G.S. Sarkisov

Investigation of Magnetic Fields in 1-MA Wire Arrays and

A Faraday rotation diagnostic was applied for the investigation of magnetic fields in plasma of 1-MA wire arrays and X-pinches. Laser-probing diagnostics at the Zebra generator include a four-channel polarointerferometer and a four-frame shadowgraphy. The Faraday rotation diagnostic consists of shadow and Faraday channels, shearing air-wedge interferometer, and an additional schlieren channel. The implosion dynamics of the wire arrays were studied. A current in the plasma column of Al low-wire number arrays was found by the Faraday rotation diagnostic. Optical diagnostics showed a turbulent plasma and bubblelike objects in the plasma column of Al wire arrays. The Faraday rotation diagnostic demonstrated a complicated structure of magnetic fields in X-pinch plasma

X

Experimental results on Joule energy deposition upon initiation of a fast electrical explosion of 16-μm tungsten wire in vacuum at current densities of more than 108 A/cm2 are reported. We have found that explosion with a fast current rise time (∼170 A/ns into a short) results in homogeneous and enhanced deposition of electrical energy into the tungsten before surface flashover. The maximum tungsten wire resistivity reaches a value of up to ∼185 μΩ cm before surface flashover that significantly exceeds the melting boundary and corresponds to a temperature of ∼1 eV. The highest values for light radiation and expansion velocity of wire ∼1 km/s were observed for the fast explosion. For the explosion mode with a slower current rise time (∼22 A/ns into a short), we observed the existence of an “energy deposition barrier” for tungsten wire. In the slow explosion mode, the current is reconnected to the surface shunting discharge before melting. The maximum tungsten wire resistivity in this case reaches the value of ∼120 μΩ cm, which is less than indicative of melting. Also, the energy deposition along the wire is strongly inhomogeneous, and wire is disintegrated into parts. We attribute the early reconnection of the current to the surface discharge for the slow explosion to high electron emission from the wire surface, which starts before melting.

-Pinches

Time-integrated and laser-probing imaging diagnostics provide insight about the physics of wire explosions. Most interesting results have been obtained for refractory metal wires. In these cases, imaging diagnostics provide insight into the structure and value of the energy deposited into metal.

Homogeneous electrical explosion of tungsten wire in vacuum

Laser probing diagnostics at the wavelength of 266 nm were developed for investigation of the 1-MA z-pinch plasmas. The absorption and refraction in plasma are significantly smaller at 266 nm than at the regular wavelength of 532 nm. These features allow observation of fine details in the z-pinch plasma at the ablation, implosion, and stagnation phases. Two-color shadowgraphy at 532/266 nm presents a structure of ablating wires and implosion bubbles in wire arrays. Plasma distribution and dynamics in compact cylindrical, star, and planar wire arrays can be studied at the wavelength of 266 nm. An electron density Ne > 5 · 1019 cm-3 was reconstructed with interferometry at 266 nm in the stagnated z-pinch. Further development of laser probing diagnostics of the z-pinch plasmas is discussed.

Imaging of exploding wire phenomena

The dynamics of large- and small-scale plasma structures is investigated in the precursor of 1-MA wire array Z-pinches by laser probing diagnostics. It is found that plasma streams from the wires induce density perturbations in the precursor. Small-scale perturbations and large-scale cells arise in the nonlinear stage before implosion. The spatial and temporal scales of the observed structures are in agreement with the theoretical investigation for current-driven excitation of electromagnetic flute modes.

