Z-pinch Generator Research Articles

Instability growth mitigation study of a dielectric coated metallic wire in a low current Z-pinch configuration

An experimental and numerical study of the nanosecond electrical explosion dynamics of bare and dielectric coated metallic wires in vacuum is reported. A table top Z-pinch generator down-tuned to generate a peak current of 40 kA with a rise time of 60 ns is implemented for the experiments in the skin effect mode. Optical probing diagnostics as well as multiphysics–multiphase simulations are used to study the spatiotemporal dynamics of the exploded wire from the solid to the plasma phase. The results show that the inclusion of the dielectric coating mitigates the electro–thermo–mechanical instability growth developed prior to plasma formation, which results in a subsequent reduced magnetohydrodynamic instability growth in the plasma phase. The study offers valuable insights into the understanding of the seeding mechanisms for the generation of plasma instabilities and the efforts for their growth rate suppression.

Electron temperature and electron number density measurements in laser ablation Z-pinch experiments of Al2O3

Aluminum oxide (Al2O3) plasma was generated using the Ti:Sapphire/Nd:glass laser and Z-pinch pulse-power generator at the University of Nevada, Reno, Nevada Terawatt Facility. To determine the electron temperature, we use the Boltzmann plot method for the transitions (Al III 4d 2D3/2 → 4p 2P1/2) at 451.2564 nm, (Al III 4d 2D5/2 → 4p 2P3/2) at 452.9194 nm, (Al III 4p 2P3/2 → 4s 2S1/2) at 569.6603, and (Al III 4p 2P1/2 → 4s 2S1/2) at 572.2728 nm. Simultaneously, Mach–Zehnder laser interferometry was used to measure the electron number density. We compared simulated spectra obtained from PrismSPECT with electron number density and temperature measurements, and the electron temperature was found to range from 1.8 to 2.8 eV, while the electron number density was found to range from 2.3 × 1017 to 6.9 × 1017 cm−3 early in the pinched plasma formation.

Studies of Implosion and Radiative Properties of Tungsten Planar Wire Arrays on Michigan’s Linear Transformer Driver Pulsed-Power Generator

Wire arrays were widely studied as loads for Z-pinch generators in order to be used for multiple scientific applications. More recently, tungsten double planar wire arrays (DPWAs), which consist of two parallel planes of wires at a distance of a few millimeters, were suggested and tested for indirect drive inertial confinement fusion. Tungsten DPWAs have previously demonstrated the highest (among planar wire arrays) radiation yield (up to 30 kJ), compact size (few millimeters), and strong electron beam production on university-scale high-impedance Marx bank Zebra generator at University of Nevada, Reno. During the last few years, we have reported on the outcome of the experiments with uniform and mixed Al and stainless steel DPWAs on the low-impedance linear transformer driver (LTD) Michigan accelerator for inductive Z-Pinch experiments (MAIZE) generator at University of Michigan. Here, we present the results of the most recent campaigns with tungsten DPWA loads, where the successful implosion of W wire arrays on a university-scale LTD MAIZE generator was demonstrated and analyzed. These implosions were recorded using filtered X-ray diodes, X-ray spectrometers, and pinhole cameras, and a 12-frame optical shadowgraphy system. In particular, tungsten DPWAs with a mass up to $87~\mu \text{g}$ /cm arranged in various configurations were successfully imploded at a peak current of 0.5 MA during ~190–215 ns. The experimentally estimated changes of tungsten DPWA plasma region inductance and total load inductance were correlated qualitatively in time with X-ray bursts. In addition, on shots that demonstrated strong plasma pinching process and Faraday cup signals time were correlated with the appearance of the minimum current-carrying radius of the plasma column. In addition, analysis of soft (4–7 A) and hard (1–2.4 A) line radiation indicate keV M-shell tungsten (W) plasma and the presence of electron beams.

Radiative signatures of Z-pinch plasmas at UNR: from X-pinches to wire arrays

University-scale Z-pinch generators are able to produce High Energy Density (HED) plasmas in a broad range of plasma parameters under well-controlled and monitored experimental conditions suitable for radiation studies. The implosion of X-pinch and wire array loads at a 1 MA generator yields short (1-20 nsec) x-ray bursts from one or several bright plasma spots near the wire cross point (for X-pinches) or along and near Z-pinch axis (for wire arrays). Such X- and Z-pinch HED plasma with scales from a few µm to several mm in size emits radiation in a broad range of energies from 10 eV to 0.5 MeV and is subject of our studies during the last ten years. In particular, the substantial number of experiments with very different wire loads was performed on the 1 MA Zebra generator and analyzed: X-pinch, cylindrical, nested, and various types of the novel load, Planar Wire Arrays (PWA). Also, the experiments at an enhanced current of 1.5-1.7 MA on Zebra using Load Current Multiplier (LCM) were performed. This paper highlights radiative signatures of X-pinches and Single and Double PWAs which are illustrated using the new results with combined wire loads from two different materials.

