INTRODUCTION
We present a direct experimental evidence4 of long-predicted nonlinear aspects of the Richtmyer-Meshkov1,2 process in the high energy density regime, in which new modes first arise from the coupling of initially-present modes, and in which shorter-wavelength modes are eventually overtaken by longer-wavelength modes. The experiment is using a long driving laser pulse on the OMEGA-EP laser facility to create a strong (M ~ 8) shock across a well-characterized two mode sinusoidal perturbation (having wavelengths of 50 and 100 µm) seeded in a material interface, separating two materials having different densities. The results are compared to full, 2D numerical simulations performed in DAFNA,3,4 with good agreement evident. This technique further permits the shock to be sustained, without decay of the high-energy-density flow conditions, long enough for the system to evolve into the nonlinear phase.
RESULTS
A radiographic image (Left), at t=30ns after the laser pulse initiation is presented in Fig. 1. The perturb interface is located at x~550 µm.
Figure 1: Radiographic (log scaled) image taken at 30ns after laser initiation.
The corresponding Fourier transform of the perturb interface shape from the experiment and the numerical simulation as a function of time is given in Fig. 2. The evolution of the different mode numbers can be seen, showing their growth and early saturation behavior, including the evolution of l=12, corresponding to a wavelength of 33 µm. This mode evolution is a result of the primary modes (l=4,8) coupling.
Figure 2: Time evolution of the primary modes (l=4, 8, shown in red and black, respectively), plus one harmonic (l=12, shown in blue). The open circles indicate estimated postshock amplitudes. The solid lines correspond to data, and the dashed lines to simulation output.
CONCLUSIONS
Results indicate the presence of important theoretically-predicted, nonlinear aspects of RM physics: mode interaction producing new modes, and the growth and saturation of those modes over time5. Further, the experimental platform presents several directions in which the results presented here can be extended. For example, current OMEGA-EP laser performance for the opportunity to increase both the power and duration of the driving laser pulse, which would enable the extension of the steady shock conditions by roughly 5 ns and allow the probing of RM structure further into its development.
REFERENCES
1. R. D. Richtmyer, Commun. Pure Appl. Math. 13, 297 (1960).
2. E. E. Meshkov, Sov. Fluid Dyn. 4, 101 (1969).
3. G. Malamud, C. A. Di Stefano, Y. Elbaz, C. M. Huntington, C. C. Kuranz, P. A. Keiter, and R. P. Drake, High Energy Density Phys. 9, 122 (2013).
4. Di Stefano, C. A., G. Malamud, C. C. Kuranz, S. R. Klein, C. Stoeckl, and R. P. Drake. "Richtmyer-Meshkov evolution under steady shock conditions in the high-energy-density regime." Applied Physics Letters 106, no. 11 (2015): 114103.
5. J. Hecht, U. Alon, and D. Shvarts, Phys. Fluids 6, 4019 (1994).
