Our research focuses on the investigation of high energy-density plasmas using a wide range of experimental and theoretical techniques.

High energy-density plasmas are systems at temperatures above about 1 eV (∼11600 K) and at densities at or above that of a typical solid (1022–1023 atoms per cm3). Such systems are of broad interest because they are common throughout the universe, forming the cores or giant planets, brown dwarfs and stellar interiors. They are also transiently created in a wide range of intense laser-matter interactions, and the understanding of their equation of state, material response and transport properties is crucial for the interpretation, modelling and design of inertial confinement fusion experiments. Experimental research investigating the fundamental properties of high energy-density plasmas has been historically confronted by two major challenges. The first is the difficulty of creating simultaneously hot and dense plasmas controllably in homogeneous conditions over thermodynamically meaningful temporal and spatial scales. The second is the requirement for advanced diagnostics with sufficient spatial and temporal resolution to study the vast range of transient plasma dynamics ranging from attosecond electron processes all he way through to large atomic displacements and compressions taking place on nanosecond timescales. Diagnostics also should be able to penetrate deep into a dense system, which further limits the available set of experimental techniques and makes x-rays particularly suitable for this line of research.

X-ray free-electron lasers (XFELs), with their high brightness, narrow bandwidth and tunable wavelength, are ideal to support this research, both by enabling us to create and to investigate extreme states of matter. Our research focuses on using FEL facilities around the world – the Linac Coherent Light Source (LCLS) at Stanford (USA), the FLASH XUV FEL and the European XFEL in Hamburg (Germany), and the FERMI FEL in Trieste (Italy) – to develop new and improve existing techniques to create high energy-density plasmas controllably in the lab, and investigate their structure and dynamics on the fundamental timescales of electron interactions.

High Intensity X-Ray Interaction with Matter

We are interested in investigating how matter responds to, and interacts with, extreme X-ray intensities on ultra-short timescales. At x-ray wavelengths the ponderomotive energy of the laser field remains small for even the highest achievable intensities approaching 1020 Wcm-2, and plasma non-linearities and hot-electron generation, commonly observed at optical wavelengths with conventional lasers, become negligible. However, the high x-ray intensities afforded by FELs have lead to the observation of several novel non-linear process, including the observation of saturable absorption at XUV and x-ray wavelengths in a range of systems from Al to Cu, lasing in the x-ray regime, resonant plasma dynamics, and resonant inelastic scattering.

X-Ray Plasma Spectroscopy

We continue to develop and deploy emission spectroscopy and resonant inelastic scattering techniques to explore the charge-stage-resolved electronic structure of dense plasmas.

This effort has led to novel investigations of the physics of continuum lowering in solid-density systems at temperatures exceeding a million Kelvin, and enabled the first direct experimental measurement of ionization potential depression in a hot dense plasma.
We have also used x-ray-driven emission spectroscopy to measure electron dynamics via a novel core-hole atomic Auger clock: by tuning an XFEL beam to specific bound-bound resonances in Mg and MgF2 we were able to measure sub-femtosecond electron impact ionization rates in solid-density systems for the first time.

Most recently, we demonstrated the viability of single-shot resonant inelastic scattering, and used it to determine the temperature and valence density of states of nickel heated by x-rays to temperatures exceeding 200,000K.

Quantum Electronic Structure Simulations

Our research relies on first principles simulations of electronic structure based on density functional theory to support and enhance our experimental efforts.

We have developed novel core-hole excited-state Projector Augmented Wave (PAW) potentials to model hot-dense plasmas, and have used this to shed light on the physical process of continuum lowering.

We also explore local plasma properties at high densities, including how states re-localize and de-localize as a function of electron density, and on how such dynamics influences the equation of state of ionized matter in extreme conditions.