Search for Drive Parameter of Neutron-Optimized Z-Pinches and Dense Plasma Foci

Dense plasma foci and deuterium gas puff Z-pinches are studied as sources of fusion neutrons. To design a neutron-optimized plasma focus, the drive parameter I0/(a(ρ0)1/2) has been discussed in the last 15 years. Here, I0, a, and ρ0 represent the peak discharge current, the inner electrode radius, and the initial gas density, respectively. In this paper, similarities between a dense plasma focus and a gas puff Z-pinch are shown. The drive parameter of dense plasma foci seems to be closely related to the I/(m)1/2 parameter of Z-pinches, where I and m stand for the current during stagnation and linear mass, respectively. On the 3-MA Z-pinch generator, it was found that the optimal I/(m)1/2 parameter reached the value of about 230 kA · (cm/μg)1/2. This optimal value can be explained by a tradeoff between the energy of fast deuterons and the number of fast and target deuterons. Obtained experimental results show that the optimal drive parameter cannot be fully explained without the consideration of ion acceleration mechanisms to fusion energies.

Producing Kiloelectronvolt L-Shell Plasmas on Zebra at UNR

Experiments with various wire loads from mid-atomic-number wires, which were performed on the university-scale 1-MA Zebra generator at the University of Nevada, Reno, during the last few years, are analyzed to assess the highest electron temperature reached. In particular, the results from experiments with planar wire arrays (PWAs) were considered. Load materials from mid-atomic-number such as stainless steel, Alumel, Cu, brass, Mo, and up to Ag were used to generate L-shell plasmas and to study plasma parameters. Though the full diagnostic set was utilized, the main focus was on X-ray spectroscopic data and on the non-local thermodynamic equilibrium kinetic modeling. As a result, the scaling of the maximum Te with the load material atomic number is presented for the first time in the range from Fe to Ag for L-shell plasmas from PWAs. The highest values of the electron temperature in L-shell plasmas, which are estimated from the modeling, were from both Ag PWAs and X-pinches. This work is important for the development of efficient X-ray radiators on university-scale Z-pinch generators.

Development of the 50 TW laser for joint experiments with 1 MA z-pinches

A 50 TW high-intensity laser (aka "Leopard" laser) was developed for experiments with the 1 MA z-pinch generator at the University of Nevada, Reno. The laser produces short pulses of 0.35 ps; energy is 15 J. Long pulses are 1 ns; energy is 30 J. The output beam diameter is 80 mm. The Leopard laser applies chirped pulse amplification technology. The laser is based on the 130 fs Ti:Sapphire oscillator, Öffner-type stretcher, Ti:Sapphire regenerative amplifier, mixed Nd:glass rod and disk amplifiers, and vacuum grating compressor. An adaptive optics system ameliorates focusing ability and augments the repetition rate. Two beam terminals are available for experiments: in the vacuum chamber of the z-pinch generator (aka "Zebra"), and a laser-only vacuum chamber (aka "Phoenix" chamber). The Leopard laser coupled to the Zebra z-pinch generator is a powerful diagnostic tool for dense z-pinch plasma. We outline the status, design, architecture and parameters of the Leopard laser, and its coupling to Zebra. We present the methods of laser-based z-pinch plasma diagnostics, which are under development at the University of Nevada, Reno.

Radiation from wire arrays imploding on an axial target

Adding thin wires on the axis of cylindrical and conical wire arrays significantly affected the x-ray emission of the conical wire arrays, while having practically no effect on that of the cylindrical ones. The experimental study was performed using Al wire arrays with central Al and Cu wires on the 1 MA Zebra Z-pinch generator at the Nevada Terawatt Facility. Adding wires on the axis of cylindrical wire arrays led to a decrease in plasma temperature and produced practically no change in the total radiation yield. In contrast, for conical wire arrays, both the plasma temperature and the total radiation yield increased approximately proportional with the central wire thickness. Both effects were stronger for Cu center wires than for Al center wires. When using Cu, radiative cooling played a substantial role in the implosion dynamics, leading to a more compact pinch than for Al wires. Adding Al wires on the axis of conical arrays produced a cocoon-shaped emissivity distribution, with a minimum on axis. For the case of Cu wires, a short-lived, straight, and narrow filament emitting photons with energy greater than 2.5 keV formed on the axis of the cocoon. This hot filament is produced by a transient heating mechanism intense enough to fully ionize Al, and the cocoon represents plasma heated by a conical shock formed in the conical array.

