Experiments and simulations of isochorically heated warm dense carbon foam at the Texas Petawatt Laser

R Roycroft,B M Hegelich,P Hakel,S Blouin,E L Vold,P A Bradley,B Bowers,L Yin,E Mccary,B J Albright,H Smith,H J Quevedo,G M Dyer

doi:10.1063/5.0026595

Abstract

An experimental and simulation study of warm dense carbon foams at ambient density (ne ∼ 1021 cm−3) is presented. This study of isochorically heated foams is motivated by their potential application in carbon-atmosphere white-dwarf envelopes, where there are modeling uncertainties due to the equation of state. The foams are heated on an approximately picosecond time scale with a laser-accelerated proton beam. The cooling and expansion of the heated foams can be modeled with appropriately initialized radiation-hydrodynamics codes; xRAGE code is used in this work. The primary experimental diagnostic is the streaked optical pyrometer, which images a narrow band of radiation from the rear surface of the heated material. Presented are xRAGE modeling results for both solid aluminum targets and carbonized resorcinol-formaldehyde foam targets, showing that the foam appears to cool slowly on the pyrometer because of partial transparency. So that simulations of cooling foam are processed properly, it is necessary to account for finite optical depth in the photosphere calculation, and the methods for performing that calculation are presented in depth.

Highlights

We are motivated to study warm dense matter (WDM) because of its prevalence in many physical systems relevant to high-energydensity physics, including imploding inertial-confinement-fusion capsules and astrophysical bodies such as stellar and giant-planet interiors
WDM is defined as a state of matter in which the Coulomb coupling parameter and the electron degeneracy parameter are both of order unity, thereby making it difficult to calculate the equation of state (EOS) and other transport properties
While these two shots were taken under similar conditions, a much higher peak brightness temperature was achieved on shot 11 477 because of more ions being accelerated and better coupling to the Texas Petawatt Laser (TPWL) short-pulse beam

Summary

Introduction

We are motivated to study warm dense matter (WDM) because of its prevalence in many physical systems relevant to high-energydensity physics, including imploding inertial-confinement-fusion capsules and astrophysical bodies such as stellar and giant-planet interiors. In the present experiment on heated carbon foam, the coupling parameter was near unity and the degeneracy parameter (the ratio of the kinetic energy to the Fermi energy) was between 2 and 8 depending on the shot. The motivation for the present work was to achieve laboratory conditions that are analogous to those in the atmospheres and envelopes of white-dwarf stars with carbon lines [DQ white dwarfs (DQWDs)] by heating carbon foams to between ∼1 eV and 2 eV. DQWDs constitute a class of helium-rich white dwarfs with substantial concentrations of carbon in their atmospheres, which is relevant to our carbon-foam experiments. DQWD atmospheres have electron densities of up to 1018 cm−3, while their envelopes have electron densities of up to 1023 cm−3 This density– temperature regime is within the WDM parameter space, laboratory studies can provide benchmarks for theoretical models of scitation.org/journal/mre white-dwarf atmospheres and envelopes.. Kritcher et al. emphasized recently the current interest in white-dwarf EOS properties, they examined the behavior of the principal Hugoniot at pressures and temperatures far higher than those that we achieve with isochoric heating

Methods

Results

Conclusion