Fusion Science and Technology / Volume 49 / Number 3 / April 2006 / Pages 278-296
Technical Paper / Fast Ignition
In the fast ignition scheme, the compressed core is surrounded by a 1-mm-scale coronal plasma. The critical density where the laser deposits energy is still more than 100 m away from the core. The distance is much longer than the laser focus radius or the core size. This situation raises an important question: How can we couple laser energy to the core from such a distance? One of the techniques that has been proposed to overcome this problem is hole boring by the ponderomotive pressure of the incident laser light. In this paper, the physics related to the laser hole boring, including the parametric instabilities, the channel formation, and the hot electron acceleration by ultraintense laser light, are discussed. The maximum density where the laser can propagate by hole boring is obtained as a function of the intensity. This agrees well with experimental observations, and it is confirmed by numerical simulations. The acceleration mechanism of hot electrons in the magnetic channel is also identified. The hot electrons are characterized by the numerical simulations. In summary, the critical issue of energy coupling in this scheme is raised and discussed.