Extreme Physics

In Chapter 4 we introduced the concept of shock waves. There we discussed how, when a fluid or plasma is set into motion at a particular spatial position by a pressure pulse that increases with time, the resulting acceleration can be approximated as a sequence of small velocity jumps. Each small jump in velocity launches a compression wave that travels into the fluid or plasma at the local adiabatic sound speed. As the pressure and hence the fluid velocity increases, each successive compression wave that is launched travels faster than the one before. Eventually all these compression waves “pile up,” forming a single wave, a shock wave, that has a narrow wave front. The wave front is simply the narrow spatial region over which the flow variables transition from the undisturbed state ahead of the wave and the compressed state behind the wave.

In Chapter 1 we learned that one of the principal ways we create matter at extreme conditions in the laboratory is by dynamically compressing it with shock waves created by a “driver” like a high-power laser, a Z-pinch, or a gas-gun projectile. Shock physics is at the very heart of the simulation codes for describing the properties and behavior of matter at extreme conditions. Shocks also occur in nature. One example is the shock created in the gravitational collapse of a massive star at the end of its hydrogen-burning lifetime. It is this shock that ejects the envelope of the star into space that we see as a supernova explosion. Shocks play a prominent role also in the physics of astrophysical jet formation, in accretion processes, and in cosmic ray acceleration.