San Diego, California, 17-19 June 2002

Analysis of Core Plasma Heating by Relativistic Electrons in Fast Ignition

T. Johzaki, K. Mima, H. Nagatomo

Institute of Laser Engineering, Osaka University, Suita Osaka 565-0871, Japan

tjohzakiile.osaka-u.ac.jp

Y. Nakao, T. Yokota, H. Sumita

Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka

812-8581, Japan

One of the basic issues of fast ignition is clarification of the ignition process, i.e., (a) the relativistic laser-plasma interaction, (b) the propagation of relativistic fast electrons to the dense core region, and (c) the core plasma heating to ignition. The processes (a) and (b) have been actively studied by means of particle simulation. On the other hand, little is reported about the process (c). Since the collisions between the fast electrons and the bulk particles become significant in the dense core, the Fokker-Planck (F-P) calculation seems appropriate for analyzing the core heating.

For the purpose of investigating the possibility of plasma heating and ignition, we have recently developed a one-dimensional (1-D) code system which consists of F-P transport codes for relativistic electrons [2] and alpha-particles and a hydrodynamic one for bulk plasma. In the F-P calculation, the magnetic field is neglected, while the electric field is taken into account by assuming a current neutrality condition [3]. For Coulomb interactions, binary collisions and Langmuir wave excitation are considered after the model of Deutsch, et al. [4].

Fig.1 shows the electron beam requirement for ignition evaluated on the basis of coupled transport-hydrodynamic simulations. As initial target configuration, we assumed a stationary, isochoric D-T planar plasma (core density = 200g/cm³, temperature = 0.25keV, thickness = 150 mm). The electron beam having relativistic Maxwellian distribution at a given temperature T_REB was assumed to be injected into the plasma from the left boundary with the beam duration of 10ps.

In the case of T_REB= 0.5MeV, the fast electrons efficiently heat the left side of dense core plasma, so that the target can be ignited with the lowest beam intensity, I_REB = 5 10¹⁹ W/cm². With increasing fast electron energy, the range becomes long, and thus the energy deposition rate per unit volume decreases. The beam intensity required for ignition hence increases with increasing T_REB. In the case of T _REB= 3 MeV, I_REB  210²⁰ W/cm² are required for ignition. It was also found that when T_REB  2 MeV, a fairly fraction of the fast electrons escape from the right boundary of the core plasma before thermalization. (When T_REB = 2MeV, about 25% of the initial kinetic energy of the fast electrons leaks out of the plasma.) These results imply that for achieving the fast ignition efficiently, the relatively low energy electrons beams (T_REB  1MeV) is desirable.

Presently, we are carrying out the calculations for ignition-experiment grade plasmas by assuming more realistic energy distribution of electron beams (as obtained from experiments or PIC simulations). We will also present these results and further discuss the requirement for fast ignition.

1. M. Tabak, et al., Phys. Plasmas, 1 (1996) 1626.

2. A 1-D Fokker-Planck code for relativistic electrons was developed also by K. Nakashima, et al.; the paper is to be published in Phys.Plasmas.

3. C. Deutsch,et al., Phys.Rev.Lett., 77 (1996) 2483.

4. A. R. Bell, et al., Phys. Rev., E56 (1997) 7193.