San Diego, California, 17-19 June 2002

Effect of Porous Lining on the Reduction of Growth Rate of
Rayleigh-Taylor-Instability in IFE Target *

N. Rudraiah

National Research Institute for Applied Mathematics 492/G, 7th cross, 7th block
(West Jayanagar), Bangalore-560082

In inertial fusion energy (IFE) the physical target filled with deuterium and tritium (DT) is symmetrically irradiated by multiple high power laser beams. This laser radiation interacts on the surface of the spherical targets and generates few mega bars of pressure which isotropically compresses the DT. Here the core plasma serves as an igniter. In the case of directly driven ICF scheme the ignitor size is actually about 2% of initial target radius. Therefore for efficient fusion reaction the igniter surrounded by isotropically compressed plasma requires a high uniformity for the stability of implosion. The implosion symmetry of an ablatively accelerated target is affected by RTI during implosion phase which appear in the following two stages:

1. Acceleration of high density plasma (i.e. porous lining) by low density ablating plasma.

2. Deceleration of high density plasma by the ignitor.

In either of these cases, if the RT1 grow, the symmetry of implosion is lost. For example, in the acceleration phase the symmetry is lost by causing shell break up and in the deceleration phase it is lost by degrading the plasma compression with pusher plasma mixing.

In IFE the plasma state is formed upto the ablation laser due to thermal conduction from the critical density layer to the target material. Any non-uniformity in the energy transport profile having origin either in laser illumination or target surface will affect the uniformity of the ablation surface. The early works aim to prevent the non-uniformities reaching the ablation surface. However, the RT1 is originating independently at the ablation surface still remains a major threat to IFE objective. The work of Pant et. al., (1989) on the reduction of RT1 growth rate was based on the experiments involving x-ray radiation transport pertaining to the use of plastic foil targets. Later Borisenko and Merkuiev (1990) used visco elastic forces and the self generated magnetic field to reduce the growth rate of RT1. Recently Mikaelian (1993) has studied the effect of surface tension on the growth rate of RTI. Later Rudraiah et. al., (1996) have considered the combined effect of surface tension and magnetic field as additional factors in stabilizing the shell during acceleration. A good estimation of the growth rate of linear RT1 has been reported by Takabe (1985).

In IFE it is necessary to control the growth of RT1 and means to suppress it. Although there are several methods to control non-uniformities of the surface of the implosion targets in IFE but much work has not been done using a porous lining as means of suppressing RT1. Therefore, the primary objective of this paper is to use porous lining to suppress the growth rate of RT1.

For this purpose the basic equations and the relevant boundary conditions using suitable approximations are discussed in detail. The velocity distribution and the dispersion relation are obtained analytically using linear stability analysis. An important conclusion of this paper is the suppression of the RTI growth rate, by a proper choice of porous parameter.

References

Pant, H.C et. al., in Proc. 3rd Tropical College on Appl. Phys, Kaulampur, Malaysia, 389, 1989, Exs C.S. Wong, S.Lec, B.C. Toua, A.C. Chan, Low, K.S. and Hoo S.P. (world scientific, Singapore).

Borisenko, N.G Merkuiev, Yu, A, 1990 preprint 47, P.N. Lebedev phy. Institute, Moscow.

Mikaelian, K.O. 1993, Phys. Rev. E, 47, 375.

Rudraiah, N, Krishnamurthy, B.S., and Mathad, R.D., The effect of oblique magnetic field on the surface instability of a finite conducting fluid layer, Acta. Mech., Vol 119, 165-180, 1996.

Takabe, H., 1985, Phys. Fluids, 28, 3676.