High-energy lasers have been one of the workhorses of high energy density experiments. They typically have a flexibility that allows a wide range of high energy density conditions to be achieved. The pulse duration, focal spot size, energy, and, in the case of multiple beam systems, the pointing can all be varied to optimize the experimental conditions. Lasers with energies of even a few joules and nanosecond durations can produce high energy density conditions in targets. Multibeam lasers with energies in the kilojoule range have been used to produce extremely high energy density conditions. Many of these conditions are described in this chapter, with astrophysically relevant high energy density conditions described in Chapter 2. The largest currently operating laser system is the 60-beam, 30-kJ 0.35-µm wavelength OMEGA laser system at the University of Rochester. It can be configured in many ways, allowing a wide variety of high energy density research to be carried out. This research includes inertial confinement fusion (as described above) in both direct- and indirect-drive modes, strongly shocked materials, the evolution of hydrodynamic instabilities relevant to both laboratory and astrophysical plasmas, material opacities under extreme conditions, and generation of intense x-ray radiation sources. The National Ignition Facility, currently under construction at Lawrence Livermore National Laboratory, will extend the available laser energy range by almost 2 orders of magnitude, producing ~2 MJ of laser energy in 192 beams at 0.35-µm wavelength. It will maintain the flexibility afforded by a multibeam laser system but will generate much more extreme conditions than currently possible, including the possibility of obtaining fusion ignition.

Lasers couple their energy to a solid material or a plasma primarily through the collisional and/or noncollisional absorption of the light in the coronal region of the plasma. Electromagnetic radiation cannot propagate at electron densities higher than its wavelength-dependent critical electron density (n_cr~10²¹ cm⁻³/λ² µm²). This is a relatively low density region at the edge of a target. Thermal conduction carries the laser energy into the higher density regions. The next step depends on whether the goal is to directly couple the laser energy to the target (direct drive) or to convert the energy to x rays that are subsequently coupled to the target (indirect drive). In direct drive, the hot, high-density region expands (ablates), causing a pressure to be applied to the remaining target material. This ablation process is similar to the process that drives a rocket, acting through the conservation of momentum. This pressure can launch shock waves into the material, bringing it to higher pressure and/or accelerating it. In indirect drive, the laser energy is converted to x rays, often in an x-ray oven (hohlraum) that produces nearly blackbody radiation