Marcus D. Knudson

Bio: Marcus D. Knudson is an academic researcher from Washington State University. The author has contributed to research in topics: Shock (mechanics) & Equation of state. The author has an hindex of 19, co-authored 49 publications receiving 1488 citations. Previous affiliations of Marcus D. Knudson include Sandia National Laboratories.

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

Abstract: 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 ...

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

Abstract: 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. Furthermore, the present results exhibit increased accuracy over the previous techniques used to obtain data in the higher-stress range. This improved accuracy enhances our understanding of the response of aluminum to 500 GPa, and lends increased confidence to the use of aluminum as a standard material in future impedance matching experiments.

Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments

Abstract: 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 acceleration to peak velocity. In addition, we present results pertaining to plate impact, shock wave experiments in which the aluminum flyer plates were magnetically accelerated across a vacuum gap and impacted z-cut, α-quartz targets. Accurate measurements of resulting quartz shock velocities are presented and analyzed through high-fidelity MHD simulations enhanced using optimization techniques. Results show that a fraction of the flyer remains at solid density at impact, that the fraction of material at solid density decreases with increasing magnetic pressure, and that the observed abrupt decrease in the quartz shock velocity is well correlated with the melt transition in the aluminum flyer.

Magnetically driven isentropic compression experiments on the Z accelerator

Abstract: Isentropic compression experiments (ICE) have been performed on the Z accelerator facility at Sandia National Laboratory. We describe the experimental design that used large magnetic fields to slowly compress samples to pressures in excess of 400 kbar. Velocity wave profile measurements were analyzed to yield isentropic compression equations of state (EOS). The method can also yield material strength properties. We describe magnetohydronamic simulations and results of experiments that used the “square short” configuration to compress copper and discuss ICE EOS experiments that have been performed with this method on tantalum, molybdenum, and beryllium.

Electrothermal instability growth in magnetically driven pulsed power liners

Abstract: This paper explores the role of electro-thermal instabilities on the dynamics of magnetically accelerated implosion systems. Electro-thermal instabilities result from non-uniform heating due to temperature dependence in the conductivity of a material. Comparatively little is known about these types of instabilities compared to the well known Magneto-Rayleigh-Taylor (MRT) instability. We present simulations that show electrothermal instabilities form immediately after the surface material of a conductor melts and can act as a significant seed to subsequent MRT instability growth. We also present the results of several experiments performed on Sandia National Laboratories Z accelerator to investigate signatures of electrothermal instability growth on well characterized initially solid aluminum and copper rods driven with a 20 MA, 100 ns risetime current pulse. These experiments show excellent agreement with electrothermal instability simulations and exhibit larger instability growth than can be explained by MRT theory alone.

Experimental astrophysics with high power lasers and Z pinches

Abstract: With the advent of high-energy-density (HED) experimental facilities, such as high-energy lasers and fast Z-pinch, pulsed-power facilities, millimeter-scale quantities of matter can be placed in extreme states of density, temperature, and/or velocity. This has enabled the emergence of a new class of experimental science, HED laboratory astrophysics, wherein the properties of matter and the processes that occur under extreme astrophysical conditions can be examined in the laboratory. Areas particularly suitable to this class of experimental astrophysics include the study of opacities relevant to stellar interiors, equations of state relevant to planetary interiors, strong shock-driven nonlinear hydrodynamics and radiative dynamics relevant to supernova explosions and subsequent evolution, protostellar jets and high Mach number flows, radiatively driven molecular clouds and nonlinear photoevaporation front dynamics, and photoionized plasmas relevant to accretion disks around compact objects such as black holes and neutron stars.

X-ray Thomson scattering in high energy density plasmas

Abstract: Accurate x-ray scattering techniques to measure the physical properties of dense plasmas have been developed for applications in high energy density physics. This class of experiments produces short-lived hot dense states of matter with electron densities in the range of solid density and higher where powerful penetrating x-ray sources have become available for probing. Experiments have employed laser-based x-ray sources that provide sufficient photon numbers in narrow bandwidth spectral lines, allowing spectrally resolved x-ray scattering measurements from these plasmas. The backscattering spectrum accesses the noncollective Compton scattering regime which provides accurate diagnostic information on the temperature, density, and ionization state. The forward scattering spectrum has been shown to measure the collective plasmon oscillations. Besides extracting the standard plasma parameters, density and temperature, forward scattering yields new observables such as a direct measure of collisions and quantum effects. Dense matter theory relates scattering spectra with the dielectric function and structure factors that determine the physical properties of matter. Applications to radiation-heated and shock-compressed matter have demonstrated accurate measurements of compression and heating with up to picosecond temporal resolution. The ongoing development of suitable x-ray sources and facilities will enable experiments in a wide range of research areas including inertial confinement fusion,more » radiation hydrodynamics, material science, or laboratory astrophysics.« less

The National Ignition Facility: Ushering in a new age for high energy density science

Abstract: The National Ignition Facility (NIF) [E. I. Moses, J. Phys.: Conf. Ser.112, 012003 (2008); https://lasers.llnl.gov/], completed in March 2009, is the highest energy laser ever constructed. The high temperatures and densities achievable at NIF will enable a number of experiments in inertial confinement fusion and stockpile stewardship, as well as access to new regimes in a variety of experiments relevant to x-ray astronomy, laser-plasma interactions, hydrodynamic instabilities, nuclear astrophysics, and planetary science. The experiments will impact research on black holes and other accreting objects, the understanding of stellar evolution and explosions, nuclear reactions in dense plasmas relevant to stellar nucleosynthesis, properties of warm dense matter in planetary interiors, molecular cloud dynamics and star formation, and fusion energy generation.

Forming a Moon with an Earth-like composition via a Giant Impact

Abstract: In the giant impact theory, the Moon formed from debris ejected into an Earth-orbiting disk by the collision of a large planet with the early Earth. Prior impact simulations predict that much of the disk material originates from the colliding planet. However, Earth and the Moon have essentially identical oxygen isotope compositions. This has been a challenge for the impact theory, because the impactor’s composition would have likely differed from that of Earth. We simulated impacts involving larger impactors than previously considered. We show that these can produce a disk with the same composition as the planet’s mantle, consistent with Earth-Moon compositional similarities. Such impacts require subsequent removal of angular momentum from the Earth-Moon system through a resonance with the Sun as recently proposed.