Energy transport in short-pulse-laser-heated targets measured using extreme ultraviolet laser backlighting

The accurate characterization of thermal electron transport and the determination of heating by suprathermal electrons in laser driven solid targets are both issues of great importance to the current experiments being performed at the National Ignition Facility, which aims to achieve thermonuclear fusion ignition using lasers. Ionization, induced by electronic heat conduction, can cause the opacity of a material to drop significantly once bound-free photoionization is no longer energetically possible. We show that this drop in opacity enables measurements of the transmission of extreme ultraviolet (EUV) laser pulses at 13.9 nm to act as a signature of the heating of thin (50 nm) iron layers with a 50-nm thick parylene-N (CH) overlay irradiated by 35-fs pulses at irradiance $3\times {10}^{16}$ Wcm${}^{-2}$. Comparing EUV transmission measurements at different times after irradiation to fluid code simulations shows that the target is instantaneously heated by hot electrons (with approximately 10$\%$ of the laser energy), followed by thermal conduction with a flux limiter of $\approx$0.05.

Figure 1

(a) Schematic showing the measurement of EUV laser transmission through a short pulse (35 fs) laser heated iron layer (50 nm) buried in plastic (CH). The EUV laser is focused onto the target and the target EUV transmission is imaged onto a CCD camera. (b) A schematic of the iron buried layer target.

Figure 2

Image of the EUV laser radiation at 89 eV transmitted through a CH tamped 50-nm thick iron target at time 130 ps. The EUV radiation is transmitted through a largely unirradiated portion of the target (over $200\times 150$ $\mu$m${}^2$) with a smaller section of higher transmission where irradiation with a 35-fs laser pulse has heated the iron (encircled). On the line-out, A represents the level of EUV radiation transmitted through the unheated target, while B represents the base level of background signal found to be associated with self-emission from the tamped iron target. The large, over $600\times 200$ $\mu$m${}^2$ area of emission is due to scattered self-emission.

Figure 3

Transmission of EUV laser radiation at 89 eV as a function of time through a tamped 50-nm thick iron target. The target is irradiated at 0 ps by a $3\times {10}^{16}$ Wcm${}^{-2}$ peak irradiance pulse of 35-fs duration. Simulation results using the hyades code and a revised IMP opacity model postprocessor are superimposed on the measured transmissions (shown with error bars). Simulation results are shown for various values of flux limiter (as labeled) calculated assuming (i) energy transport occurs via thermal conduction only (dotted curves) and (ii) hot electrons dump a fraction 10$\%$ of the laser pulse energy through the target at 0 ps and then energy transport occurs via thermal conduction (solid curves).

Figure 4

Cross sections of (a) sample electron temperature $T_e$, (b) electron density $n_e$, (c) average ionization $Z^{*}$, and (d) target transmission $T$ as a function of distance from the target surface at 10 ps (full line) and 100 ps (broken line) after the irradiation of the target as illustrated in Fig. 1b with peak irradiance $3\times {10}^{16}$ Wcm${}^{-2}$ in a 35-fs pulse (with prepulse). The results are simulated using the hyades code with a flux limiter of 0.05 and an assumed dump of 10$\%$ of the laser energy into hot electrons which are deposited proportionally to the mass density in the target. The position of the iron component of the target is indicated by superimposed circles. The heating laser pulse is assumed incident from the left (negative distance positions). A prepulse starting 70 ps before the 35-fs pulse as measured in the experiment is assumed for the simulations.

Figure 5

(a) Sample target electron temperature $T_e$, (b) average ionization $Z^{*}$, and (c) electron density $n_e$ within the iron layer as a function of time from the arrival time of the heating pulse simulated using the hyades code at a peak intensity of $3\times {10}^{16}$ Wcm${}^{-2}$ calculated with a flux limiter of 0.05 and 10$\%$ hot electron energy dump in the target. A prepulse starting at time (70 ps as measured in the experiment followed by the 35-fs pulse at time “0 ps”) is assumed for the simulation.

Figure 6

Comparison of different opacity models of iron in the 85–95 eV photon energy region at a density of 0.008 gcm${}^{-3}$ and a temperature of 25 eV. The solid line is a detailed line accounting the prediction from the York opacity model [43] using atomic data from the Opacity Project [42]. Data from the IMP opacity model [38] which is accurate for spectrally broad backlighter calculations is also shown (dotted line revised IMP data, dashed line original IMP data). The vertical line shows the photon energy of the EUV laser.