Publications

Wire explosion experiments have been carried out at the University of Nevada, Reno. These experiments investigated the explosion phase of wires with properties and current-driving conditions comparable to that used in the initial stage of wire array z-pinch implosions on the Z machine at Sandia National Laboratories. Specifically, current pulses similar to and faster than the pre-pulse current on Z (current prior to fast rise in current pulse) were applied to single wire loads to study wire heating and the early development of plasmas in the wire initiation process. Understanding such issues are important to larger pulsed power machines that implode cylindrical wire array loads comprised of many wires. It is thought that the topology of an array prior to its acceleration influences the implosion and final stagnation properties, and therefore may depend on the initiation phase of the wires. Single wires ranging from 4 to 40 pm in diameter and comprised of material ranging from AI to W were investigated. Several diagnostics were employed to determine wire current, voltage, total emitted-light energy and power, along with the wire expansion velocity throughout the explosion. In a number of cases, the explosion process was also observed with x-ray backlighting using x-pinches. The experimental data indicates that the characteristics of a wire explosion depend dramatically on the rate of rise of the current, on the diameter of the wire, and on the heat of vaporization of the wire material. In this report, these characteristics will be described in detail. Of particular interest is the result that a faster current rise produces a higher energy deposition into the wire prior to explosion. This result introduces a different means of increasing the efficiency of wire heating. In this case, the energy deposition along the wire and its subsequent expansion, is uniform compared to a ''slow'' current rise (170 A/ns compared to 22 A /s current rise into a short circuit) and the expansion velocity is larger. The energy deposition and wire expansion is further modified by the wire diameter and material. Investigations of wire diameter indicate that the diameter primarily effects the expansion velocity and energy deposition; thicker wires explode with greater velocities but absorb less energy per atom. The heat of vaporization also categorizes the wire explosion; wires with a low heat of vaporization expand faster and emit less radiation than their high heat of vaporization counterparts.

Abstract not provided.

Numerical calculations have been performed to investigate the role that load thickness may play in the performance of fast annular z pinch implosions. In particular, the effects of load thickness on the mitigation of the magnetically-driven Rayleigh-Taylor (RT) instability and energy coupling between the load and generator are addressed. using parameters representative of the Z accelerator [R.B.Spielman et al., Phys.Plasmas, 5, 2105 (1998)] at Sandia National Laboratories, two dimensional magnetohydrodynamic (MHD) simulations show that increased shell thickness results in lower amplitude, slightly longer wavelength RT modes. In addition, there appears to be an optimum in load velocity which is directly associated with the thickness of the sheath and subsequent RT growth. Thin, annular loads, which should couple efficiently to the accelerator, show a large reduction in implosion velocity due to extreme RT development and increased load inductance. As a consequence, thicker loads on the order of 5 mm, couple almost as efficiently to the generator since the RT growth is reduced. This suggests that z-pinch loads can be tailored for different applications, depending on the need for uniformity or high powers.

The authors have constructed a quasi-analytic model of the dynamic hohlraum. Solutions only require a numerical root solve, which can be done very quickly. Results of the model are compared to both experiments and full numerical simulations with good agreement. The computational simplicity of the model allows one to find the behavior of the hohlraum temperature as a function the various parameters of the system and thus find optimum parameters as a function of the driving current. The model is used to investigate the benefits of ablative standoff and axial convergence.