  We investigate the triggering and guiding of DC high-voltage discharges over
a distance of 37 cm by filaments produced by ultraviolet (266 nm) laser pulses
of 200 ps duration. The latter reduce the breakdown electric field by half and
allow up to 80% discharge probability in an electric field of 920 kV/m. This
high efficiency is not further increased by adding nanosecond pulses in the
Joule range at 532 nm and 1064 nm. However, the latter statistically increases
the guiding length, thereby accelerating the discharge by a factor of 2. This
effect is due both to photodetachment and to the heating of the plasma channel,
that increases the efficiency of avalanche ionization and reduces electron
attachment and recombination.
