Full-Trajectory Diagnosis of Laser-Driven Radiative Blast Waves in Search of Thermal Plasma Instabilities

A. S. Moore, E. T. Gumbrell, J. Lazarus, M. Hohenberger, J. S. Robinson, R. A. Smith, T. J. A. Plant, D. R. Symes, M. Dunne
2008 Physical Review Letters  
Experimental investigations into the dynamics of cylindrical, laser-driven, high-Mach number shocks are used to study the thermal cooling instability predicted in astrophysical radiative blast waves. A streaked Schlieren technique measures the full blast wave trajectory on a single-shot basis, which is key for observing shock-velocity oscillations. Electron density profiles and deceleration parameters associated with radiative blast waves were recorded, enabling the calculation of important
more » ... on of important blast wave parameters as a function of time for comparison with radiation hydrodynamics simulations. PACS numbers: 52.35.Tc, 52.50.Jm, 52.72.+v An understanding of the role of thermal and dynamical instabilities in plasmas is crucial in creating accurate numerical models of heating and mixing in complex astrophysical systems. In many such systems radiative effects strongly influence the dynamics and can lead to instabilities which are responsible for many of the complex astrophysical structures observed. Radiative blast waves present a physical system that can be instructively studied through observations, numerical modeling and laboratory experiments. Blast waves result when the rarefaction behind a shock front overtakes it, forming a thin shell that contains all the swept-up mass. If the temperature of the post-shock material creates conditions for efficient cooling then energy is radiated through the optically thin shock front. The upstream material is partially preheated, being opaque at some photon energies, but also transmits a considerable fraction of the radiated energy causing the deceleration of the blast wave shell to increase since the system loses energy. In astrophysics this describes the third phase in the evolution of the Supernova Remnant (SNR), the pressure-driven snowplow, and it is this radiative phase that has been particularly linked to observed unstable phenomena such as unsteady ultraviolet and optical line emission from the Cygnus Loop and Vela. In the laboratory, hydrodynamic conditions, which may be closely scalable to astrophysics, can be reproduced through utilization of the efficient absorption of high-power, short-pulse laser pulses by atomic clusters [1] [2] [3] . This decouples the short-timescale (<ps) cluster expansion from the late-time (many ns) hydrodynamic expansion of interest. Experiments of this type can be used as a powerful aid to understanding astrophysical systems since they enable repeatable access to all phases of the radiation-driven hydrodynamics and have been highlighted as an important area of laboratory astrophysics research [4, 5] . Non-radiative, energy conserving blast waves follow a self-similar expansion as identified by Sedov and Taylor [6] . The blast wave radius R follows a power law in time R ∝ t n in which n = 0.5 in cylindrical and 0.4 in spherical geometry. In addition to decelerating more quickly, radiative blast waves can also be identified through observation of an ionized precursor region ahead of the shock front. Two similar instabilities exist for blast waves in which shock compression is enhanced by the effects of radiation: the dynamical overstability (DO) and the oscillating thermal cooling instability (TCI) [7, 8] . To date experimental research has looked for the effects of the first [2, 9, 10], but the second has not been experimentally investigated and requires a different diagnostic approach. Both mechanisms rely on strong post-shock cooling, but exhibit very different characteristics. The first results from a mismatch in the direction of the thermal and ram pressure vectors either side of the shock and can cause ripples in the position of the shock front to grow. In contrast the TCI depends on the temperature scaling of the cooling function. The cooling function describes the power radiated by a gas of certain density, as a function of electron temperature, and is typically described in terms of a power-law: Λ(T ) ∝ ρ 2 T α , where ρ is the density, T is the temperature and α is the power-law exponent. Provided that α ≡ d(lnλ)/d(lnT ) ≤ 1 then the TCI is predicted to cause an oscillation of the shock velocity D [8]. Its occurence is dependent solely on the shape of the cooling function. If a gas radiates more strongly with increasing temperature, the situation can arise where shock-heated gas radiates with such efficiency that the shock front collapses, but is then reformed by the high pressure of the freshly shocked gas. In order to meet these criteria in an astrophysical shock, it has been shown that D ≥ 120km s −1 , but since the instability is
doi:10.1103/physrevlett.100.055001 pmid:18352379 fatcat:nvgxov3ge5elvnc7dt4aflohba