Pulse Design for Relaxation Adiabat-Shaped Targets in Inertial Fusion Implosions

Introduction Controlling the seeds and the growth of Rayleigh–Taylor (RT) instability during the acceleration phase of imploding shells is crucial to the success of inertial confinement fusion (ICF). Since the RT growth is damped by the ablative flow off the shell’s outer surface, target performances are greatly improved by target designs with enhanced ablation velocity. A significant increase in ablation velocity and shell stability can be achieved by shaping the entropy inside the shell. Following the standard ICF notation, we measure the entropy through the so-called “adiabat” defined as the ratio of the plasma pressure to the Fermi-degenerate DT pressure: α ρ ≡ ( ) ( ) P Mb g cc 2 18 5 3 . , where the pressure is given in megabars and the density in g/cc. The optimum adiabat shape in the shell consists of a profile that is monotonically decreasing from the outer to the inner surface as qualitatively shown in Fig. 101.8 on p. 14. Large adiabat values on the shell’s outer surface increase the ablation velocity Va, which follows a power law of the outer-surface adiabat α α out out 3 5 , ~ , Va [ ] while low adiabat values on the inner surface lead to improved ignition conditions and larger burn.1–5 A more detailed history and target design implications of adiabat shaping can be found in the introduction of Ref. 6 by the same authors, which is devoted mostly to the adiabat shape induced by a strong decaying shock. Shaping by a decaying shock was introduced in Ref. 7 and requires a very strong prepulse aimed at launching a strong shock. This strong shock decays inside the shell shortly after the prepulse is turned off; the picket pulse is followed by the low-intensity foot of the main pulse. The decaying shock (DS) leaves behind a monotonically decreasing adiabat profile, which follows a power law of the mass coordinate