Hydrodynamic optically-field-ionized (HOFI) plasma channels up to 100mm long are investigated. Optical guiding is demonstrated of laser pulses with a peak input intensity of $6times10^{17}$ W cm$^{-2}$ through 100mm long plasma channels with on-axis densities measured interferometrically to be as low as $n_{e0} = (1.0pm0.3)times10^{17}$cm$^{-3}$. Guiding is also observed at lower axial densities, which are inferred from magneto-hydrodynamic simulations to be approximately $7times10^{16}$cm$^{-3}$. Measurements of the power attenuation lengths of the channels are shown to be in good agreement with those calculated from the measured transverse electron density profiles. To our knowledge, the plasma channels investigated in this work are the longest, and have the lowest on-axis density, of any free-standing waveguide demonstrated to guide laser pulses with intensities above $>10^{17}$ W cm$^{-2}$.
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