Session VO06: ICF: Dense Plasma Focus, X and Z Pinches

Live

Multiple pinching actions are observed as a consequence of gradually increasing the working pressure in a kilojoule plasma focus device, PF-2kJ. The signatures of pinching action are observed in the voltage and current derivative signals. A sharp fall in the current derivative near to the maximum current (usually known as dip) and a sharp rise in the voltage signals are considered the evidence of pinching action in plasma focus devices. In the present work, at 4 mbar only one pinch is observed near to the maximum current, and at pressures higher than 9 mbar multiple pinches are observed. A sharp rise in the current derivative is observed near to the minimum current at higher pressures that suggest a pinching action near to the minimum current. Low energy x-rays that are detected using BPX65 PIN diodes appear at the time of the multiple pinches. Ion emission that is detected using a Faraday cup, shows two peaks and does not correlate with the pinch that is observed near to the minimum current. It was found that the same physical phenomenon is responsible for both pinches that have opposite appearances.   

Live

We present results for a novel flyer acceleration method in which a planar wire array in a waterbath is exploded using a current pulse, generating an approximately planar shock in the water that reaches and accelerates the flyer. Flyer plate impact is of interest for material equation of state research, and more recently for a novel fusion ignition scheme being developed at First Light Fusion Ltd. This method may allow more control over the spatial and temporal profile of the force accelerating the flyer than other methods such as magnetic stripline acceleration by controlling the current path using wires of varying characteristics and utilizing shock reflections. Results indicate velocities of around 1100 m/s for a 10 mm$^{\mathrm{2}}$, 1 mm thick aluminium flyer, using a current pulse with peak current of 600 kA and rise time of 500 ns. Effect of flyer thickness and material have also been investigated. Reflected shock waves have been shown to provide initial acceleration as in a reverberation cavity. Work is in progress to further diagnose the characteristics of the planar shock launched from the wire array using x-ray radiography, and to simulate the system to further understand material conditions.   

Live

Ion acoustic wave (IAW) feature broadening in collective Thomson scattering in neon gas-puff z-pinch plasmas is investigated on the COBRA pulsed power generator (rise time ~240 ns to 0.9 MA peak current). A 526.5 nm, 10 J, 2.3 ns Thomson scattering diagnostic laser enables probing of the plasma conditions with < 1 mm spatial and < 1 ns temporal resolution. Electron temperature and plasma flow velocity can be obtained routinely from IAW spectra, but the width of the IAW peaks depends on both ion temperature, $T_i$, and on fluid velocity distributions within the scattering volume. In some cases, electron temperature ($T_e$) and density ($n_e$) can be obtained from the high-frequency electron plasma wave spectral feature (EPW). The width of the EPW depends on $T_e$, but is also affected by fluctuations in $n_e$. By comparing the values of $T_e$ derived from both scattering features, 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 IAW improves the fit quality for a range of times before stagnation; with density fluctuations included in the analysis, the presence of non-thermal, small scale hydromotion in the scattering volume may be indicated.   

Live

Linear Transformer Drivers (LTDs) are pulse generators with low intrinsic impedance and potential for high energy coupling to gas puff Z-pinch loads. The CESZAR LTD, recently commissioned at UC San Diego, is used to drive gas puff Z-pinch experiments at 0.5 MA current levels with a 160 ns rise time. The Z-pinch load consists of a hollow shell (liner) of different materials (H2, O2, Ne, Ar, Kr) and a central D2 jet (target). An external axial magnetic field (Bz) can be pre-embedded in the plasma to mitigate magneto-Rayleigh-Taylor (MRT) instability. We present a parametric study of the pinch performance in terms of Bz0 required for MRT stability, radial convergence, and radiation yield (X-rays from the liner and neutrons from the target) as a function of liner gas species. The Z-pinch implosion is characterized with multiple diagnostics, including time-gated XUV emission images, laser interferometry and schlieren imaging, time-resolved X-ray detectors, time-integrated spectroscopy, and neutron detectors. Magnetohydrodynamic (MHD) simulations are performed for the different configurations and compared with the experimental pinch dynamics, e.g. Bz threshold for MRT mitigation and neutron production.   

Live

Transient high density plasmas, and particularly Z-pinches, are known to emit outflows of energetic particles and/or plasma jets. When interacting with surfaces, these emissions are able to modify their structures by ablation, implantation, and/or deposition. In this work, the axial outflows from W conical wire array Z-pinches interact with targets located at different distances with respect to the array. The experiments are driven in the Llampudken generator (\textasciitilde 350kA in \textasciitilde 350ns) using Si(100) targets located at distances ranging from 10 cm to 21 cm measured from the top of the wire array. The targets are later analyzed using surface science techniques such as SEM, XPS, AFM amongst others. The results indicate differences in the surface modifications according to the relative positioning of the target with the array. For instance, SEM images show surface morphologies in the form of micropores and stripe-like (wrinkle-like) structures. The micropores abundance decreases as the axial distance increases, in contrast to the stripe-like structures where both prevalence and uniformity increase. Further details on the characterization of the W plasma outflows and their effects on the silicon targets will be shown and discussed.   

