Coherent x-ray - ultraviolet radiation can be generated by the highly nonlinear interaction between a gas target and high intensity ultrafast laser pulses using the high harmonic generation (HHG) process. Guiding the fundamental laser field inside a hollow capillary waveguide improves phase matching and extends the potential interaction length. However, the propagation of an intense pulse within a capillary waveguide filled with ionizable gas is complex, as the pulse creates a plasma, which in turn strongly affects the propagation. Previous work has used emission from the excited gas to study propagation of nanosecond pulses in capillary waveguides. In this work we demonstrate spectrally resolved imaging of the plasma created by intense femtosecond pulses within a capillary during an HHG experiment. The spectral and spatial resolution is used to separate contributions from ions and neutral species, and is an effective diagnostic for the local pressure and modal intensity variations along the waveguid
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