Observation of self-sustaining relativistic ionization wave launched by a sheath field.

We present experimental evidence supported by simulations of a relativistic ionization wave launched into a surrounding gas by the sheath field of a plasma filament with high energy electrons. Such a filament is created by irradiating a clustering gas jet with a short pulse laser (115  fs) at a peak intensity of 5×10(17)  W/cm2. We observe an ionization wave propagating radially through the gas for about 2 ps at 0.2-0.5 c after the laser has passed, doubling the initial radius of the filament. The gas is ionized by the sheath field, while the longevity of the wave is explained by a moving field structure that traps the high energy electrons near the boundary, maintaining a strong sheath field despite the significant expansion of the plasma.