In-Flight Measurements of Capsule Shell Adiabats in Laser-Driven Implosions

We present the first x-ray Thomson scattering measurements of temperature and density from spherically imploding matter. The shape of the Compton downscattered spectrum provides a first-principles measurement of the electron velocity distribution function, dependent on $T_e$ and the Fermi temperature $T_F\sim n_e^{2/3}$. In-flight compressions of Be and CH targets reach 6–13 times solid density, with $T_e/T_F\sim 0.4–0.7$ and ${\Gamma }_{ii}\sim 5$, resulting in minimum adiabats of $\sim 1.6–2$. These measurements are consistent with low-entropy implosions and predictions by radiation-hydrodynamic modeling.

Figure 1

(top) Illustration of the target and self-emission images of imploding Be shells. (bottom) Photo of a Be cone-in-half-shell target and radiation-hydrodynamic modeling of the imploding shell mass density vs radius and time. The white data points are measured radii from self-emission images and the white curve is the laser power profile vs time.

Figure 2

Measured (black) and best fits (red) to the Compton feature of the scattered x-ray spectra from Be implosions at various times, yielding $n_e$ and $T_e$.

Figure 3

(top) Example of rms fits of the experimental Compton feature for Be scattering data taken at $3.9\pm 0.1\mathrm{ns}$. (bottom) Fits plotted together with the measured Compton red wings for varying electron temperature and density.

Figure 4

Adiabat (top) and ion-ion coupling parameter (bottom) plotted as function of time for compressed Be and CH ablators. Also plotted are 1D radiation-hydrodynamic calculations.