Hydrodynamic instability experiments with three-dimensional modulations at the National Ignition Facility

Abstract

The first hydrodynamic instability growth measurements with three-dimensional (3D) surface-roughness modulations were performed on CH shell spherical implosions at the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004)]. The initial capsule outer-surface amplitudes were increased approximately four times, compared with the standard specifications, to increase the signal-to-noise ratio, helping to qualify a technique for measuring small 3D modulations. The instability growth measurements were performed using x-ray through-foil radiography based on time-resolved pinhole imaging. Averaging over 15 similar images significantly increased the signal-to-noise ratio, making possible a comparison with 3D simulations. At a convergence ratio of ∼2.4, the measured modulation levels were ∼3 times larger than those simulated based on the growth of the known imposed initial surface modulations. Several hypotheses are discussed, including increased instability growth due to modulations of the oxygen content in the bulk of the capsule. Future experiments will be focused on measurements with standard 3D 'nativeroughness' capsules as well as with deliberately imposed oxygen modulations.

Mix and hydrodynamic instabilities on NIF

“…These oxygen irregularities could significantly vary form shot-to-shot and could hypothetically be the reason of the stronger than expected growth mitigation with HF drive, discussed above. Another unexpected discovery in these experiments was the presence of large coherent features in the measured xray radiographs [10][11][12]. The leading hypotheses to explain these observations were based on the increased instability amplitudes due to modulations of the oxygen content in the bulk of the capsule [14].…”

“…The HGR platform was extended to measure 3-D perturbations from the capsule outer surface roughness during the acceleration phase of implosions [10][11][12]. Examples of these experiments are described in figure 3 showing measured optical-depth images of 3-D surface roughness perturbations at convergence ratio of ~3 for LF and HF drives.…”

An improved model of damage depth of shock-melted metal in microspall under triangular wave loading*

“…[1] If the material is shocked above its melting pressure or melted on release from a lower shock pressure, tensile stress emerges in the melted materials and the dynamic damage process is referred to as "microspall". [2] Microspall has attracted great attention from scientists and engineers over the past decades because it has been observed in inertial confinement fusion [3][4][5][6] and laser-driven surface micromachining. [7][8][9] Experimental, theoretical and computational studies of microspall have been carried out over the past decades.…”

Optical measurements and analytical modeling of magnetic field generated in a dieletric target

“…These currents can generate megagauss magnetic fields, which will in turn influence the propagation of the energetic electron beam, thereby diminishing the energy deposition in the precompressed ignition core. As electrons penetrate into the target, a low-energy electron backflow will be generated in order to ensure electric neutrality [9]. In such a system Weibel instability [10] or the current filamentation instability can grow efficiently and give rise to an ultra-intense magnetic field in a manner that is akin to the formation of an ultraintense magnetic field from unmagnetized plasmas in astrophysics [11].…”