Observation of a Velocity Domain Cooling Instability in a Radiative Shock

Abstract

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...

Influence of atomic kinetics in the simulation of plasma microscopic properties and thermal instabilities for radiative bow shock experiments

“…The fitting of the RPL and these proposed criteria have been already used to predict the possibility of thermal instabilities in experiments of convergent radiative shocks in argon and neon produced in a cylindrical liner Z-pinch configuration [24] and also in experiments of blast waves launched in clusters of xenon [17,26,60]. In both cases, the results obtained theoretically were consistent with the experimental observations.…”

“…Thermal instabilities due to radiative cooling develop over the characteristic radiative cooling time which was given in Eq. (26).…”

“…For that purpose we compared the cooling time calculated using Eq. (26) in the optically thin approximation and the convective time calculated as the ratio between the FWHM of the bow shock and the sound speed in the medium, that was obtained taking the temperature just behind the front shock. The convective times obtained at 300 and 400 ns were around 100 ns-1 μs, respectively.…”

“…The first one is the analysis of the validity of LTE assumption in the calculation of average microscopic properties of argon plasmas in mass densities and electron temperatures ranging from 10 −6 to 10 −1 g cm −3 and from 1 to 100 eV, respectively. Argon is an element commonly used in laboratory astrophysics experiments on radiative shocks generated using either pulsed power devices [19,[21][22][23][24] or ultraintense lasers [17,[25][26][27][28][29][30], and the ranges of plasma conditions of these experiments fall within the ones before mentioned, hence, the interest of this study. For this analysis, we have made calculations assuming the plasma either in LTE and therefore using the Saha-Boltzmann (SB) equations, or in non-LTE (NLTE) in steady state, in which we have solved the rate equations implemented in our collisional-radiative model.…”

Time-dependent and radiation field effects on collisional-radiative simulations of radiative properties of blast waves launched in clusters of xenon

“…In this case, a shock moves into the surrounding medium creating a blast wave, usually defined as an expanding shock that sweeps up the material that is ahead of the shock. One manner to launch the shock in this context is to irradiate a gas formed by atomic clusters by intense lasers directly [8,9,10,11,12,13,14,15,16,17,18,19]. Clustered gases exhibit extremely efficient absorption of intense laser light creating a hot, high energy density plasma in a low average density target.…”