Summary of previously reported experimental results for Al target excited at . (a) Experimental setup for femtosecond microscopy with time-integrated x-ray spectrum (top) and selected images of the heated plasma using a -polarized probe (bottom). (b) Lineouts from probe images at two pump laser intensities as indicated. Radial expansion of the low-reflectivity region is observed only at the higher pump intensity (left).
Diameter of hot region vs time delay for . Data points: measurements of diameter of region of reduced probe reflectivity. Error bars reflect variations from measuring diameter through different cross sections, as well as run-to-run variations. Solid curve: calculated diameter of surface region heated to by lateral diffusion and inelastic collisions of runaway electrons. Dashed line: diameter of directly photoexcited region.
Probe images as in Fig. 1(a), but for Al target excited at . (a) . Probe light illuminates the entire frame; the bright central spot is scattered SHG by tightly focused pump. (b) . Reduced reflectivity region, outlined by dashed curve, is in diameter, well beyond central bright spot showing pump-irradiated area. Horizontal dashed arrow denotes plane and direction of pump incidence. (c) 3D plot of grayscale image in (b) showing central spot from pump SHG in center of surrounding reflectivity minimum.
Calculated temperatures vs pump intensity of (a) ions in the heated zone and (b) runaway electrons. Temperatures plotted are at the end of the laser pulse at the surface of the target averaged transversely within the intensity contour of the focused pump spot. Horizontal dashed line in (a): critical temperature for production of runaway electrons.
Diameter of surface region with temperature exceeding vs pump intensity. Data points: measured diameter of region of reduced probe reflectivity. Solid curve: Estimated diameter after stoppage of runaway electrons from model calculations. Upper left inset: drawing of diffusive motion of runaway electrons.
Calculated values of hot electron range , space charge electric field at , and plasma conductivity at surface and averaged over for incident pump intensities listed in first column.
Calculated depth profiles of temperature and space charge electric field for incident pump intensity .
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