Spectroscopic features of implosions of Mo single- and double-planar wire arrays produced on the 1 MA Z-pinch generator

The spectroscopic features of implosions of Mo single- and double-planar wire arrays are studied. The experiments were performed on the 1 MA Zebra generator at UNR. Implosions of Mo planar wire arrays radiate high peak powers and produce high-temperature L-shell plasmas. In particular, plasma electron temperature of single-planar wire arrays of Mo reached ∼1375 eV. To estimate and analyze the evolution of plasma parameters in space and time, spatially resolved, time-integrated L-shell Mo X-ray spectra as well as time-gated and time-integrated pinhole images were collected and analyzed. A non-LTE kinetic model was applied to study the spatial structures, temperatures and densities of different wire loads of Mo planar wire arrays. Effects of non-Maxwellian hot electrons on L-shell radiation of Mo are discussed. Furthermore, Mo planar wire arrays generate bright spots or clusters of bright spots along the axial directions. Temperature and density gradients inside these bright spots or its clusters are also investigated. The results are compared with results of previous experiments with X-pinches on UNR Zebra and nested wire arrays at SNL-Z. Radiation magnetohydrodynamics modeling was performed to analyze the mechanisms of Z-pinch plasma heating in the presence of strong density gradients.

Radiative properties of implosions of combined X-pinches and planar wire arrays composed from different wire materials on the UNR 1 MA Z-pinch generator

The radiation from implosions of X-pinches and planar wire arrays, which consist of alloyed Al and Mo wires, on the UNR 1MA ZEBRA generator was studied. Combined, planar-loop X-pinches are good surrogate sources for studying radiative properties of Z-pinch plasmas that can be of small size, high density and high temperature as are standard X-pinches. Further, the X-pinches consist of the wires from different materials and can also exhibit distinct radiation properties. In particular, combined X-pinches studied in this work used an Al 5056 (5% Mg) wire in the anode (cathode) loop and a Mo wire in the cathode (anode) loop. Combined planar wire arrays had from 10 to 15 wires arranged in one plane and consisted of the same two wire materials as used in the X-pinches. We find that planar wire arrays radiate significant peak power. Spatially-resolved and spatially integrated X-ray spectral data as well as time integrated pinhole X-ray images were analyzed. Non-LTE kinetic models were applied to study the axial gradients of electron temperatures and densities in these X-pinch and Z-pinch plasmas. Specifically, X-ray spectra of K-shell Al and Mg, and L-shell Mo were modeled. As a result, we discuss the dependence of the plasma spatial structure, temperature, density, and opacity. The importance of using different materials or alloys for Z-pinch plasma diagnostics is illustrated.

Laboratory Simulation of Magnetospheric Plasma Shocks

An experimental simulation of planetary magnetospheres is being developed to investigate the formation of collisionless shocks and their effects. Two experimental situations are considered. In both, the solar wind is simulated by laser ablation plasmas. In one case, the “solar wind” flows across the magnetic field of a high-current discharge. In the other, a transverse magnetic field is embedded in the plasma flow, which interacts with a conductive obstacle. The ablation plasma is created using the “Tomcat” laser, currently emitting 5 J in a 6 ns pulse at 1 μm wavelength and irradiance above 1013 W/cm2. The “Zebra” z-pinch generator, with load current up to 1 MA and voltage up to 3.5 MV produces the magnetic fields. Hydrodynamic modeling is used to estimate the plasma parameters achievable at the front of the plasma flow and to optimize the experiment design. Particle-in-cell simulations reveal details of the interaction of the “solar wind” with an external magnetic field, including flow collimation and heating effects at the stopping point. Hybrid simulations show the formation of a bow shock at the interaction of a magnetized plasma flow with a conductor. The plasma density and the embedded field have characteristic spatial modulations in the shock region, with abrupt jumps and fine structure on the skin depth scale.