  The photoelectron spectrum in the ultra-relativistic limit of tunneling
ionization is strongly affected by wave-particle resonance and finite spot-size
effects, in contradistinction with the usual assumptions of strong field
physics. Near term laser facilities will access a regime where ionized
electrons are abruptly accelerated in the laser propagation direction, such
that they stay in phase with the laser fields through a substantial portion of
the confocal region. The final momentum of the electron depends significantly
on where in the confocal region it originated. By manipulating the target and
collection geometry, it is possible to obtain low emittance, low energy spread,
gigavolt photoelectrons. Radiation reaction effects play a negligible role in
near term scenarios, but become interesting in the multi-exawatt regime. A
significant advance in numerical particle tracking is introduced.