Development of UV Laser Probing Diagnostics for 1-MA Z-Pinches

A high-repetition-rate, 2-TW Z-pinch (Zebra or HDZP-II from LANL: 2 MV, 1.2 MA, 100 ns, 200 kJ, 1.9 ohm) has been assembled to investigate the early-time evolution of a current-driven wire, the plasma turbulence around and between wires, the acceleration of a plasma current sheet by a magnetic field, and the suppression or reduction of plasma instabilities, and to generate radiation for applications. The heating, expansion, and dynamics of wires driven by current prepulses similar to those at SNL-Z is being examined in isolated wires and soon in SNL-Z wire arrays. 290 trillion watts of X-rays can now be generated by a few cubic millimeters of plasma. The source of this plasma is the Z-pinch. This plasma confinement device drives a giant current through a tiny load, compressing and heating it with extreme current-produced magnetic fields. The Z-pinch suffers from plasma instabilities that limit its performance. The ultimate performance limit of the Z-pinch is unknown: another order of magnitude increase in X-ray power levels may be possible. Such an improvement would open up new applications. Understanding the dense Z-pinch is vital to the search to ameliorate it. This article describes the activation of the 2-TW Zebra Z-pinch, the development of diagnostics, and an initial single-wire experiment.

Experimental Study of the Dynamics of Large- and Small-Scale Structures in the Plasma Column of Wire Array

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.

Z

Important results of initial stage of nanosecond exploding wire in vacuum are presented. Polarity effect and current rate effect are responsible for homogeneity of energy deposition into wire core before breakdown and later after breakdown axial ablation symmetry of the wire. It was shown experimentally that symmetry of plasma cylinder as well as final X-ray yield affected by initiation phase of exploding wires during current prepulse. Atomization enthalpy of metal significantly effects property of electrical explosion of wires. Experimental results based on set of electrical, laser probing and spectroscopic diagnostics. Magneto hydro dynamic simulation successfully reproduced main feature of the fast and hot electrical explosion. A thermodynamic calculation demonstrates.

-Pinches

Summary form only given. The effect of current prepulse on wire array core temperature on the 1-MA z-pinch installation ZEBRA was investigated using current, light emission, 4-channel laser shadowgraphy, visible ICCD camera and PCD/XRD diagnostics. A vacuum flashover switch reduced the original ~250ns prepulse on ZEBRA to as short as ~50ns. Shortened prepulse increases the initial current rate through the individual wires and this in turn leads to more energy deposited into the wire cores (higher temperature) at breakdown. Shorter current prepulse also improved the axial homogeneity for deposited energy. Better axial homogeneity of deposited Joule energy improves the axial symmetry of precursor plasma and reduces its typically observed m=l instability. Reduction of the precursor instability results in the generation of a single X-ray pulse rather than the multiple pulses observed for unstable precursor. Multi-frame shadowgraphy shows that discrepancy in the initial wire-electrode contacts results in differences in energy deposition and ablation from wire to wire. Keeping similar contacts for all wires in the array is critical for azimuth ablation uniformitv. Shortening the current prepulse using a flashover switch results in an increase of light emission peak and wire core temperature. Core temperature rises from ~800K to ~1100K and finally to ~3100K. at moment of breakdown. Moment t=0 corresponding to the beginning of main current pulse.

Two-terawatt Zebra Z-pinch at the Nevada terawatt facility

Summary form only given. Dynamics of the implosion stages in cylindrical, nested, and linear wire arrays were compared in the 1-MA Zebra generator. Using multi-frame laser probing and gated X-ray imaging in wire array Z-pinches, measurements of the microscopic origins of mass flow instabilities and current distribution were performed. In cylindrical wire arrays, plasma bubbles bring material to the axis of the array with a speed 200-500 km/s and produce a shock during collision with the precursor plasma column. A bubble-like mass transport is observed in all types of wire arrays (cylindrical, nested, linear, etc). In linear wire arrays, ablation and implosion begins on the outermost wires. Imploding plasma moves to the center of the array cascading from wire to wire. Nested arrays with the equal wire length were tested. Two-step implosions were observed in double nested arrays. Implosion begins on the outer array. The plasma bubbles then hit the plasma columns at the locations of the inner array wires, which then collapses to the center. Rescaling of plasma perturbations was observed in linear and nested arrays. This rescaling was also observed in Al nested double and triple wire arrays. In Al triple wire arrays, smoothing of plasma instabilities was observed in the last phase of the implosion.