Materials science

Over the past few years materials science has rapidly become a burgeoning area of study with high-power-lasers. There are two main reasons for this. Firstly, high energy lasers, such as the proposed HIPER system, afford the opportunity to create states of matter which are still in the solid phase, but otherwise completely unobtainable. Secondly, such states can be interrogated directly by short burst of X-radiation that can be synchronously generated with separate laser beams, allowing the structure of the material to be directly interrogated by X-ray diffraction and/or EXAFS techniques.

The energy capacity of HIPER, combined with ramped drives in long pulse mode will mean that it will be capable of compressing materials quasi-isentropically. In this mode of compression it is proposed that a ramped pressure wave, several nanoseconds in duration, is launched into the material of interest. Before the ramp has had time to steepen into a shock, the material is compressed along the isentrope, rather than the Hugoniot (strictly speaking it is a quasi-isentrope, with differing strength properties than true isentropic compression). Sophisticated target designs with layers of differing impedance can also keep the compression of a specific layer close to isentrope. The main point here is that for solid materials it is predicted that a machine such as HIPER should be able to compress crystals to pressures of order 10-15Mbar, whilst still keeping them in the solid phase. It should be noted that along the Hugoniot, a typical crystal melts about 2Mbar, so the very high shock pressures produced in the past (tens, or even hundreds of MBar) have all produced matter in the liquid or plasma, rather than solid state.

Schematic diagram for the experimental set up to perform x-ray diffraction on sub-nanosecond timescales from laser-shock-compressed targets.  One set of laser beams, focussed to a tight spot, produces a hot plasma which is a copious source of K-shell x-radiation.  These quasi-monochromatic x-rays diffract from a driven crystal, and are recorded on a wide angle detector as shown.