Relativistic electron drift in underdense plasma produced by a super-intense femtosecond laser pulse

Simple analytic expressions have been obtained for the relativistic electron drift in an ultra-short laser pulse along the pulse propagation in vacuum, or in underdense plasma. If an electron initially is at rest, the drift is exponentially small even in a super-intense laser pulse. The drift velocity becomes a relativistic quantity only when an electron is produced suddenly during the laser pulse at the tunnelling, or barrier-suppression ionization. Analytical estimates are obtained also for the drift velocity in this case.