Experimental observations of the expansion of an optical-field-induced ionization channel in a gas jet target

The evolution of an optical-field-ionized channel created by a 3.5 ps KrF laser pulse in a low density nitrogen gas jet target (10¹⁷ ions cm^-3) has been studied using Moir\'e deflectometry. This allowed the density profiles in the channel and its lateral expansion to be quantitatively measured for the first time. The results were reproduced well by hydrocode simulations and analytical models. In addition, structured Raman backscatter signals indicated very low longitudinal electron velocities in the rising part of the pulse, as confirmed by 2D PIC code simulations.