Molecular dynamics studies of static and dynamic melting points, using numerical modelling and simulated tampers

The pressures required for high energy density physics are achieved experimentally using laser shock compression. This is a dynamic process, with picosecond time scales. Establishing a melt temperature in a dynamic system is made non trivia by overshoot of the melting point due to the kinetic barrier for the phase change. There is therefore a need to be able to model dynamic melt temperatures in such a way that a static melt curve can be predicted, and to link these simulations to an experimental method.

By tracking liquid growth to create a numerical melt model, static melt curves of Cu and Ta (in the range 0-10 GPa) were obtained from dynamic heating simulated using molecular dynamics. These were in agreement, within an error of 1%, to literature melt curves and also to the values found using the established static coexistence method, with the computational cost reduced by two orders of magnitude.

A typical shock target consists of an ablator, the target material and a tamper. A shock impedance method was developed to model the tamper as a mobile wall which returned the expected release wave through the bulk material. Different tamper materials could therefore be used to alter the cooling rate during release. With Cu as the test material, the literature melt curve (in the range 50-110 GPa) was recreated, with a 7% error in temperature. This method of modelling a tamper in molecular dynamics removes the need to know the potential of the tamper material.

Both novel methods developed in this thesis have succeeded in reproducing established static melt curves, by modelling dynamic systems. These methods could be applied to laser shock experiments, in order to verify simulated melt curves at high
pressures.