (16 wires each) are analyzed from 12 ns before and up to 27 ns after the x-ray burst. In addition, our results were compared with cold atom and warm dense matter results on Gamble II (Seely et al, HEDP, 2013). Future work is discussed. This research was supported by NNSA under DOE grant DE-NA0003877 and in part by DE-NA0002075.   

Implicit and Hybrid Techniques for the Simulation of High-Density Electrode Plasmas for Pulsed Power Accelerator Design

Recent advances in implicit and hybrid techniques have demonstrated that finite-difference-time-domain particle-in-cell (PIC) simulation codes can effectively model volumetric and electrode plasmas at high density. Plasmas generation and evolution can seriously affect the efficiency of pulsed power delivery as well as microwave sources and gas switch performance. Energy-conserving implicit kinetic algorithms greatly relax the spatial Debye length and temporal plasma frequency constraints allowing for larger simulations volumes and times. Including PIC hybrid techniques further accelerates the computational speed. These new capabilities allow for more accurate simulation of pulse-power accelerators, high power diodes, microwave sources, and gas switch performance. We will describe PIC methodologies for kinetic, multi-fluid and hybrid techniques for blending the various PIC descriptions into a single integrated simulation. Finally, practical examples of these techniques in stressing plasma physics environments will also be presented using the L\textsc{sp} and C\textsc{hicago} codes.   

Development of linear transformer driver technologies at the University of Rochester

The High Amperage Driver for Extreme States (HADES) being built at the University of Rochester is a compact linear transformer driver (LTD) designed to produce 1 MA of current in 250 ns rise time. HADES is designed to be modular and portable allowing for the whole machine to be relocated easily next to an XFEL. In developing HADES, we identify design obstacles and engineering hurdles typically connected with the construction of compact high voltage systems. In this talk I will present the different solutions we used to overcome these limitations using a cavity containing 22 bricks. Each brick is composed of two 60 nF capacitors and a high voltage gas switch. Investigation into the lifespan of the LTD components such as the charging resistors, were rested to determine time between LTD maintenance. HADES will be used to produce large volumes of matter under extreme conditions in the z-pinch geometry.   

Stabilizing Liner Implosions with a Dynamic Screw Pinch

Pulsed power driven liner implosions are susceptible to instabilities like the magneto Rayleigh-Taylor (MRT) instability. One proposed method for mitigating MRT uses the rotating magnetic field of a dynamic screw pinch, which can be generated using a twisted return current structure. This method has been examined in computer simulations [1] and now in experiments as well. Using the COBRA pulsed power driver, both straight and twisted return current paths were tested on imploding thin-foil liners, made from 650 nm thick aluminum foil. Each implosion was driven by a current pulse that rose from 0 to 1 MA in 100 ns. Three different twisted return current structures were tested with peak axial magnetic fields ranging from 2 T to 20 T. These experiments revealed remarkable differences in the instability structures between the cases. Helical modes were observed for the twisted return can cases and were absent from the normal z-pinch case. The amplitudes of the MRT spikes were also reduced, by up to a factor of two, at the time of liner stagnation on a central support rod, at a convergence ratio of about two. [1] P.F. Schmit et al., PRL 117, 205001 (2016).   

Fielding First X Pinches at New 300-kA 150-ns Pulser for High-Energy Density Laser-Plasma Experiments.

X pinches are unique radiation sources often used as a diagnostic in high-energy density physics. Given the right conditions, they produce \textmu m-sized ``hot spots'', which generate x-ray bursts less than a nanosecond long. To improve our understanding of x-pinch dynamics, a new, compact and portable pulser was recently built at the University of Rochester. Simulations predict it can deliver up to 300-kA of peak current into an inductive x-pinch load with less than 150-ns time-to-peak when the pulser is fully charged to \textpm 100 kV. We measured the pulser total internal inductance to be about 40 nH, and it is currently undergoing the final engineering short-circuit test. In this work we present the first measurements of the x pinches fielded inside the vacuum chamber of the new pulser.   

Analysis of Laser-Cut Foil X-Pinch X-Ray Line Emission for Radiography Applications

Previous studies of laser-cut foil x-pinches have shown that they are an improved radiation source over conventional wire x-pinches [1]. We present an analysis of additional foil x-pinch experiments on the GenASIS (150 ns, 250 kA) and LTD-III (150 ns, 800 kA) drivers located at the University of California, San Diego (UCSD). Various foil materials, such as Al, Ti, Mo, and W are tested to observe their x-ray line emission characteristics, particularly from K-shell and L-shell transitions. A comparison of the foil x-pinch's performance between the two drivers provide an assessment of its scaling properties. The radiation is studied with time-integrated x-ray spectrometers and filtered photo-conducting diodes (PCD) for time-resolved measurements. The global characteristics of the foil x-pinches are studied with optical laser probing techniques and extreme ultraviolet (XUV) framing images. [1] G.W Collin IV, et al., Physics of Plasmas 23, 101212 (2016)