preplasma

Preplasma is low-density plasma that forms on the surface of a solid target before the arrival of the main high-intensity laser pulse. It is typically generated by pre-pulses and the amplified spontaneous emission present in many high-intensity laser systems. The preplasma creates a gradient of decreasing density extending into the target.

It arises when the initial part of the laser pulse or its background light ionizes surface material and drives hydrodynamic expansion. The resulting density profile is often described as an exponential or power-law gradient, characterized by a scale length L, which depends on the laser contrast, pulse duration, and target material. Typical scale lengths range from sub-micrometer to several tens of micrometers; high-contrast, ultrashort pulses can produce much shorter preplasma.

The presence of preplasma strongly affects laser-plasma coupling. It can change the dominant absorption mechanism (for example, resonant absorption or vacuum heating), alter hot-electron generation, and influence ion acceleration and x-ray emission. In inertial confinement fusion and laser-driven ion sources, preplasma can modify energy coupling, implosion symmetry, and instability growth.

Diagnostics include interferometry and shadowgraphy to map density profiles, and Thomson scattering for temperature and density. Mitigation strategies include improving laser contrast, for example with plasma mirrors or other contrast-enhancement techniques, to minimize preplasma length and create steeper density gradients when desired.