Measurements of Electron Transport in Foils Irradiated with a Picosecond Time Scale Laser Pulse

The heating of solid foils by a picosecond time scale laser pulse has been studied by using x-ray emission spectroscopy. The target material was plastic foil with a buried layer of a spectroscopic tracer material. The laser pulse length was either 0.5 or 2 ps, which resulted in a laser irradiance that varied over the range ${10}^{16}–{10}^{19}\mathrm{W}/{\mathrm{cm}}^2$. Time-resolved measurements of the buried layer emission spectra using an ultrafast x-ray streak camera were used to infer the density and temperature conditions as a function of laser parameters and depth of the buried layer. Comparison of the data to different models of electron transport showed that they are consistent with a model of electron transport that predicts the bulk of the target heating is due to return currents.

Figure 1

The peak temperature as a function of depth for various laser parameters. The results are for (1) a $P$-polarized, bare infrared beam, ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$; (2) $S$, $2\omega$, $6\times {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$; (3) $P$, $2\omega$, $6\times {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$; (4) $S$, $2\omega$, ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$; (5) $P$, $1\omega$ with a plasma mirror, $6\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$; (6) $P$, $2\omega$, ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$; and (7) data from germanium (blue) and titanium (red) layers.

Figure 2

X-ray emission spectra measured with an ultrafast streak camera with time going from left to right. The data shown are representative examples from experiments at different irradiance and polarization. These are (a) $S$-polarized ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$, $4\mu \mathrm{m}$ layer separation; (b) $P$-polarized ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$, $15\mu \mathrm{m}$ layer separation; (c) $P$-polarized $5\times {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$ separation $10\mu \mathrm{m}$; and (d) $P$-polarized $1.4\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$, separation $4\mu \mathrm{m}$.

Figure 3

Comparison of the THOR2 model predictions and experimental results. The black data points are aluminum $P$ $2\omega$ ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ and at $6\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$ using infrared light and a plasma mirror at $P$ polarization. The red and blue data points are from titanium and germanium buried layers, respectively. The solid blue and red lines are the predictions of thor2 for $T_h=300\mathrm{keV}$, 35% absorption 35° and 25° divergences, respectively. The dotted line is the 25° case with the magnetic field off. The solid black line is the collisional heating due only to the forward current.