The propagation of a 1-ps laser pulse at intensities exceeding 10(19) Wcm(-
2) in a low-density plasma channel was experimentally tested. The channel w
as produced by a lower intensity preceding pulse of the same duration. Plas
ma electrons were accelerated during the propagation of the main pulse, and
high energy gamma -ray detectors were used to detect their bremsstrahlung
emission. The gamma -ray yield was studied for different channel conditions
, by varying the delay between the channel forming pulse and the high inten
sity pulse. These results are correlated with the interferograms of the pro
pagation region into the plasma.