Behavior of the Thermal Cooling Instability at High-Energy-Density Scales

The thermal cooling instability has been the subject of one and two-dimensional numerical studies at astrophysical scales, but numerical simulations that directly capture its behavior at high-energy-density scales (L ∽ 1 mm, τ ∽ 1 ns) have been scarce. We have recently completed a series of studies of the thermal cooling instability using CRASH, the University of Michigan's predictive radiative hydrodynamic code. We will present the results of study of the thermal cooling instability using artificial cooling functions of the form Λ ∝ T_e^α and varied α (Λ has units energy per volume per time). This was intended to test an analytic prediction: systems are expected to transition from unstable to stable at α > α_crit, where α_crit = 2. At incoming velocities above 300 km/s, we found α_crit ≈ 2.3. However, as the incoming velocity dropped, α_crit shifted to higher values, suggesting that the thermal cooling instability may be occurring in regimes where it was previously thought impossible.