An Empirical model for the interaction of ultraintense laser pulses with fully ionized plasmas including electrostatic effects

Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2009.The fast ignitor approach to inertial confinement fusion offers an efficient route to produce higher energy gain for less driver energy and compressed fuel density than the conventional hydrodynamic ignition scheme. Over the last decade, serious efforts have been expended towards the goal of achieving controlled fusion using this new approach. However, until now no simple physical plasma model for this idea has been available and the feasibility of the fast ignition project by petawatt laser pulses is not yet clear. We have investigated the capability of ultrafast lasers with irradiance I ≥ 1018 W cm-2 to produce highly energetic electron beams both in a planar wave and in a Gaussian focus in a low-density plasma and within a physical model of electrostatic effects in relativistic plasmas. The trajectory of a free electron in a plane wave with arbitrary initial conditions has been derived. From the complete solutions for the particle trajectory, we have also determined the initial velocities required to produce gure-of-eight motions for arbitrary initial particle positions. A new expression for the relativistic ponderomotive force has been developed. It compares very well with earlier work by Quesnel and Mora. The new expression promises to speed up particle-in- cell simulations. It has been found that free electrons escape from the Gaussian focal region of a 10-ps petawatt laser pulse very quickly before the field reaches its maximum amplitude. In this case very small net energy transfer occurs during the complete interaction of the electrons with the laser beam, indicating that (in the absence of collective electrostatic effects) free electrons cannot extract enough energy from the ignition laser pulse for ignition. This thesis presents a novel simulation model for predicting the large-scale dynamic behavior of the high intensity laser-plasma interaction. We have developed a simple particle simulation code to explore collective electrostatic effects in plasmas. In this work, it has been demonstrated that the electrostatic field generated by the electron flow is able to strongly modify the range and direction of the laser-generated MeV electrons by allowing the trapped electrons to experience much higher intensity peaks along their trajectories and, thus, be accelerated to higher velocities, drifting along the laser direction. However, the ignition spark requires the generation of a more sharply collimated electron beam than is predicted by this modelling. This indicates that some additional physics is still needed to validate the fast ignition concept. A self-generated magnetic field might be a promising candidate for generating ballistically collimated MeV electrons. Other experiments with gas jets and foil targets performed on problems related to collimated electron beams have also been reviewed in this work