Live

The use of a copper X-pinch as backlighting source for Talbot-Lau X-ray Deflectometry (TXD) is presented. The TXD technique can provide information about density gradients and elemental composition in HED plasmas, through single-image x-ray refraction and attenuation. In order to test the system in pulsed power environments, a TXD was implemented using a Cu X-pinch as X-ray source in the Llamp\"{u}dke\~{n} generator (\textasciitilde 350kA in \textasciitilde 350ns). A minimum source size of \textasciitilde 50um was measured at the crossing point, with pulses of \textless 2ns; as well as an extended x-ray source from the anode side of the array. Characteristic x-rays, as well as a broad continuum under 5keV were detected. A Be object is used as probing object, measuring its density with a difference \textless 13{\%}. No damage from debris or magnetic field was observed in the gratings used for TXD, but it is shown that a protective filter is required. These results are relevant in order to adapt and design further pulsed power experiments that aim to use the Talbot-Lau technique to characterize pulsed plasmas.   

Live

Three types of Cu X-pinches were studied as X-ray sources for refraction-based imaging. In the deflectometer configuration, Talbot-Lau X-ray (TLX) Interferometry can provide electron density, elemental composition, and scatter information from a single image. TLX backlighters must meet specific source requirements to accurately diagnose HED experiments. Wire, hybrid, and laser-cut foil X-pinches were compared on the GenASIS driver ($\sim $ 200 kA, 150 ns). All configurations produced short (\textasciitilde 1 ns), small ($\le $ 5 $\mu $m) Cu L-shell ($\sim $ 1 keV) sources with comparable peak fluxes. Laser-cut foil X-pinches produced the brightest ($\sim $ 1 MW) and smallest ($\le $ 5 $\mu $m) Cu K-shell (\textasciitilde 8-9 keV) sources. While Moire fringe formation was demonstrated for all X-pinches, laser-cut foils delivered the highest fringe contrast and spatial resolution, making them the ideal candidate for pulsed-power based X-ray backlighting for TLX refraction diagnostics. Moreover, spectroscopic data indicate foil X-pinches reached temperatures \textgreater 2 keV, produced no temporally or spatially separated electron beams, and produced single sources with the highest and most localized K-shell flux of any configuration.   

Live

Penetrating X-rays are one of the most effective tools for diagnosing high energy density experiments, whether through radiographic imaging or X-ray diffraction. To expand the X-ray diagnostic capabilities at the 26-MA Z Pulsed Power Facility, we have recently developed a new diagnostic X-ray source called the inductively driven X-pinch (IDXP). This X-ray source is powered by a miniature transmission line that is inductively coupled to fringe magnetic fields in the final power feed. The transmission line redirects a small amount of Z's magnetic energy into a secondary cavity where 150$+$ kA of current is delivered to a hybrid X-pinch. In this paper, we describe the multi-stage development of the IDXP concept, through experiments both on Z and in a surrogate setup on the 1-MA Mykonos facility. Initial short-circuit experiments to verify power flow were followed by X-ray source development experiments on Mykonos and then on Z. The creation of an X-pinch hot spot is verified through a combination of X-ray diode traces, laser shadowgraphy, and fiducial radiographs. When fully implemented on Z, IDXPs will greatly increase the number of available radiography frames and lines of sight for diagnosing high energy density experiments.   

Live

Deflections of multi-MeV ions can be used for measurements of path-integrated B-fields in high-temperature plasmas. Ion deflectometry has been successfully employed in laser-produced plasmas. Ions accelerated by laser-target interactions were also utilized for a study of Z-pinch plasmas. Here, we present results of the first Z-pinch-driven ion deflectometry experiments using MeV ion beams accelerated within a hybrid gas-puff Z-pinch plasma on the GIT-12 pulse power generator. During a disruption of 3 MA current, hydrogen ions were accelerated up to 50 MeV. In our configuration, an inserted fiducial grid separated an internal ion source from studied B-fields below it. A shadow of this grid, backlighted by the ions, was distorted by the B-fields and recorded by an ion pinhole camera. Analysis of experimental images (deflectograms) provided a radial distribution of path-integrated B-fields near the axis (within a 13-mm radius). Moreover, a 2D topological map of the B(r,z) was obtained by numerical reconstruction of the deflectograms.   

Live

Production of energetic protons, deuterons, and neutrons up to 60 MeV is observed in z-pinch experiments on the GIT-12 generator at 3 MA current and 0.6 MV driving voltage. Efficient ion acceleration is obtained with a hybrid gas-puff z-pinch, i.e., with an inner deuterium gas puff surrounded by an outer hollow cylindrical plasma shell. The behavior of the hybrid gas-puff z-pinch on GIT-12 can be characterized as a high-density plasma opening switch with a microsecond conduction time, 3 MA conduction current, nanosecond opening, \textgreater 20 $\Omega $ impedance after opening, and \textgreater 60 MV stand-off voltage [1]. These are unique properties that can be employed in the fast transport of current into an on-axis load. For this purpose, we place Al or Ti wires on the axis of the hybrid gas-puff z-pinch. The experimental results on K-shell radiation and plasma-on-wire dynamics are presented and discussed. [1] D. Klir, et al., NJP \textbf{20}, 053064 (2018).   

