Measuring the microscopic properties of warm dense matter and driven solids

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Under this project a student will develop the technique of inelastic x-ray scattering (IXRS) to diagnose both warm dense matter and laser-shocked and ramp-compressed solids. IXRS is a technique whereby an incoming hard x-ray scatters from a dense system, creating or destroying a quantum of an acoustic mode. In the case of a dense plasma, this is scattering from the so-called ion-acoustic modes, whereas in the case of a solid, one scatters from the phonons within the system. As the frequency of the acoustic modes is small (a few tens of meV for phonons, and unto a few 100 meV for ion acoustic waves) the energy gain or loss experienced by the x-ray (which is of order 10 keV or so) is small. This necessitates the use of highly monochromatic x-rays (with a bandwidth of order 1 part in a million), which in turn requires an extremely bright x-ray source if transient matter is to be interrogated-that is to say we must use a 4th generation light source- an x-ray laser.
The project comprises several components. (1) Using x-ray scattering from ion acoustic waves in dense plasmas to provide information on the pressure and equation of state. In these experiments the frequency/k-vector relationship of the ion acoustic waves are recorded by measuring the frequency and wave-vector gain/loss of the x-ray incident on a dense laser-produced plasma. This provides the speed of sound in the system, which in turn is related to the pressure. Moreover, the shape of the spectrum itself contains a wealth of additional information. The frequency broadening of the plasmon resonances is related to the viscosity, and the ratio between the intensity of the light scattered by waves to that scattered by randomly moving ions is a measurement of the thermal conductivity.
(2) Using the same technique in ramp compressed and shocked materials to directly measure the temperature-this being a long-standing problem in shock physics. In this case the up and down-shifted x-rays (Stokes and anti-Stokes) are related in their intensity by the Boltzmann factor according to the law of detailed balance. For ramp compressed materials typical temperatures are of order the Debye temperature, and thus there is a good measurable difference in the intensity of the two scattered peaks.
(3) The technique, as discussed above, provides a unique tool for the measurement of transport coefficients - namely viscosity, thermal conductivity and sound speed. These are notoriously difficult to obtain in compressed matter and they affect how the compressed matter evolves. Evolutionary models for planets strongly depend on the choice of the transport coefficients and so our experiments can give useful experimental constraints.In addition, the values of such coefficients under warm dense matter conditions are believed to reach the bounds set by string theory methods-that is, the AdS/CFT correspondence. Such correspondence postulate that under a conformal transformation, a strongly coupled fluid can always be remapped onto as a weakly interacting fluid onto the event horizon of a black hole. These techniques are new in our field, but the work proposed here could offer different avenues to be explored.The experiments described above will be performed on the HED end-station at the European XFEL in Hamburg. We envisage the first year of the project will also involve detailed experimental design, assessing the best trade off between spectral resolution and collection efficiency (the proof of principle experiments at LCLS have used a resolution of order 70 meV, where a figure closer to 15 -20 meV may be more suitable for direct temperature measurements).Within the first part of the project the student will also asses which materials will be the best candidates for early experiments. We will apply for time to use the HED end-station at XFEL in both parasitic x-ray only mode (to test the optics), and then from mid 2019 onwards we will request time to use it in conjunction with the DIPOLE laser.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R513295/1 01/10/2018 30/09/2023
2117021 Studentship EP/R513295/1 01/10/2018 31/03/2022 Oliver Roland Karnbach