A. Haboub

A Zebra Experiment to Study Plasma Formation by Megagauss Fields

An alternative concept for fusion energy production is magnetized target fusion using metal liners to compress a mixture of magnetic flux and plasma fuel. In liner flux compression experiments, megagauss fields are produced at peak compression that heats the surfaces of aluminum walls of the liner cavity. Some radiation magnetohydrodynamic (MHD) modeling indicates that plasma formation should occur between 3 and 5 MG; however, such modeling depends on assumed material properties, which are a topic of ongoing research. Load hardware and diagnostics have been developed to study metal vapor and plasma formation on aluminum surfaces subjected to pulsed megagauss fields on the University of Nevada Zebra facility. The experiment is designed to study this interesting threshold for plasma formation. A current of 1 MA is pulsed along a stationary central rod to generate magnetic fields of 2-4 MG. The goal is to observe and diagnose the formation of metal vapor and plasma in the vicinity of the rod. The simple geometry enables easy access by diagnostics, which include magnetic sensors, filtered photodiode measurements, optical imaging, and laser schlieren, shadowgraphy, and interferometry. From these measurements, the magnetic field, the temperature of the surface metal plasma, the radiation field, and the growth of instabilities can be inferred. The diagnostics are time resolved to individually examine the distinct phases of heating, surface plasma formation predicted by radiation MHD modeling, and instability.

Investigation of ablation and implosion dynamics in linear wire arrays

Ablation and implosion dynamics were investigated by optical probing in linear wire arrays of different geometry. Formation of ablation jets begins on the outermost wires. In the beginning of implosion plasma bubbles arise in breaks on the outer wires. Implosion bubbles move to the next wire in the array and hit the plasma column with the speed >250km∕s. Imploding plasma moves to the center of the array cascading from wire to wire. Configuration of magnetic fields in the linear array can be changed by variation of wire spacing. The regimes of ablation and implosion in the wire arrays are found to differ with different wire spacing.

Effect of current prepulse on wire array initiation on the 1-MA ZEBRA accelerator.

Experiments on the 1-MA ZEBRA accelerator with reduced current prepulse duration, using a flashover switch, demonstrate a significant increase of initial energy deposition into the tungsten wire array before breakdown, and of total radiation energy from the Z pinch. Shorter current prepulse raises the current rate through each individual wire in the array and results in an increase of the energy deposition into wire cores before breakdown. In our experiments, the inferred tungsten wire temperature increases from ∼800K (with 250ns prepulse) up to ∼3700K (with 60ns prepulse). Total radiation energy increases from 12 to 16kJ. Our experimental results relate wire-array initiation to heating of the individual array wires up to the time of breakdown.

Initiation of aluminum wire array on the 1-MA ZEBRA accelerator and its effect on ablation dynamics and x-ray yield

The effect of current prepulse on the initiation of Al wire arrays, ablation dynamics and x-ray production was investigated on the 1-MA ZEBRA accelerator (University of Nevada, Reno). It is shown that increasing the number of wires lowers the temperature of the wire cores at the time of breakdown. Al arrays with cold wire cores demonstrate long and inhomogeneous ablation, and a less intense, wider x-ray pulse. Shortening the current prepulse by a flashover switch causes an increased wire-core temperature, symmetrization and synchronization of the wires’ ablation, and improvement of the amplitude and shape of the x-ray pulse. Application of a vacuum flashover switch can be important for shortening the current prepulse on the upcoming 28-MA ZR-accelerator at Sandia National Laboratories to optimize the x-ray production and shot-to-shot reproducibility from wire-array Z pinches.

Observation of Cascade Implosions in “Star”-Like Wire-Array

The dynamics of implosions in ldquostarrdquo-like wire arrays were investigated in the 1-MA Zebra generator. The hydrodynamic mode of implosion was confirmed by several plasma diagnostics, including laser probing, an optical streak camera, and a time-gated charge-coupled-device camera. Implosion begins on the edge wires and cascades from wire to wire accelerating toward the center.

