Non-equilibrium atomic physics and X-Ray lasers

The penetrating power of a radiation increases when the wavelength of the radiation decreases. Whenever matter is under extreme conditions of temperature and density (beyond 106 K and 0.1 g/cm3), its plasma state implies spectacular radiating properties with a propensity for the X-ray range and if the geometry favours a direction of emission an amplification effect appears (laser effect). The designing of X-ray laser operating below 500 angstroms was a challenge for the first teams working on inertial confinement fusion in the eighties. Now the latest developments of this technology offer a broad range of wavelengths from 500 to 38 angstroms. The corresponding radiation is quasi-monochromatic and quasi-coherent, its duration is monitored from 10 to 500 ps, its brilliancy is exceptional (up to a 105 factor compared with equivalent synchrotron radiation) but its efficiency remains weak (2.10-5). X-ray laser is used in plasma diagnostics and X-ray laser radiography is a powerful tool to study the growth of the Rayleigh-Taylor instability or the interface between 2 media with very different densities. (A.C.)