Strong shocks and blast wave collisions are commonly observed features in astrophysical objects such as nebulae and supernova remnants. Numerical simulations often underpin our understanding of these complex systems, however modelling of such extreme phenomena remains challenging, particularly so for the case of radiative or colliding shocks. This highlights the need for well-characterized laboratory experiments both to guide physical insight and to provide robust data for code benchmarking. Creating a sufficiently high-energy-density gas medium for conducting scaled laboratory astrophysics experiments has historically been problematic, but the unique ability of atomic cluster gases to efficiently couple to intense pulses of laser light now enables table top scale (1 J input energy) studies to be conducted at gas densities of >10 19 particles cm −3 with an initial energy density >5 × 10 9 J g −1 . By laser heating atomic cluster gas media we can launch strong (up to Mach 55) shocks in a range of geometries, with and without radiative precursors. These systems have been probed with a range of optical and interferometric diagnostics in order to retrieve electron density profiles and blast wave trajectories. Colliding cylindrical shock systems have also been studied, however the strongly asymmetric density profiles and radial and longitudinal mass flow that result demand a more complex diagnostic technique based on tomographic phase reconstruction. We have used the 3D magnetoresistive hydrocode GORGON to model these systems and to highlight interesting features such as the formation of a Mach stem for further study.

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We report on experimental investigations into strong, laser-driven, radiative shocks in noble-gas cluster media. Cylindrical shocks launched with several J exhibit strong radiative effects such as increased deceleration and radiative preheat. Using time-resolved propagation data from single-shot streaked Schlieren measurements we observe temporal modulations on shock position and velocity, which we attribute to the thermal cooling instability, an instability which until now has not been observed experimentally. PACS numbers: Valid PACS appear hereShocks are a common phenomenon in astrophysics and high-energy-density (HED) environments in general. A shock forms when material expands with supersonic speed into an ambient medium, faster than the surrounding material can adapt to the expansion. If the energy deposition initially launching the shock is limited in time, the shock is followed by a rarefaction which eventually catches up with the shock front and a blast wave is formed, often consisting of a thin shell containing much of the swept-up material [1].An understanding of shocks and the dynamics of thermal and dynamical instabilities in HED plasmas is vital for numerical models of complex plasma systems. In such environments, radiation can lead to fundamental structural and dynamical changes in the system evolution. A shock becomes radiative if the post-shock conditions lead to an efficient cooling rate through radiative energy losses. The radiation is transmitted through the shock shell and, in an optically thin case, is lost from the system. In contrast, if the upstream material ahead of the shock front is optically thick to parts of the emission spectrum, radiation can be reabsorbed leading to preheat and ionization of the material ahead of the shock front. This modifies the shock propagation dynamics and can lead to growth of instabilities [2,3].The temporal expansion of a shock radius is often described as a power-law type function of the formwhere E 0 denotes the deposited energy per unit length (in cylindrical geometry) and ρ is the mass density. The parameter α is the deceleration parameter determined by the geometry and the energy dissipation in the system, which for cylindrical, adiabatic blast waves is α = 0.5 [4]. Dissipative processes such as radiation or ionization necessarily reduce the polytropic index, γ, of the system, * Electronic address: M.Hohenberger@Imperial.ac.uk and therefore α, to a value below the adiabatic solution and the blast wave decelerates more quickly. In case where radiative losses in the shell are sufficiently large such that the shell cannot support itself any longer, it is pushed by the low-density but high-pressure interior of the shock and collapses to high densities. Specifically the transition to this pressure driven snowplow regime and the associated shell-thinning is thought to make the shock more susceptible to radiation-driven instabilities, one of which we address in detail in this paper. Radiative shocks can be studied experimentally by utilizing the efficient a...

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 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 (

We describe experiments that investigate the capability of an experimental platform, based on laser-driven blast waves created in a medium of atomic clusters, to produce results that can be scaled to astrophysical situations. Quantitative electron density profiles were obtained for blast waves produced in hydrogen, argon, krypton and xenon through the interaction of a high intensity (I ≈ 10 17 Wcm −2 ), sub-ps laser pulse. From this we estimate the local post-shock temperature, compressibility, shock strength and adiabatic index for each gas. Direct comparisons between blast wave structures for consistent relative gas densities were achieved through careful gas jet parameter control. From these we investigate the applicability of different radiative and Sedov-Taylor self-similar solutions, and therefore the (ρ, T ) phase space that we can currently access.