Z

Star-like and closely spaced nested wire array configurations were investigated in precursor and non-precursor implosions. Closely spaced nested cylindrical arrays have inner and outer arrays with equal wire numbers, and inner and outer wires aligned to each other. The gap between the outer and inner wires is not more than 1 mm. Calculation of magnetic fields shows that the small gap results in a reversed, outward j × B force on the inner wires. Closely spaced arrays of 6–16 wires with outer diameter of 16 mm and with gaps of ΔR = 0.25–1 mm were tested. 6–8-wire arrays with a gap of ΔR = 0.4–1 mm imploded without precursor, but precursor was present in loads with 12–16 wires and ΔR = 0.25–1 mm. Implosion dynamics of closely spaced arrays was similar to that of star-like arrays. Implosion time was found to decrease with decreased wire numbers. Star array configurations were designed with a numerical scheme to implode with or without precursor. The lack of precursor resulted in a marginal improvement in tot...

-Pinches on the Zebra Generator

Plasma dynamics in cylindrical closely coupled and star wire arrays at ablation and implosion stages were studied. Formation of the Z-pinch and its radiative properties were studied in the non-precursor regime and compared with regular ablation regime with a precursor.

Study of the precursor and non-precursor implosion regimes in wire array Z-pinches

Time-resolved laser-probe diagnostics and gated x-ray imaging of star-like wire array z-pinch implosions have shown implosion characteristics that are more stable than those of other types of wire arrays on the Zebra generator. Here, we study the plasma conditions achieved in star-like wire arrays implosions using time-integrated and spatially-resolved x-ray spectroscopy. To this end, spectroscopic data have been recorded along a line of sight perpendicular to the z-pinch axis using a convex KAP crystal spectrometer, with a spectral resolution power of 350 in a series of shots performed at the Nevada Terawatt Facility. The target loads were star-like wire arrays comprised of 9 to 15 wires, 7.6–25 microns in diameter each, and driven by the Zebra pulsed-power accelerator. The wire array material combined aluminum with wires made out of titanium, stainless steel, and tungsten. The implosion dynamics in wire arrays was observed with laser probing at wavelengths of 532 and 266 nm and time-gated x-ray pinhole camera. The spectroscopic analysis focuses on the K-shell emission of aluminum, but the total energy radiated in soft and K-shell x-rays was also measured. The observed aluminum K-shell spectra show line transitions in Li-, He- and H-like Al ions; analysis of the spectra using detailed collisional-radiative atomic kinetics and radiation transport calculations yields electron temperature and density in the plasma. We discuss the plasma conditions achieved for several types of star-like wire array configurations, and its connection with soft and K-shell radiated energy.

The study of ablation and implosion dynamics in closely coupled nested cylindrical and star wire array Z pinches

Summary form only given. Optical laser diagnostics are widely used for probing Z-pinch plasmas. Measurement of the electron plasma density with regular laser interferometry meets the zero-number fringe issue on the axis of the Z-pinch. From the ablation stage on, the density of the inhomogeneous plasma increases quickly and produces very complicated structures of fringes. Even two-frame interferometry cannot derive the plasma density because it does not include non-perturbed reference fringes, and the plasma near the electrodes and the wires frame the axial area of the pinch. We suggest a new diagnostic to record a continuous history of the interferograms and the individual evolution of the fringes. In this case, the plasma density could be measured by deriving the shift of the fringes on the slit of a streak camera. This diagnostic is based on the Nd:YAG laser with a long probing pulse of 300 ns, at a fundamental wavelength of 1064, Mach Zehnder interferometer, and an optical streak camera. A CW laser at the wavelength of 1064 nm with a power 0.7 W, seeds two multi-pass amplifiers. A Pockels Cell cuts a 300-500 ns pulse from the CW radiation for amplification. A sevenpass amplification provides a total gain >10 s. Faraday rotators are used to prevent self-oscillation in the laser system. The laser pulse at the fundamental frequency will be converted to the second and fourth harmonics. This diagnostic has been developed in order to provide precision measurements of the plasma density during the ablation and the implosion phases in wire arrays.

X-ray spectroscopy of z-pinches in implosions of wire arrays with combined materials

Star-like wire arrays [1] with two closely located wires (“gates”) on the inner cylinder were studied on the 1-MA Zebra generator. The hydrodynamic mode of collision is typical for regular star-like wire arrays [2] and it was observed with shadowgraphy and optical streak cameras. In this mode, star-like wire arrays generate a short 8-12 ns x-ray pulse [1]. The star-like wire arrays were modified to reach a transparent mode typical for multiwire nested arrays [3–4].