Study of Ablatively Imploded Spherical Shells

Abstract

Compression of plasma in laser-driven implosions has been characterized as "ablative" or "exploding pusher" according to the relative importance of surface ablation pressure and bulk pressure due to preheating by electrons. To date, experimental study of ablative implosions has been limited, 1 partly because their high-density low-temperature plasma yields little diagnostic information in the form of x-ray or fusion-product emission compared with exploding-pusher implosions. 2 The development of x-ray probing techniques 3 has alleviated this problem and permitted study of the implosion dynamics of spherical-shell targets under ablative acceleration with use of streak time-resolved x-ray shadowgraphy. 4 Elementary mechanics show that for negligible mass loss, a spherical shell with ratio of radius to wall thickness (aspect ratio) r/Ar and density p can be accelerated to a velocity v of the order (p a r/&r) l/2 by a surface ablation pressure P a . Since the stagnation pressure P f of the implosion is of the order pv 2 it follows that P f^Pa r/&r. High aspect ratio therefore leads to high stagna-reduce direct fast-electron preheat of the fuel for laserfusion targets. See for example, K 0 Lee, Do W.

One-dimensional hydrodynamic simulations of low convergence ratio direct-drive inertial confinement fusion implosions

“…Numerous authors have identified that ICF experiments performed at lower convergence ratios can show improved agreement with simulations; a (by no means complete) selection can be found in references [28][29][30][31][32][33][34][35]. Such a trend was evident as far back as 1980 [36] when it was observed that lower initial aspect ratios (thicker shells, and as such lower implosion velocities and lower convergence ratios) resulted in more 1D-like performance; however, the laser energies available at the time made the pursuit of high gain at low convergence ratios unfeasible. This trend was again observed recently in the work performed by Olson et al, looking at indirect drive shots on wetted foam capsules on the NIF [5].…”

Simulations of shock generation and propagation in laser-plasmas

“…(1) At the shortest laser wavelengths, there is still some incertitude concerning the scaling versus laser intensity. For instance, measurements at 0.351 mm (Key et al, 1983(Key et al, , 1979 showed a scaling % I 0.3 , very different from what is predicted by theoretical models (usually % I 0.7 ). Such experimental results were dominated by 2D and hot spots effects.…”

Toward a theory of thin-layer lasers

“…An important contribution to diagnostics was the first use of X-ray back-lighting (or radiographic) techniques for the study of implosions. Two-dimensional pulsed radiography was demonstrated initially [9] and then streak camera imaging was used to show the temporal evolution of compression in one dimension [10]. Interest in compression physics shifted from exploding pushers to ablative compression, and sixbeam experiments at X= 1.05 nm showed degraded compression of high-aspect-ratio shells using streaktime-resolved radiography [10].…”

“…Two-dimensional pulsed radiography was demonstrated initially [9] and then streak camera imaging was used to show the temporal evolution of compression in one dimension [10]. Interest in compression physics shifted from exploding pushers to ablative compression, and sixbeam experiments at X= 1.05 nm showed degraded compression of high-aspect-ratio shells using streaktime-resolved radiography [10]. With six green beams (X = 0.53 Mm), streak radiographic methods showed improvement in drive pressure at the shorter wavelength [11] (Fig.…”