Professor Justin Wark

Abstract:

This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.

Abstract:

We present measurements of the spectrally-resolved X-rays emitted from solid-density magnesium targets of varying sub-μm thicknesses isochorically heated by an X-ray laser. The data exhibit a largely thickness-independent source function, allowing the extraction of a measure of the opacity to K-shell X-rays within well-defined regimes of electron density and temperature, extremely close to local thermodynamic equilibrium (LTE) conditions. The deduced opacities at the peak of the K-α transitions of the ions are consistent with those predicted by detailed atomic-kinetics calculations.

Abstract:

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption (RSA). It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast non-radiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10^16~17 W/cm2. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in extreme state of matters, as well as affording the potential to regulate ultrafast XFEL pulses.