  We demonstrate through experiments and numerical simulations that
low-density, low-loss, meter-scale plasma channels can be generated by
employing a conditioning laser pulse to ionize the neutral gas collar
surrounding a hydrodynamic optical-field-ionized (HOFI) plasma channel. We use
particle-in-cell simulations to show that the leading edge of the conditioning
pulse ionizes the neutral gas collar to generate a deep, low-loss plasma
channel which guides the bulk of the conditioning pulse itself as well as any
subsequently injected pulses. In proof-of-principle experiments we generate
conditioned HOFI (CHOFI) waveguides with axial electron densities of
$n_\mathrm{e0} \approx 1 \times 10^{17} \; \mathrm{cm^{-3}}$, and a matched
spot size of $26 \; \mathrm{\mu m}$. The power attenuation length of these
CHOFI channels is $L_\mathrm{att} = (21 \pm 3) \; \mathrm{m}$, more than two
orders of magnitude longer than achieved by HOFI channels. Hydrodynamic and
particle-in-cell simulations demonstrate that meter-scale CHOFI waveguides with
attenuation lengths exceeding 1 m could be generated with a total laser pulse
energy of only $1.2$ J per meter of channel. The properties of CHOFI channels
are ideally suited to many applications in high-intensity light-matter
interactions, including multi-GeV plasma accelerator stages operating at high
pulse repetition rates.

Accelerator Physics;Plasma Physics