Experimental measurement of two copropagating shocks interacting with an unstable interface

In this work, we present results from experiments capable of producing and measuring the propagation of multiple successive, copropagating shocks across an unstable planar interface, where the shocks are independently driven and separately controllable, enabling the study of this important phenomenon. Copropagating shocks play a significant role in a wide range of systems involving stratified media subject to a shock, and exhibit different physical characteristics compared to counterpropagating shocks. Existing techniques, however, preclude copropagating shocks, so experiments to date have been limited to the study of counterpropagating shocks. We address this previous limitation and open a physical parameter space for study using a new hohlraum platform on the National Ignition Facility. Initial experimental results are presented together with comparisons from numerical simulations.

Figure 1

Simulated comparison of the growth rate, which is dramatically different and includes an extra phase shift, of a sinusoid subject to two copropagating shocks of equal energy, compared to the equivalent counterpropagating case. The amplitude $a$ is normalized by the wavelength $\lambda$; the time $t$ is normalized by the postshock interface velocity $u_{\mathrm{int}}$, the wave number $k$, the Atwood number $A_t$, and the shock crossing time $t_{\mathrm{s}}$.

Figure 2

(a) Sketch of the experimental system. (b) Diagram showing the incidence of the direct- and indirect-drive laser beams on the ablator and halfraum walls, respectively, for the halfraums shown in the next two frames. (c) Sketch of the variation of the system for imaging measurements.

Figure 3

Radiation temperature measurement made by the Dante diagnostic, for NIF shot N190415-001, shown as a solid black curve. An estimated prediction from the simulation is included, as a red dashed curve. The laser pulse shape is shown as a dotted blue curve. The simulation predicts the temperature to within a few eV for most of the duration of the experiment.

Figure 4

(a) VISAR data for NIF shot N190415-001; (b) SOP data for the same shot. Breakout of the first, directly-driven shock (1) and the merging of this shock with the second, indirectly-driven shock (1-2) can clearly be seen by the disturbance these events produce on the VISAR fringes and the simultaneous jumps in thermal emission detected by the SOP. The blackened area corresponds to distorted data due to the axial seam [Fig. 2].

Figure 5

Shock velocity measurement, starting when the shock breaks out of the W witness material, made using the VISAR diagnostic. The equivalent prediction from xRAGE calculations is also included. The two shock events can be clearly identified by the jumps in velocity at about 10 and 25 ns.

Figure 6

Simulated interface and shock positions for the imaging system, shown as solid black and dashed red lines, respectively. The trajectory in the absence of the second, indirect drive is shown as a dotted gray line, and experimental data are shown as points. The second shock forms a few ns after the indirect pulse turns on and eventually merges with the first shock at about 23 ns. The predicted trajectory is consistent with the experimental measurement and would be measurably different in the absence of a second shock.