Abstract Details
Femtosecond imaging of surface heat transport in solid target excited at relativistic intensity
Author: Michael C Downer
Requested Type: Oral Only
Submitted: 2009-04-18 17:05:54
Co-authors: H. Langhoff, B. T. Bowes, J. Nees, B. Hou
Contact Info:
University of Texas at Austin
1 University Station C1600
Austin, TX 78712
United States
Abstract Text:
We present an extension of recent fs microscopy experiments [1] in which a planar Al target is excited by a 24 fs pump focused to intensity up to 3e18 W/cm2 in a ~ 1 micron radius spot; subsequent heat propagation along the target surface appears as a region of reduced reflectivity that is imaged by a delayed probe pulse. Isotropic expansion of the surface heated region out to 15 microns within 500 fs is observed for pump intensities above e18 W/cm2. We present a semi-empirical theoretical model in which the pump drives hot electrons into the target via collisionless interactions. A return current heats the target and, above a critical temperature, includes runaway electrons that return to the surface before dissipating their energy. Ultrafast radial expansion of the heated surface layer is explained by lateral diffusive motion of returning runaway electrons that undergo damped oscillations between vacuum and a sub-surface layer confined by space charge. Isotropy of the observed expansion is consistent with dominance of RA over jxB heating, indicating prepulse heating is important.
[1] B. Bowes et al., Opt. Lett. 31, 116 (2006).
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