Improved performance of direct-drive inertial confinement fusion target designs with adiabat shaping using an intensity picket

Abstract

Hydrodynamic instabilities seeded by laser imprint and surface roughness limit the compression ratio and neutron yield in the direct-drive inertial confinement fusion target designs. New improved-performance designs use adiabat shaping to increase the entropy of only the outer portion of the shell, reducing the instability growth. The inner portion of the shell is kept on a lower entropy to maximize shell compressibility. The adiabat shaping is implemented using a high-intensity picket in front of the main-drive pulse. The picket launches a strong shock that decays as it propagates through the shell. This increases the ablation velocity and reduces the Rayleigh–Taylor growth rates. In addition, as shown earlier [T.J.B. Collins and S. Skupsky, Phys. Plasmas 9, 275 (2002)], the picket reduces the instability seed due to the laser imprint. To test the results of calculations, a series of the picket pulse implosions of CH capsules were performed on the OMEGA laser system [T.R. Boehly, D.L. Brown, R.S. Craxton et al., Opt. Commun. 133, 495 (1997)]. The experiments demonstrated a significant improvement in target yields for the pulses with the picket compared to the pulses without the picket. Results of the theory and experiments with adiabat shaping are being extended to future OMEGA and the National Ignition Facility’s [J.A. Paisner, J.D. Boyes, S.A. Kumpan, W.H. Lowdermilk, and M.S. Sorem, Laser Focus World 30, 75 (1994)] cryogenic target designs.

Improved Performance of High Areal Density Indirect Drive Implosions at the National Ignition Facility using a Four-Shock Adiabat Shaped Drive

“…The longer picket in this new "adiabat-shaping" (AS) pulse launches a first shock similar to that of HF, but with lower laser power and lower risk of laser-plasma instabilities during the picket. The approach is similar to the AS [24][25][26][27][28] techniques previously fielded in direct drive implosions [29][30][31]. In this case, however, the stability benefits are a consequence of the RichtmyerMeshkov (RM) oscillations [32][33][34][35] during the shock transit phase, rather than a reduction of the RT growth rate directly [21].…”

Time-resolved compression of a capsule with a cone to high density for fast-ignition laser fusion

“…The m-error is inferred based on an estimated 10% temperature uncertainty. ARTICLE hydrodynamic stability 15 , but those implosions use thick shells and a low implosion velocity so the slight reduction in ablation velocity does not significantly affect hydrodynamic stability. The main effect of the reduced picket is a lower a inn and reduced mass and temperature in the hot spot.…”

“…A low adiabat (a ¼ B1 to B2) is a prerequisite for high-target compression and is achieved by shaping the laser pulse in such a way that the inner portion of the shell, which is not ablated, remains on a low a, whereas the laser raises the a in the ablated outer material in order to reduce the growth of hydrodynamic instabilities 15 . Imploding massive shells with a low implosion velocity (r2 Â 10 7 cm s À 1 ) and a shaped low-a profile substantiates a viable path for achieving highly compressed cores for FI 16 .…”

“…In the experiments reported here, plastic shells with an embedded hollow cone made out of high-Z material were imploded with good laser illumination uniformity using the adiabatic shaping technique 15 . Figure 1 shows a schematic of a direct-drive cone-in-shell experiment.…”

Bell-Plesset effects for an accelerating interface with contiguous density gradients

“…where ( ) and is driven by a pure entropy change across the interface [24]. In the small wavenumber limit, we obtain with aid of Eqs.…”