  Ultra-low emittance (tens of nm) beams can be generated in a plasma
accelerator using ionization injection of electrons into a wakefield. An
all-optical method of beam generation uses two laser pulses of different
colors. A long-wavelength drive laser pulse (with a large ponderomotive force
and small peak electric field) is used to excite a large wakefield without
fully ionizing a gas, and a short-wavelength injection laser pulse (with a
small ponderomotive force and large peak electric field), co-propagating and
delayed with respect to the pump laser, to ionize a fraction of the remaining
bound electrons at a trapped wake phase, generating an electron beam that is
accelerated in the wake. The trapping condition, the ionized electron
distribution, and the trapped bunch dynamics are discussed. Expressions for the
beam transverse emittance, parallel and orthogonal to the ionization laser
polarization, are presented. An example is shown using a 10-micron CO2 laser to
drive the wake and a frequency-doubled Ti:Al2O3 laser for ionization injection.
