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 ($115\mathrm{fs}$) at a peak intensity of $5\times {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$. 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.

• Received 11 February 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.045002