  We study hollow plasma channels with smooth boundaries for laser-driven
electron acceleration in the bubble regime. Contrary to the uniform plasma
case, the laser forms no optical shock and no etching at the front. This
increases the effective bubble phase velocity and energy gain. The longitudinal
field has a plateau that allows for mono-energetic acceleration. We observe as
low as 10^{-3} r.m.s. relative witness beam energy uncertainty in each
cross-section and 0.3% total energy spread. By varying plasma density profile
inside a deep channel, the bubble fields can be adjusted to balance the laser
depletion and dephasing lengths. Bubble scaling laws for the deep channel are
derived. Ultra-short pancake-like laser pulses lead to the highest energies of
accelerated electrons per Joule of laser pulse energy.
