Experimental methods for warm dense matter research

Katerina Falk

doi:10.1017/hpl.2018.53

Abstract

The study of structure, thermodynamic state, equation of state (EOS) and transport properties of warm dense matter (WDM) has become one of the key aspects of laboratory astrophysics. This field has demonstrated its importance not only concerning the internal structure of planets, but also other astrophysical bodies such as brown dwarfs, crusts of old stars or white dwarf stars. There has been a rapid increase in interest and activity in this field over the last two decades owing to many technological advances including not only the commissioning of high energy optical laser systems, z-pinches and X-ray free electron lasers, but also short-pulse laser facilities capable of generation of novel particle and X-ray sources. Many new diagnostic methods have been developed recently to study WDM in its full complexity. Even ultrafast nonequilibrium dynamics has been accessed for the first time thanks to subpicosecond laser pulses achieved at new facilities. Recent years saw a number of major discoveries with direct implications to astrophysics such as the formation of diamond at pressures relevant to interiors of frozen giant planets like Neptune, metallic hydrogen under conditions such as those found inside Jupiter’s dynamo or formation of lonsdaleite crystals under extreme pressures during asteroid impacts on celestial bodies. This paper provides a broad review of the most recent experimental work carried out in this field with a special focus on the methods used. All typical schemes used to produce WDM are discussed in detail. Most of the diagnostic techniques recently established to probe WDM are also described. This paper also provides an overview of the most prominent examples of these methods used in experiments. Even though the main emphasis of the publication is experimental work focused on laboratory astrophysics primarily at laser facilities, a brief outline of other methods such as dynamic compression with z-pinches and static compression using diamond anvil cells (DAC) is also included. Some relevant theoretical and computational efforts related to WDM and astrophysics are mentioned in this review.

Highlights

Interest in the experimental study of warm dense matter (WDM) especially in relation to planetary interiors and high pressure states has been around since the invention of the diamond anvil cell (DAC) that could be used to reach high pressures and temperatures[1]
During the past two decades the research of WDM has experienced an accelerated development thanks to the availability of novel high power laser and accelerator facilities. This field gained a lot of interest among scientists from various fields including astrophysics, geophysics, material science, shock physics and energy
WDM is dominant inside large gaseous, and rocky planets, ice giants, brown dwarfs or crusts of old stars. It appears in many dynamic processes like collisions of celestial bodies and astrophysical shock waves

Summary

Introduction

Interest in the experimental study of warm dense matter (WDM) especially in relation to planetary interiors and high pressure states has been around since the invention of the diamond anvil cell (DAC) that could be used to reach high pressures and temperatures[1]. The first interest of the high power laser community in the study of WDM equation of state (EOS) was triggered by the initial research carried out on the ICF technology, since it soon became apparent that the EOS of WDM would play an important role in the efficiency of implosion of the DT ice pellets[6] It was not until mid-1990s, when the large scale laser and z-pinch facilities like the OMEGA laser or the Zmachine were built, that the field truly boomed thanks to novel technology capable of generating WDM. With the development of novel facilities such as short-pulse optical lasers and X-ray free electron lasers (FELs) capable of delivering pulses with femtosecond duration and high repetition rates, the field is currently undergoing another major transition With these new generation radiation sources, experiments can achieve a wider range of WDM conditions and probing of nonequilibrium states. Developments in the theoretical description of WDM with emphasis on EOS models, transport properties such as conductivity and their application for simulations of planetary structures and formation are briefly reviewed at the end of this publication

Planetary interiors

Generation of WDM

Diagnostic methods

Theoretical models

Findings

Summary