A comparison of shock isentropic compression: experiment and theory
Supervisor: Professor Justin Wark
It has long been known that high pressure shock waves can be generated in materials by irradiation of their surfaces by high power nanosecond laser pulses. Indeed, shock pressures well in excess of TPa have been produced by such techniques. However, shock compression of matter to such high pressures inevitably, with the large entropy increases associated with shock compression, melts the material (and indeed, at the high pressures cited, produces a plasma). However, what has recently emerged as an interest in laser-driven high pressure research has been the field of isentropic compression. This novel field has, in principle, the potential to revolutionise several areas of high pressure research.
In its simplest form the argument runs thus: the highest pressures to which solid materials have hitherto been subject (and then interrogated) in the laboratory is approximately 350GPa - which, interestingly, is about the pressure thought to exist at the centre of the earth. This limit is due to the fact that such static pressures are produced in diamond anvil cells (DACs), and to date DACs subject to higher pressures have fractured, owing to the finite strength of the diamonds. Whilst materials can be subjected to much higher pressures transiently by laser-ablation, in the past almost all such experiments have resulted in shock waves being launched into the material of interest, and shocks produce large amounts of entropy, and considerably raise the temperature of the material. As a rule of thumb, thypical metals will melt on being subject to shock pressures between 100 and 200 GPa.