Plasma channels in doubly-ionized helium

Summary form only given. Plasma channels can be used to guide high intensity laser pulses over distances much longer than the diffraction limit, which is necessary for applications such as laser wakefield accelerators. Previous plasma channels have been formed only in incompletely ionized gases, in which a sufficiently intense guided pulse suffers ionization-induced phase distortion. In the paper we demonstrate formation of a plasma channel in almost fully doubly-ionized helium. The key enabling technique is use of an electrical discharge to pre-ionize the helium, thus lowering the threshold for ionization heating and channel formation by a line focused laser pulse by an order of magnitude. In our experiment, a 100 ps, 120 mJ Nd:YAG laser pulse is focused to a line by an axicon to intensities of /spl sim/10/sup 13/W/cm/sup 2/ into 300-600 torr Helium gas.