R.W. Lemke

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 ...

Investigation of a load-limited, magnetically insulated transmission line oscillator (MILO)

The magnetically insulated transmission line oscillator (MILO) is a crossed-field microwave tube that is closely related to the linear magnetron. We discuss research involving a MILO whose DC operating characteristics (voltage and impedance) are determined by a current-carrying load. The load also forms part of a unique power extraction scheme. We show that maximum power is obtained from the load-limited MILO when an RF choke is used for the upstream boundary. This efficiency-enhanced version of the load-limited MILO yields gigawatt-level output power in both experiments and numerical simulations, results of which are also presented. In addition, we use the parapotential theory of magnetically insulated transmission lines in conjunction with observed simulation results to obtain estimates of maximum spoke current, spoke drift velocity, and maximum efficiency.

Near-absolute Hugoniot measurements in aluminum to 500 GPa using a magnetically accelerated flyer plate technique

Hugoniot measurements were performed on aluminum (6061-T6) in the stress range of 100–500 GPa (1–5 Mbar) using a magnetically accelerated flyer plate technique. This method of flyer plate launch utilizes the high currents, and resulting magnetic fields produced at the Sandia Z Accelerator to accelerate macroscopic aluminum flyer plates (approximately 12×25 mm in lateral dimension and ∼300 μm in thickness) to velocities in excess of 20 km/s. This technique was used to perform plate-impact shock-wave experiments on aluminum to determine the high-stress equation of state (EOS). Using a near-symmetric impact method, Hugoniot measurements were obtained in the stress range of 100–500 GPa. The results of these experiments are in excellent agreement with previously reported Hugoniot measurements of aluminum in this stress range. The agreement at lower stress, where highly accurate gas gun data exist, establishes the magnetically accelerated flyer plate technique as a suitable method for generating EOS data. Furth...

Direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium

Driving liquid deuterium into metal Quick and powerful compression can force materials to change their properties dramatically. Knudson et al. compressed liquid deuterium to extreme temperatures and pressures using high-energy magnetic pulses at the Sandia Z-machine (see the Perspective by Ackland). Deuterium began to reflect like a mirror during compression, as the electrical conductivity sharply increased. The observed conditions for metallization of deuterium and hydrogen help us to build theoretical models for the universes most abundant element. This a our understanding of the internal layering of gas giant planets such as Jupiter and Saturn. Science, this issue p. 1455; see also p. 1429 Magnetic compression drives an insulator-to-metal transition in dense liquid deuterium. [Also see Perspective by Ackland] Eighty years ago, it was proposed that solid hydrogen would become metallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary science. Here, we show direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium. Experimental determination of the location of this transition provides a much-needed benchmark for theory and may constrain the region of hydrogen-helium immiscibility and the boundary-layer pressure in standard models of the internal structure of gas-giant planets.Eighty years ago, it was proposed that solid hydrogen would become metallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary science. Here, we show direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium. Experimental determination of the location of this transition provides a much-needed benchmark for theory and may constrain the region of hydrogen-helium immiscibility and the boundary-layer pressure in standard models of the internal structure of gas-giant planets.

Magnetically driven isentropic compression to multimegabar pressures using shaped current pulses on the Z accelerator

A technique has previously been developed on the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] to generate ramped compression waves in condensed matter for equation-of-state studies [C. A. Hall, J. R. Asay, M. D. Knudson, W. A. Stygar, R. B. Spielman, T. D. Pointon, D. B. Reisman, A. Toor, and R. C. Cauble, Rev. Sci. Instrum. 72, 3587 (2001)] by using the Lorentz force to push on solid electrodes rather than to drive a Z pinch. This technique has now been extended to multimegabar pressures by shaping the current pulse on Z to significantly increase the sample thickness through which the compression wave can propagate without forming a shock. Shockless, free-surface velocity measurements from multiple sample thicknesses on a single experiment can be analyzed using a backward integration technique [D. B. Hayes, C. A. Hall, J. R. Asay, and M. D. Knudson, J. Appl. Phys. 94, 2331 (2003)] to extract an isentropic loading curve. At very high pressures, the accuracy of this method is dominat...

