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UBC Theses and Dissertations

Shock wave as a probe of flux-limited thermal transport in laser-heated solids Smith, Kristin A.


Classical Spitzer-Harm treatment of electron thermal conduction breaks down in the steep temperature gradients found in laser-heated solids. A phenomenological flux-limiter, which reduces heat flow, is incorporated into computer codes which model laser-target interactions. There is disagreement in what the correct value of the flux-limiter, ∫, should be. A simple method to determine the best value is presented. It involves comparing experimental shock speed data with predicted values for the range of possible values of ∫, i.e. between 0.03 and 0.6, where 0.6 represents the free-streaming limit. Three different laser intensity regimes are investigated (2x10¹³, 2 x 10¹⁴, and 1 x 10¹⁵ W/cm²) using a trapezoidal (100 ps rise and fall times, 400 ps flat top) 0.532 μm laser pulse. Two laser absorption models are also compared: the traditional inverse bremsstrahlung (IB) absorption and an electromagnetic wave solver (EMS). The first calculates the local absorption factor, ⍺, as the laser light penetrates into the target, and requires a free parameter to start the simulation. This parameter legislates a fraction of the penetrating laser energy to be deposited at the critical density surface. The second method solves the time evolution of the Helmholtz equations for electromagnetic waves in an inhomogeneous dielectric. It is shown that the predicted shock speed is sensitive to ∫ in the range 0.03≲∫≲0.08 at 2x10¹⁵ W/cm², and 0.03≲∫≲0.15 at 1x10¹⁵ W/cm². A shock speed of (3x10⁶±5%) cm/s is predicted using the EMS method and laser irradiance of (2x10¹⁴ ± 10%) W/cm² with ∫≃0.090[sup +0.095]/[sub -0.022]. The IB method does not give a unique solution. The same irradiance gives the required speed with ∫≃[sup +0.006]/[sub -0.007] assuming 1% energy dump at the critical density, and with ∫≃0.03 assuming 10% dump. For this reason, the EMS method is preferred.

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