[en] We diagnose the self-channeled propagation of intense femtosecond pulses over an extended distance in a N₂O cluster gas target using high resolution kilovolt x-ray pinhole images of the channel and spatially resolved x-ray spectra. The x-ray images are consistent with femtosecond optical scattering, shadowgraphy, and interferometry images. We observe extended plasma channels (∼9 mm) limited either by the cluster jet length or by absorption, for injected laser intensities in the range of 10¹⁶-10¹⁷ W/cm². Spectral line shapes for the OVII 1s²-1s3p and OVIII 1s-2p transitions (at 1.8627 and 1.8969 nm, respectively) show significant broadening to the blue side and with truncated emission on the red side. We attribute this effect to Doppler blueshifted emission from fast ions from exploding clusters moving toward the spectrometer; redshifted emission from the opposite side of the cluster is absorbed