arXiv subjects

James M. McNaney

Publications and source records attributed to James M. McNaney.

4 recordsLinked to original sources

arXiv2021

Equation of state and strength of diamond in high pressure ramp loading

Diamond is used extensively as a component in high energy density experiments, but existing equation of state (EOS) models do not capture its observed response to dynamic loading. In particular, in contrast with first principles theoretical EOS models, no solid-solid phase changes have been detected, and no general-purpose EOS models match the measured ambient isotherm. We have performed density functional theory (DFT) calculations of the diamond phase to ~10TPa, well beyond its predicted range of thermodynamic stability, and used these results as the basis of a Mie-Greuneisen EOS. We also performed DFT calculations of the elastic moduli, and calibrated an algebraic elasticity model for use in simulations. We then estimated the flow stress of diamond by comparison with the stress-density relation measured experimentally in ramp-loading experiments. The resulting constitutive model allows us to place a constraint on the Taylor-Quinney factor (the fraction of plastic work converted to heat) from the observation that diamond does not melt on ramp compression.

cond-mat.mtrl-sci↗

arXiv2020

Comment on J. Fernandez et al, "Requirements and sensitivity analysis for temporally- and spatially-resolved thermometry using neutron resonance spectroscopy," Rev. Sci. Instrum. 90, 094901 (2019)

The article by Fernandez et al argues that, because of advances made in laser-produced neutron pulses, it should now be possible to measure temperature in samples loaded to elevated pressures, by neutron resonance spectrometry with neutrons accelerated using a single sub-picosecond laser pulse in the 300J range. We point out that the configuration used for neutron source and detector would require an infeasibly large sample, potentially requiring tens of megajoules of laser energy to drive the sample. The laser-driven neutron technology is thus not yet mature enough to consider for new facilities, and the previously-demonstrated experiments at LANSCE still represent the only credible point design. We summarize some possible strategies for coupling brighter neutron sources with dynamic loads.

cond-mat.mtrl-sci↗

arXiv2009

Approximate, analytic solutions of the Bethe equation for charged particle range

By either performing a Taylor expansion or making a polynomial approximation, the Bethe equation for charged particle stopping power in matter can be integrated analytically to obtain the range of charged particles in the continuous deceleration approximation. Ranges match reference data to the expected accuracy of the Bethe model. In the non-relativistic limit, the energy deposition rate was also found analytically. The analytic relations can be used to complement and validate numerical solutions including more detailed physics.

cond-mat.other↗

arXiv2008

Shock formation and the ideal shape of ramp compression waves

We derive expressions for shock formation based on the local curvature of the flow characteristics during dynamic compression. Given a specific ramp adiabat, calculated for instance from the equation of state for a substance, the ideal nonlinear shape for an applied ramp loading history can be determined. We discuss the region affected by lateral release, which can be presented in compact form for the ideal loading history. Example calculations are given for representative metals and plastic ablators. Continuum dynamics (hydrocode) simulations were in good agreement with the algebraic forms. Example applications are presented for several classes of laser-loading experiment, identifying conditions where shocks are desired but not formed, and where long duration ramps are desired.

cond-mat.mtrl-sci↗