M. Keith Matzen

Z pinches as intense x-ray sources for high-energy density physics applications

Fast Z-pinch implosions can efficiently convert the stored electrical energy in a pulsed-power accelerator into x rays. These x rays are produced when an imploding cylindrical plasma, driven by the magnetic field pressure associated with very large axial currents, stagnates upon the cylindrical axis of symmetry. On the Saturn pulsed-power accelerator [R. B. Spielman et al., in Proceedings of the 2nd International Conference on Dense Z Pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3] at Sandia National Laboratories, for example, currents of 6–8 MA with a rise time of less than 50 ns are driven through cylindrically symmetric loads, producing implosion velocities as high as 108 cm/s and x-ray energies exceeding 400 kJ. Hydromagnetic Rayleigh–Taylor instabilities and cylindrical load symmetry are critical, limiting factors in determining the assembled plasma densities and temperatures, and thus in the x-ray energies and...

Pulsed-power-driven high energy density physics and inertial confinement fusion research

The Z accelerator [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] at Sandia National Laboratories delivers ∼20MA load currents to create high magnetic fields (>1000T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10MJ∕cm3 and temperatures >1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers >200TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3Mbar and accelerates flyer plates to >30km∕s for equation of state ...

Structure and observable characteristics of laser driven ablation

Numerical hydrodynamic‐heat flow simulations of spherical ablation have been done with sufficient spatial resolution to show the details of the ablation front structure. These simulations show a continuous qualitative change in the velocity spectrum of expanding ions with increasing laser pulse length, ranging from approximately isothermal behavior from short pulse to ablative behavior from longer pulses. The ablative behavior is characterized by an energetic peak in the ion spectrum, a characteristic of ablation that is experimentally observable. The longer pulse cases are found to be in good agreement with the stationary flow model of ablation.

Non‐Maxwellian H and F velocity distributions in an H2–F2 reaction

Time‐dependent and quasi‐steady‐state solutions for the velocity distribution functions of H and F atoms in a burning, homogeneous, and isotropic H2–F2 gas mixture show an increase in the rates of the H+F2 and F+H2 chemical reactions that approaches a factor of two larger than the thermalized rates. The mixture is ignited by dissociating a small fraction of the F2 molecules. Approximations to the time‐dependent Boltzmann equation for the system reduce it to a form which is linear in the H and F distribution functions. This limits the solutions to early time behavior and small initial F‐atom concentrations, but it allows a general picture of the burn rates to be developed from a moderate number of calculations.

A classical trajectory study of inelastic collisions between highly vibrationally excited KBr and Ar

We present a study of inelastic collisions between highly vibrationally excited KBr and Ar based on three‐dimensional classical trajectory calculations. Calculations are performed for three closely related potential energy surfaces chosen to allow determination of the importance of an attractive well. Comparison with experiment indicates that both depth and shape of an attractive well are important for calculating detailed scattering distributions. Analysis of selected trajectories indicates that double impact collisions play an important role in the overall vibrational deactivation mechanism. Double impact collisions occur frequently for all three surfaces and, unlike the situation in one dimension, transfer energy very efficiently.

Impact of nonequilibrium ionization and recombination processes on the evaluation of laser‐produced plasmas

A Lagrangian hydrodynamics code is used to provide the temperature and density parameterization for a coupled set of rate equations which describe the number densities of the ionization stages in a laser‐produced, expanding plasma. Comparison of the steady state and rate equation ion density distributions indicates that time delays in the ionization of the plasma can affect x‐ray calculations whereas the calculation of the freezing out of the recombination processes in the plasma is necessary to predict signals from ion diagnostics.

Z pinches as intense x-ray sources for inertial confinement fusion applications

Abstract Fast z-pinch implosions can convert more than 10% of the stored electrical energy in a pulsed-power accelerator into X-rays. On the Saturn pulsed-power accelerator at Sandia National Laboratories, currents of 6–8 MA with a risetime of less than 50 ns have been used to drive cylindrically-symmetric arrays of wires, producing X-ray energies greater than 400 kJ with X-ray pulsewidths less than 5 ns and peak X-ray powers of 75±10 TW. Using similar loads, PBFA Z has produced >1.5 MJ and >150 TW of X-rays in the first 4 months of operation in the z-pinch mode. These X-ray energies and powers are records for laboratory X-ray production. The X-ray output can be thermalized into a near-Planckian X-ray source by containing it within a cylindrical radiation case (a hohlraum). These energetic, intense, large volume, long-lived hohlraum X-ray sources have recently been used for ICF-relevant ablator physics experiments and offer the potential for performing many new basic physics and fusion-relevant experiments.

Wire number dependence of the implosion dynamics, stagnation, and radiation output of tungsten wire arrays at Z driver

We report results of the experimental campaign which studied the initiation, implosion dynamics and radiation yield of tungsten wire arrays as a function of the wire number. The wire array dimensions and mass were those of interest for the Z-pinch driven ICF program. An optimization study of the X-ray emitted peak power, rise time and FWHM was effectuated by varying the wire number while keeping the total array mass constant and equal to ~5.8 mg. The driver utilized is the ~20 MA Z accelerator in its usual short pulse mode of 100 ns. We studied single arrays of 20 mm diameter and 1 cm height. The smaller wire number studied was 30 and the largest 600. It appears that 600 is the highest achievable wire number with present days technology. Radial and axial diagnostics were utilized including crystal monochromatic X-ray backlighter. An optimum wire number of ~370 was observed which is very close to the routinely utilized 300 for the ICF program in Sandia.

Z driver post-hole convolute studies utilizing MYKONOS-V voltage adder

The modern high current, high voltage pulsed accelerators utilize vacuum-post-hole convolutes to add the current of a number of parallel self Magnetic Insulated Transmission Lines (MITL) to a single one located very close to the centrally located load. The reason of course of using several parallel MITLs to transfer the current pulse from large, ~1.5 m, radii to the 1-2 cm load is to reduce the transfer inductance. For example, the vacuum chamber of the 24-26-MA Z machine has a 1.45-m radius vacuum section containing four parallel conical MITLs merging into one 6cm radial disc MITL adjacent to the centrally located load via a double post-hole convolute array located at 7.62 cm from the axis. Although special care has been taken to reduce the electrical stresses on the cathode hole surfaces in order to avoid electron emission, substantial current losses, 4-6 MA, are observed most probably due to plasma formation and the unavoidable magnetic nulls. In the proposed experiments we will study the behavior of only one convolute using the MYKONOS-V driver. MYKONOS-V is a Linear Transformer Driver (LTD) voltage adder composed of 5 nominally 1-MA cavities connected in series. The voltage adder radial A-K cavity is deionized water insulated. The experimental set-up is designed in such a way to reach conditions on the convolute very similar to those existing on Z. Most importantly, in contrast to Z, it provides full view of the convolute for optical and spectroscopic imaging and gives the flexibility and freedom to study various options in an effort to reduce the convolute losses without affecting the day-to-day Z experiments. This is going to be a dedicated convolute study experiment. The hardware design, numerical simulations and proposed diagnostics will be presented and discussed.