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

Abstract

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 (<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 i...

Investigations of laser-driven radiative blast waves in clustered gases

“…At later times the blast wave enters an energy recovery phase with n > ½. A deceleration parameter exceeding the Sedov-Taylor value suggests that the blast wave is sweeping up energy that has previously been deposited in the gas ahead of the shock [3,12,13,32]. Under these conditions, a blast wave can remain strongly radiative and yet follow an energy conserving trajectory of n ¼ ½ as long as energy losses are balanced by energy gain [3].…”

“…For the instability to occur the gas must have a cooling function satisfying L(T) a T b with b < 1 and should be visible as an oscillation in the velocity of the shock front [24]. The blast wave trajectory in our low energy (<1 J) cluster experiments typically exhibits three stages of evolution [12,13]. Shock formation in the first few ns is followed by the radiative phase lasting w20 ns with a deceleration parameter n < ½.…”

“…The potential of such a medium to perform shock wave studies of astrophysical importance was realized and exploited at the Lawrence Livermore National Laboratory some years ago [1][2][3][4]. Since these pioneering experiments we have performed numerous blast wave experiments using low energy (<1 J) table-top lasers [5][6][7][8][9][10][11][12][13]. We have also conducted experiments [14] using the Vulcan laser facility to launch blast waves in clusters with much higher drive energies (>10 J) than previously used.…”

The impact of recent advances in laboratory astrophysics on our understanding of the cosmos

“…This creates a focused need for the observation of such instabilities in a laboratory environment, to show if they can in fact exist. This was accomplished (Hohenberger et al 2010) by the production of cylindrical shock waves by focusing a 1.4 ps laser pulse into a medium composed of Xe gas clusters (Moore et al 2008;Symes et al 2010). Measurements of the shock trajectory clearly showed velocity oscillations attributed to this instability.…”

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.…”