P.L. Coleman

Scaling microsecond-conduction-time plasma opening switch operation from 2 to 5 MA

We describe experiments in which conduction currents were successfully scaled from 2 to 5 MA for conduction times around 1 /spl mu/s in a coaxial geometry plasma opening switch (POS) on the 4 MJ ACE 4 driver. Simple models of POS operation, derived from previous work, were used to scale successful results from drivers that operate at microsecond conduction times, but at currents below 1 MA. An MHD model for the conduction phase was verified in which the square root of the plasma density is approximately proportional to the product of conduction time and peak conduction current divided by the switch radius and length. For the opening phase, a model where the POS gap is approximately constant when the local plasma conditions at the beginning of the conduction phase are kept roughly the same is consistent with the observed POS opening voltages of at least 1 MV. The conduction current was increased by increasing the POS cathode radius from 6 to 20 cm while maintaining roughly the same plasma density. This increase in radius resulted in the observed increase in the conduction-current/conduction-time product from 2 to 5 MA /spl mu/s while maintaining MV POS voltages.

Architecture, implementation, and testing of a multiple-shell gas injection system for high current implosions on the Z accelerator

Tests are ongoing to conduct ~20 MA z-pinch implosions on the Z accelerator at Sandia National Laboratory using Ar, Kr, and D2 gas puffs as the imploding loads. The relatively high cost of operations on a machine of this scale imposes stringent requirements on the functionality, reliability, and safety of gas puff hardware. Here we describe the development of a prototype gas puff system including the multiple-shell nozzles, electromagnetic drivers for each nozzles valve, a UV pre-ionizer, and an inductive isolator to isolate the ~2.4 MV machine voltage pulse present at the gas load from the necessary electrical and fluid connections made to the puff system from outside the Z vacuum chamber. This paper shows how the assembly couples to the overall Z system and presents data taken to validate the functionality of the overall system.

PRS Load B-Dot Current Measurements on Argon Z-Pinches

Summary form only given. PRS implosion kinematics depend on the load currents time-dependent spatial distribution and magnitude. Time-dependent axial shunting of the load current to the return current posts can contribute to non-optimum assembly of the pinch on-axis, reducing the ideally achievable K-shell yield. The presence of plasma and the harsh environment in the PRS load region typically make current measurements close to the load difficult. Previously, we reported on the use of axially distributed, single loop B-dots mounted azimuthally between the return current posts. Those measurements suggested initial axial current propagation delays and exhibited plasma shielding which depended on the nozzle outer-plenum gas pressure. We present here new current measurements in the downstream PRS load region on pulsed power simulators using a 12-cm diameter argon gas puff Z-pinch. The data were taken under two drive conditions: 1) at peak currents up to 3.5 MA with implosion times exceeding 200 ns; and 2) at about 6 MA for 250-300 ns implosions. We used three different magnetic probe configurations: B-dots between current return posts (as previously reported), a hidden post B-dot (located in a recess on the back side of a current return post), and recessed in an azimuthally continuous, return current ring. A comparison of current measurements taken in the different magnetic probe configurations will be presented. All show some degree of plasma shielding of the probes. Much of the new data comes with significantly higher statistics, allowing the level of reproducibility of the measurements to be ascertained. Differences between the current measurements at the two current levels will be contrasted

A renewed argon gas puff capability on Sandia's Z machine

Summary form only given. We have reestablished gas puff z-pinch capability on Sandias 20 MA Z machine, including a Sandia-operated driver system and an imaging interferometer to characterize nozzle mass flow [1]. Initial experiments have focused on developing a 3 keV Ar K-shell x-ray source. We have pursued a design-driven approach to planning these experiments, utilizing numerical simulation to predict Ar K-shell yield for various nozzle mass profile configurations. In particular, we study coupling to the generator and how the distribution of mass between the two shells impacts magnetic Rayleigh-Taylor instability evolution. Two-dimensional radiation-magneto-hydrodynamic (MHD) simulations at NRL for a number of density profiles produced by the nozzle have predicted yields in excess of 300 kJ, and indicated that a 1:1.6 outer-to innershell mass ratio would produce the most stable implosion with high enough temperature to optimize Ar K-shell output [2]. This result was also consistent with 3D MHD modeling using the Gorgon code [3] at Sandia. Both models used tabulated non-LTE atomic models for Ar K-shell photon emission. We will present Z experimental data from the first gas puff shots on the accelerator since 2006, and compare these to the numerical models. Spectral output is measured from 1-20 keV. Electrical current measurements at different positions along the power flow section provide information on current coupling to the load. Time-gated pinhole imaging and radially-resolved spectroscopy indicate ~60 cm/μs implosion velocities and >1 keV electron temperatures.