Live

The radial implosion of one or more annular gas-puffs, or liners, onto a central on-axis jet, or target, is an efficient source of X-rays or neutrons, depending on the target material. University-scale gas-puff Z-pinches operate in a low-density regime where thermal conduction effects dominate radiative cooling, thus can significantly affect target stagnation conditions. Though the electron-ion temperature equilibration timescale is large, here we present simulation results using the radiation-MHD code HYDRA that suggest the state of the electron fluid -- which we show is significantly altered by the choice of radiation model (local thermodynamic equilibrium, LTE, vs. non-LTE) and degree of axial pre-magnetization -- affects the evolution of ion temperature gradients within the liner material. If large ion temperatures can develop at the interface between liner and target plasma, and the target electron Hall parameter is large at peak compression, thermal insulation of both target ions and target electrons can be significant. The implications of these observations are briefly discussed in the context of Z-pinch neutron sources.   

Direct Quantification of Insulator Breakdown and Plasma Sheath Formation on Pinch Quality in a 10 kJ Dense Plasma Focus.

Dense plasma foci (DPF) are intense sources of X-rays and energetic particles including neutrons. Shot-to-shot variation in yield, pulse shape, and pulse duration remain prominent outstanding issues for many DPF applications. The breakdown phase presumably plays an important role in the reproducibility and quality of the final pinch, but limited effort has quantitatively correlated the magnitude thereof. Here we report on simultaneous laser probing of breakdown and radial pinch phases on a 10-kJ DPF to systematically study how different sleeve materials and lengths affect these phases. A comparison of experimental results with MHD modeling will be presented.   

Validation of MHD Simulations Using Spectroscopically Characterized O Gas-Puff Z-Pinches.

Gas-puff z-pinches have been studied for decades for a variety of applications ranging from controlled thermonuclear fusion to its use as a bright X-ray source. Magnetohydrodynamic (MHD) simulations in conjunction with experimental data are necessary to study the physical processes at play throughout the dynamic Z-pinch implosion process. Here we present a comprehensive parameterization of gas-puff z-pinches carried out on a 300 kA peak current, 1.6 us rise time driver at the Weizmann Institute of Science. Gas discharges made with O$_{\mathrm{2}}$ included simultaneous spectroscopic measurements of electron density (n$_{\mathrm{e}})$, temperature (T$_{\mathrm{e}})$, and azimuthal (B$_{\mathrm{\theta }})$ and axial (B$_{\mathrm{z}})_{\mathrm{\thinspace }}$magnetic fields. The experimental parameters were juxtaposed against results from one and two-dimensional MHD simulations conducted using HYDRA to explore the underlying physics.   

On Demand

Plasma jets were generated by using conical-wire arrays driven by a pulsed-power system. The pulsed-power system was built for studying space sciences, particularly in simulating solar winds. The pulsed-power system consisted of twenty $1$-${\rm \mu F}$ capacitors, two rail-gap switches, two parallel plate transmission lines, and a cylindrical vacuum chamber orientated vertically. Two capacitors were first connected in series forming a brick. Five bricks were connected in parallel forming a wing. Finally, two wings were connected in parallel forming the whole capacitor bank, i.e., $5\;{\rm \mu F}$ in total. The system was charged to $20\;{\rm kV}$ storing total energy of $1\;{\rm kJ}$. When it was discharged, a peak current of $110\pm20\;{\rm kA}$ with a rise time of $1.4\pm0.2\;{\rm \mu s}$, i.e., a peak power of $\sim 700\;{\rm MW}$, was provided. The conical-wire array was formed by four tungsten wires with a diameter of $20\;{\rm \mu m}$. The opening angle and the smaller radius of the conical-wire arrays were $30^{{\rm o}}$ and $5\;{\rm mm}$, respectively. Images of the implosions were taken by an x-ray pinhole camera with an exposure time of $1\;{\rm \mu s}$, i.e., temporal-integrated images of the implosions. Images of the implosion will be shown.   

Computational Exploration of the Spatiotemporal Effects of Laser Interaction with X-Pinches.

X-pinches have been shown to be a source of extremely intense x-ray emissions useful for diagnosing plasma dynamics and imaging biological objects. The most striking feature of an x-pinch is the hotspot, the point source from where all the x-rays comes from. Unfortunately, the exact timing and location of the hotspot are still unpredictable. Since x-pinch hotspots form from instabilities (like an m$=$0 mode), we will computationally explore whether we can use a high-power laser to initiate the timing and location of these instabilities for a hybrid x-pinch setup. Our goal is to reduce the temporal and spatial jitter associated with the x-ray burst. Using an XMHD solver (PERSEUS), we explore the non-relativistic instability generation using a current profile of a 250kA LTD system and laser characteristics of SLAC's Matter in Extreme Conditions laboratory (MEC). Our results include both laser-penetration results using a boundary-defined EM-wave, and instability results from a power-deposition method.