No abstract

We present two designs relevant to ablative Rayleigh-Taylor instability in transition from weakly nonlinear to highly nonlinear regimes at the National Ignition Facility [E. I. Moses, J. Phys.: Conf. Ser. 112, 012003 (2008)]. The sensitivity of nonlinear Rayleigh-Taylor instability physics to ablation velocity is addressed with targets driven by indirect drive, with stronger ablative stabilization, and by direct drive, with weaker ablative stabilization. The indirect drive design demonstrates the potential to reach a two-dimensional bubble-merger regime with a 20 ns duration drive at moderate radiation temperature. The direct drive design achieves a 3 to 5 times increased acceleration distance for the sample in comparison to previous experiments allowing at least 2 more bubble generations when starting from a three-dimensional broadband spectrum.

Dynamic measurements of detonation velocity profiles are performed using long Chirped Fiber Bragg Gratings (CFBGs). Such thin probes, with a diameter of typically 150 µm, are inserted directly into a high explosive sample or simply positioned laterally. During the detonation, the width of the reflected optical spectrum is continuously reduced by the propagation of the wave-front, which physically shortens the CFBG. The reflected optical intensity delivers a ramp down signal type, which is directly related to the detonation velocity profile. Experimental detonation velocity measurements were performed on the side of three different high explosives (TNT, B2238 and V401) in a bare cylindrical stick configuration (diameter: 2 inches, height: 10 inches). The detonation velocity range covered was 6800 to 9000 m/s. The extraction of the detonation velocity profiles requires a careful calibration of the system and of the CFBG used. A calibration procedure was developed, with the support of optical simulations, to cancel out the optical spectrum distortions from the different optical components and to determine the wavelength-position transfer function of the CFBG in a reproducible way. The 40-mm long CFBGs were positioned within the second half of the three high explosive cylinders. The excellent linearity of the computed position-time diagram confirms that the detonation was established for the three high explosives. The fitted slopes of the position-time diagram give detonation velocity values which are in very good agreement with the classical measurements obtained from discrete electrical shorting pins.