Numerical modelling of plasmas produced by long pulse lasers

Toft, David Thomas

Abstract

The phenomena that occur when an intense laser beam interacts with matter have been of interest for some time, mainly due to the possibility of constructing a laser-driven fusion reactor. In particular a number of models describing the effect of a laser striking a plane solid target, and the behaviour of the resulting plasma, have been proposed. In 1965-66 the self-regulating model was developed independently by Caruso et al and Krokhin et al. The validity of this model has been confirmed by experiment and shown to be applicable in the case of relatively low-powered lasers. To accommodate higher intensity lasers Fauquignon and Floux,and Bobin developed the deflagration-wave model in 1970-71. The fact that these models are really two regimes of the same basic model has been demonstrated by Puell. In 1973 Pert showed that at higher intensities the effects of thermal conduction modify these models to such an extent that it is necessary to introduce two new models, these being termed thick self-regulating and thick deflagration-wave.

An alternative method of attacking the problem is to solve the hydrodynamic equations governing the plasma motion by means of a computer code. It was found that the Lagrangian form of the equations of motion was the most suitable for this treatment, and a number of one-dimensional codes of this type have been written. Notable among these are, in chronological order: Fader (l), Kidder (2), Shearer and Barnes (3), Mulser (4), Nuckolls et al (LASNEX) (5), Goldman (6) and Clarke et al (7). The production of a two-dimensional code was delayed by the fact that Lagrangian, coordinates become extremely difficult to work with in more than one dimension, and simple Eulerian techniques cannot handle the strong shocks produced in the problem. However with the advent of more sophisticated numerical methods the production of a working two-dimensional Eulerian code became feasible. There is now a code of this type due to Winsor at the Naval Research Laboratory Washington, and in this country there is the CASTOR code due to Christiansen, LASERB of Craxton, and 2DEL of Pert. There is also now a 2-D Lagrangian version of LASNEX at Livermore. All of these codes include, among other sophistications, the effects due to magnetic fields generated within the plasma.

The object of the first part of this thesis is to compare the results of a specially written two-dimensional code with steady-state versions of the theoretical models, and in particular to examine the 'thick' models of Pert and to determine the extent to which these models are lost due to flux limitation effects. To ensure that a steady-state is attained, relatively long runs of the computer code are required; therefore the code was reduced to the simplest form that could still model the physical situation adequately. Hence sophistications such as magnetic field effects were neglected. These omissions are justified since it is basically hydrodynamic effects that are of interest. In Chapter 2 the plasma model is described, the equations of motion are presented, and a description is given of the computer code. Chapter 3 examines the various analytic steady-state models and Chapter 4 presents the results of the computer code and compares them with the models of the previous chapter.

The second part of this thesis deals with the development of a ray-tracing code to examine the effects of refraction in a laser-plasma interaction. The code is described in Chapter 5 and the results obtained presented in Chapter 6.

Citation

Toft, D. T. (1979). Numerical modelling of plasmas produced by long pulse lasers. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4219260

• Thesis Type: Thesis
• Deposit Date: May 30, 2017
• Publicly Available Date: Feb 23, 2023
• Keywords: Applied physics
• Public URL: https://hull-repository.worktribe.com/output/4219260
• Additional Information: Department of Applied Physics, The University of Hull
• Award Date: Aug 1, 1979