Observations of Plasmons in Warm Dense Matter

We present the first collective x-ray scattering measurements of plasmons in solid-density plasmas. The forward scattering spectra of a laser-produced narrow-band x-ray line from isochorically heated beryllium show that the plasmon frequency is a sensitive measure of the electron density. Dynamic structure calculations that include collisions and detailed balance match the measured plasmon spectrum indicating that this technique will enable new applications to determine the equation of state and compressibility of dense matter.

Figure 1

Experimental setup for collective x-ray forward scattering is shown along with the $\mathbf{k}$-vector diagram and the x-ray probe spectrum.

Figure 2

(a) Experimental scattering data are shown along with the theoretical fit. (b) Comparison of the plasmon spectrum with calculations for three different densities indicating that the spectrum is best fit for $n_e=3\times {10}^{23}{\mathrm{cm}}^{-3}$. The dashed curve represents a calculation for a density of $n_e=3\times {10}^{23}{\mathrm{cm}}^{-3}$ that is neglecting collisions.

Figure 3

Plasma parameters from collective x-ray scattering and radiation-hydrodynamics calculations are shown. The dashed curve shows the electron density for solid-density beryllium for $Z=2.3$ measured previously in backscattering.