The split-cavity oscillator: a high-power E-beam modulator and microwave source

A compact device, called a split-cavity oscillator, whose self-excited oscillating electromagnetic field converts a large-area steady electron beam into one that is highly density modulated, is described. It does this in a short beam travel length, easing both space-charge and pinching limitations. Thus, high currents are possible without requiring a magnetic guide field. Methods for converting the modulated output beam into high-power microwaves are discussed, as are ways to phase-lock several oscillators together. Analytic theory, numerical simulations, and experiments describing the device are presented. >

Dynamic hohlraum driven inertial fusion capsules

A dynamic hohlraum is formed when an imploding annular cylindrical Z-pinch driven plasma collides with an internal low density convertor. This collision generates an inward traveling shock wave that emits x rays, which are trapped by the optically thick Z-pinch plasma and can be used to drive an inertial fusion capsule embedded in the convertor. This scheme has the potential to efficiently drive high yield capsules due to the close coupling between the intense radiation generation and the capsule. In prior dynamic hohlraum experiments [J. E. Bailey et al., Phys. Rev Lett. 89, 095004 (2002)] the convertor shock wave has been imaged with gated x-ray pinhole cameras. The shock emission was observed to be very circular and to be quite narrow in the radial direction. This implies that there is minimal Rayleigh–Taylor imprinting on the shock wave. Thus, the dominant source of radiation asymmetry is not random and in principle could be significantly decreased by proper design. Due to the closed geometry of the d...

Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments

The intense magnetic field produced by the 20 MA Z accelerator is used as an impulsive pressure source to accelerate metal flyer plates to high velocity for the purpose of performing plate impact, shock wave experiments. This capability has been significantly enhanced by the recently developed pulse shaping capability of Z, which enables tailoring the rise time to peak current for a specific material and drive pressure to avoid shock formation within the flyer plate during acceleration. Consequently, full advantage can be taken of the available current to achieve the maximum possible magnetic drive pressure. In this way, peak magnetic drive pressures up to 490 GPa have been produced, which shocklessly accelerated 850μm aluminum (6061-T6) flyer plates to peak velocities of 34km∕s. We discuss magnetohydrodynamic (MHD) simulations that are used to optimize the magnetic pressure for a given flyer load and to determine the shape of the current rise time that precludes shock formation within the flyer during ac...

Dynamics and characteristics of a 215-eV dynamic-hohlraum x-ray source on Z

A radiation source has been developed on the 20-MA Z facility that produces a high-power x-ray pulse, generated in the axial direction primarily from the interior of a collapsing dynamic hohlraum (DH). The hohlraum is created from a solid cylindrical CH2 target centered within an imploding tungsten wire-array Z pinch. Analyses and interpretation of measurements made of the x-ray generation within and radiated from the hohlraum target have been done using radiation-magnetohydrodynamic-code simulations in the r-z plane that take account of the magnetic Rayleigh–Taylor (RT) instability. These analyses suggest that a significantly reduced RT seed (relative to that used to explain targetless Z-pinch data on Z) is required to explain the observations. Although some quantitative and qualitative agreement with the measurements is obtained with the reduced RT seed, differences remain. Initial attempts to include into the simulations a precursor plasma, arising from wire material driven ahead of the main implosion,...

Compact single and nested tungsten-wire-array dynamics at 14–19MA and applications to inertial confinement fusiona)

Wire-array z pinches show promise as a high-power, efficient, reproducible, and low-cost x-ray source for high-yield indirect-drive inertial confinement fusion. Recently, rapid progress has been made in our understanding of the implosion dynamics of compact (20-mm-diam), high-current (11–19MA), single and nested wire arrays. As at lower currents (1–3MA), a single wire array (and both the outer and inner array of a nested system), show a variety of effects that arise from the initially discrete nature of the wires: a long wire ablation phase for 50%-80% of the current pulse width, an axial modulation of the ablation rate prior to array motion, a larger ablation rate for larger diameter wires, trailing mass, and trailing current. Compact nested wire arrays operate in current-transfer or transparent mode because the inner wires remain discrete during the outer array implosion, even for interwire gaps in the outer and inner arrays as small as 0.21mm. These array physics insights have led to nested arrays that...