Fast pulsed cluster jet

Summary form only given. Short burst, ultrafast laser pulses interact intensely with matter to generate beams of secondary radiation such as coherent x-rays via high harmonic generation, electron bunches via laser wakefield acceleration, and protons via laser-driven ion acceleration. These secondary radiation sources have applications in biological imaging, medical diagnostics and treatment, and nondestructive evaluation. The emerging field of laser-plasma acceleration (LPA) has demonstrated electron accelerators with unprecedented electric field gradients. Supersonic, highly collimated gas jets and gas-filled capillary discharge waveguides are two primary targets of choice for LPA. A new LPA accelerated beam energy record of >2 GeV has been recorded using the Texas Petawatt laser (150 J) focused into a 7 cm He gas cell. The electron beams were highly-collimated (<;1 mrad divergence), containing high charge (>1 nC), and had a broad energy spectrum (peaked at ~2 GeV, with electrons up to 2.4 GeV). A fast opening and closing gas valve is essential to a LPA. This paper describes a fast valve (developed under a DOE SBIR grant) that opens in <;100μs, closes in <;400μs and can run (in cooled mode) at ~10Hz rep-rates at pressures as high as 1000psia. Recently we have designed advanced versions of the nozzle to create ~10-15mm long supersonic gas jets with tailored density gradients to test the concept of phase locking in an LPA. Dense gas jets with high concentrations of clusters are also of interest for such ultra-fast laser interactions. Development of a dense cluster jet using our ultra-fast opening/closing valve is also described.

Using tomographic (“ART”) methods with an interferometer to diagnose asymmetrical gas flow

Summary form only given. A variety of plasma applications make use of pulsed gas jets. In some cases like a z-pinch1, one requirement is for cylindrical symmetry in the mass distribution delivered by a nozzle system. In other cases like astrophysical simulations2 or laser wakefield accelerators (LWA)3, controlled asymmetry is desired. Hence there is a need for diagnostic methods to assess the 3D geometry of the cold gas flow from a nozzle. This paper describes the use of an interferometer and tomographic techniques to derive the 3D characteristics of the flow.

Continued development of triple plena gas puff nozzles for Z

We have previously reported [1] on the designs of triple shell nozzles, 8 and 12 cm in diameter, for use at >20 MA on the Z generator. Here we describe the characterization of the cold gas flow, a critical input for shot preparations on Z as well as MHD modeling of the implosions.

Multi-color gated x-ray pinhole imaging of Z-pinch implosions on the Saturn and Z pulsed power generators

Magnetically driven implosions of wire array and gas puff loads on the 20 MA Z and 8 MA Saturn pulsed power drivers provide extremely intense x-ray sources. Each facility fields multi-frame, multi-color, x-ray pinhole cameras in order to assess source uniformity and study implosion dynamics. The instruments feature cameras filtered for >1 keV photons, as well as pinhole cameras in which &#60;1 keV images are reflected from planar multilayer mirror (MLM) monochromators1. Microchannel plate cameras provide &#60;1 ns time resolution, and spatial resolutions of 100–500 µm are typical when imaging >10-mm-scale z pinches. A Cr/C MLM on the Z machine is used routinely for 277 eV monochromatic imaging with &#60;10 eV bandwidth, showing a broad shell of imploding wire array plasma which generates 1–10 keV K-shell x-rays when it stagnates on the axis of symmetry. After peak x-ray power, an m=1 instability is seen to grow rapidly, followed by the pinch column breaking and disrupting late in time. This 277 eV imager may also be used for hohlraum hole closure measurements on Z. A W/Si mirror reflecting 528 eV images corresponding to an L-shell emission line of Ar has been demonstrated in Saturn gas puff experiments. A zippered implosion is observed, with K line emission generated as the plasma arrives on axis, and L-shell radiation following as the plasma cools. Collisional radiative modeling may be employed to understand line and continuum contribution to the imaged signal, and to generate synthetic images.

Modeling of gas puff Z-pinch experiments at the ZR facility

Gas puff z-pinch experiments have been proposed for the refurbished Z (ZR) facility for CY2011. Previous gas puff experiments1 on pre-refurbishment Z established a record for laboratory fusion neutron yield; new experiments will establish gas puff capability on ZR and should surpass previous neutron and X-ray yields. We present ALEGRA resistive-MHD simulations of planned experiments with deuterium and high-Z gases such as argon. We also attempt to quantify the chaotic nature of implosions by evaluating Lyapunov exponents, and consider the implications for shot-to-shot variability in experiments and for the sensitivity of simulations to initial conditions.

2D radiation MHD model assessment of initial argon gas distributions to be imploded on the Z machine

Argon z-pinch experiments are to be performed on the refurbished Z machine at Sandia National Laboratories with a new 8 cm diameter double-annulus gas puff nozzle constructed by AASC. The gas exits the nozzle from an outer and inner annulus and a central jet. The amount of gas present in each region can be varied. Here we employ a two-dimensional radiation MHD model to theoretically investigate stability and K-shell emission properties of several measured (interferometry) initial gas distributions. Of particular interest is determining the influence that the central jet and the ratio of the amount of gas in the outer to inner annulus have on K-shell emission and stability. Our model incorporates into the Mach2 two-dimensional MHD code a self-consistent calculation for non-local thermodynamic equilibrium kinetics and ray trace based radiation transport. This level of detail is necessary in order to model opacity effects and the high temperature state of K-shell emitting z-pinch loads that can be fielded on the Z machine. Comparisons of radiation properties and stability of the AASC gas profiles are made with that of the Titan 1234 nozzle that was used in numerous successful pre-refurbished Z experiments.1 Based on these comparisons, an optimal mass distribution for the AASC nozzle is theoretically determined and predictions are made for K-shell yields attainable from future Z experiments with this nozzle.