H. Takamaru

K. Mima

S. Miyamoto

T. Sato

National Institute for Fusion Science (NIFS-Repository)

§ 14. Simulation on Laser-Plasma Interaction and Fluid Dynamics in Inertial Confinement Fusion Mirna, K., Miyamoto, S. (Institute of Laser Engineering, Osaka University) Takarnaru, H., Sato, T. One of the impot1ant issues in the fast ignitor concept[ I] is the propagation of an intense short pulse laser in laser coronal pla~rna. So far, rnany theory and sirnulation works have been carried out on this subject[2-4]. By those previous works, found are the hole boring, electron heating up to a few MeV, ion acceleration, self-generated magnetic field, and so on. However very few works have been done on the effects of laser polarization on the relativistic laser plasma interaction and electron/ion acceleration. We used a two dimensional electro-rnagnetic particle (PIC) code to analyze the ion acceleration mechanisrn. We have done simulations for three types of laser polarization~ p-polarized (electric field lie in the incident x-y plane), s-polarized (electric field is perpendicular to the incident x-y plane), and circularly polarized. Simulation pararneters are as follows~ initial plasma density n = 7ncr (ncr is critical density of laser light), initi~l electron (ion) ten1perature Te (T) = 20 keY, and laser intensity IL = I 020 W/cm2 . By the laser photon pressure, a shock wave is formed in the plasma. As the result of simulations it was observed that the plasma density becam~ high at the laser-plasma interface because of the shock compression. The distinction between circularly polarized and linear polarized cases (s and p) was that in the circularly polarized case, density jmnp was much higher than those in the s and p polarized case. The maximum electron density ( nonnalized by initial density) after w t = 83 was 2. 97, 2. 05, and 2. I 0, for circul~ly polarized, p-polarized, and s-polarized cases respectively. It is explained by the oscillation of ponderomotive force with 2w (see Ref. 2) where w is the laser frequency . This oscillation of the ponderomotive force appears only in the linear polarization. Therefore, in the linear polarization, the electron density onthe plasma-laser interface oscillates with the oscillation of the ponderomotive 260 force. These oscillation will propagate into the overdense plasrna as electron plasrna waves to generate hot electrons. Therefore, the density is decreased cornpared with the circularly polarized case. . In figure I, .ion phase space plots (Px - p) for circularly polartzed (a) and p-polarized (b) are shown. Ion was accelerated to higher energy in the circularly polarized ca-;e than p-polarized case. This may correspond to the higher density cornpression by the ponderomotive force of the circularly polarized la~er. Frorn our previous works, it has been found that high energy ions were accelerated at the shock front by the potential jurnp in the shock. Since the difference of shock potential is propo11ional to the density, therefore, ions get higher energy in the case of circular polarized than p or s-polarized. Finally we refer to the energy conversion efficiency of the particle kinetic energy in plasma to the laser. From our previous work of one dirnensional particle simulation, it has been found that the energy conversion efficiency was the order of 10% over the wide la~er intensity ranae of 101l)-'"lJ J b l O- W/cm- . From two dirnensional sirnulation, energy conversion efficiency was 26. 7o/o, 40. 1 %, and 19. 9o/o for circularly polarized, p-polarized, and s-polarized respectively. These high efficiency result frorn the energy absorption around the periphery of the hole drilled by the laser. (a) (b) ~v; 0. I u ;§ () -0. I ... ,. ·· .• ~'~: 'i . .. ,~:. -0.1 0 0.1 -O.l 0 0.1 p /Me p /Me X X Fig. 1. Phase space plots of ion for circularly polruized (a) and p-polarized (b). References 1 )M. Tabak,et. al., Phys. plasma 1, }, 1626 ( 1994) 2)S. C. Wilks,et. al., Phys. Rev. Lett.l_l, 1383 ( 1992), S. C. Wilks, Phys. Fluids B }, 2603 ( 1993) 3)T. M. Anton~n, et. al., Phys. Rev. Lett. 69, 2204 ( 1 992) 4 )P. Sprangle, et. al., Phys. Rev. Lett. 64 2011 ( 1990), P. Sprangle, et. al., Phys. Flu ids B 1 2241 ( 1992) 

Simulation on Laser-Plasma Interaction and Fluid Dynamics in Inertial Confinement Fusion

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