Demonstration of a laser-driven, narrow spectral bandwidth x-ray source for collective x-ray scattering experiments

Abstract

X-ray Thomson scattering (XRTS) is a powerful diagnostic technique that involves an x-ray source interacting with a dense plasma sample, resulting in a spectrum of elastically and inelastically scattered x-rays. Depending on the plasma conditions, one can measure a range of parameters from the resulting spectrum, including plasma temperature, electron density, and ionization state. To achieve sensitivity to collective electron oscillations, XRTS measurements require limited momentum transfer where the spectral separation of elastic and inelastic scattering is small. Such measurements require an x-ray probe source with a narrow bandwidth in order to reduce the spectral overlap between scattering contributions, allowing for the different features to be more precisely deconvolved. In this investigation, we discuss the theory behind how the bandwidth for a common XRTS probe, Zn He-α emission at 9 keV, can be reduced using a Cu K-edge filter. Proof-of-principle experiments conducted at the OMEGA laser facility confirm that this is an effective method for attenuating the higher energy He-α peak in the Zn emission spectrum. Calibration measurements at the National Ignition Facility show a reduction in spectral bandwidth from 87 eV to 48 eV when using the Cu filter, which will be important to improve the spectral resolution of future XRTS measurements that will probe plasmon oscillations in strongly compressed plasmas of low-Z materials at densities of tens of g/cm3.

Analysing the dynamic structure of warm dense matter in the imaginary-time domain: theoretical models and simulations

“…This unsatisfactory situation also represents a serious obstacle for the interpretation of experiments with WDM [53], as, because of the extreme conditions, often even basic parameters such as the temperature or the density cannot be directly measured and have to be inferred from other observations. A very important method for the diagnostics of WDM is X-ray Thomson scattering (XRTS) [54,55]; here, an X-ray beam is produced either from backlighter sources [56] or using free-electron X-ray lasers (XFELs), which have become available at large research facilities such as LCLS in the USA [57], SACLA in Japan [58] and the European XFEL in Germany [59]. An XRTS measurement gives one access to the scattering intensity signal, which is given by the convolution of the dynamic structure factor (DSF) Sfalse(q,ωfalse) with the combined source and instrument function Rfalse(ωfalse) [54], Ifalse(q,ωfalse)=Sfalse(q,ωfalse)⊛Rfalse(ωfalse).Unfortunately, the numerical deconvolution of equation (1.1) is generally prevented by noise in the experimental measurement.…”

“…A particular advantage of the τ-domain is given by the well-known convolution theorem, Lfalse[Sfalse(q,ωfalse)false]=Lfalse[Sfalse(q,ωfalse)⊛Rfalse(ωfalse)false]Lfalse[Rfalse(ωfalse)false],which makes the deconvolution trivial; in practice, it is easy to compute the Laplace transform of the XRTS intensity and then divide it by the Laplace transform of the instrument function Rfalse(ωfalse). Accurate knowledge of Rfalse(ωfalse) is usually available from source monitoring at XFEL facilities or from the characterization of backlighter emission spectra [56]. In this way, equation (1.3) gives one direct access to physical information, which enables accurate inference of the temperature of arbitrarily complex systems in thermodynamic equilibrium without the use of any model, simulation or approximation [62].…”

Thermal excitation signals in the inhomogeneous warm dense electron gas

“…The aforementioned applications and the intricate nature of WDM have triggered a growing number of cutting-edge experimental activities, often in large-scale research facilities 24 – 29 . The array of experimental techniques that probe excitations in WDM include X-Ray absorption spectroscopy 30 , emission spectroscopy 31 , 32 , X-Ray Thomson scattering 33 , resonant inelastic X-Ray scattering 34 , and the most recently developed ultrafast multi-cycle terahertz measurement technique 35 .…”

The Relevance of Electronic Perturbations in the Warm Dense Electron Gas

“…Understanding transient states in warm dense matter (WDM) is one of the grand challenges of plasma physics that is currently being tackled in a number of experimental facilities [1][2][3][4] . In these experiments, WDM is generated, for example, due to laser-induced shock compression [5][6][7] .…”