Kinetic effects and nonlinear heating in intense x-ray-laser-produced carbon plasmas

The x-ray laser-matter interaction for a low-$Z$ material, carbon, is studied with a particle-in-cell code that solves the photoionization and x-ray transport self-consistently. Photoionization is the dominant absorption mechanism and nonthermal photoelectrons are produced with energy near the x-ray photon energy. The photoelectrons ionize the target rapidly via collisional impact ionization and field ionization, producing a hot plasma column behind the laser pulse. The radial size of the heated region becomes larger than the laser spot size due to the kinetic nature of the photoelectrons. The plasma can have a temperature of more than 10 000 K ($>1\mathrm{eV}$), an energy density greater than ${10}^4$ $\mathrm{J}/{\mathrm{cm}}^3$, an ion-ion Coulomb coupling parameter $\Gamma \ge 1$, and electron degeneracy $\Theta \ge 1$, i.e., strongly coupled warm dense matter. By increasing the laser intensity, the plasma temperature rises nonlinearly from tens of eV to hundreds of eV, bringing it into the high energy density matter regime. The heating depth and temperature are also controllable by changing the photon energy of the incident laser light.

Figure 1

2D picls simulation results for (a) the x-ray laser energy density ($\mathrm{J}/{\mathrm{cm}}^3$) (linear scale) at 110 fs, and (b) the electron energy density ($\mathrm{J}/{\mathrm{cm}}^3$) (logarithmic scale) at 200 fs. (c) Bulk electron temperature (red solid line) and average charge state (blue dotted line) along the laser axis at 200 fs. (d) Transverse profile of the bulk electron temperature at 200 fs averaged from $X=10$ to 30 $\mu \mathrm{m}$. (e) Time evolution of the total number of ionization events in the system.

Figure 2

Intensity dependence of the absorption, average ionization degree ($Z_{\text{avg}}$), bulk temperature ($T_e$), and the ratio of energy used for collisional ionization to the absorbed energy. $Z_{\text{avg}}$ and $T_e$ are measured along the laser axis at 200 fs, after the x-ray laser has passed through the target completely.

Figure 3

Spectrum of x-ray photons emitted backwards from the target surface integrated until $t=200$ fs for different x-ray laser intensities: ${10}^{18}$, ${10}^{19}$, $3\times {10}^{19}$, and ${10}^{20}$ $\mathrm{W}/{\mathrm{cm}}^2$.

Figure 4

The Coulomb coupling parameter $\Gamma$ (blue dashed line) and the degeneracy parameter $\Theta$ (red solid line) in the XFEL-produced plasmas.

Figure 5

2D picls simulation results of a carbon target irradiated by an x-ray laser with $I={10}^{18}$ $\mathrm{W}/{\mathrm{cm}}^2$ and 1 keV photon energy. The plots, observed at 110 fs as in Fig. 1, are (a) the x-ray energy density ($\mathrm{J}/{\mathrm{cm}}^3$) (logarithmic scale), (b) the average charge state (linear scale), and (c) the electron temperature (eV) (linear scale).