162 unifying physics of accelerators, lasers and plasma
beam phase space as illustrated (in a synectics approach of
using analogies) by a paper sheet that increases its effective
volume by orders of magnitude after being crumpled — see
Fig. 8.21.
Preservation of the emittance of an LPWA beam during its
transport from the plasma bubble into the undulator would
require properly catching the accelerated beam into a focusing system. The focusing distances of the first lenses of the
catching optics need to be comparable to the focusing in the
plasma bubble, and the focusing distances of further lenses
should gradually increase to match the focusing strength of
the undulator beamline. An alternative approach would be to
create an undulator inside of the plasma — a concept which,
if proven feasible, may create light sources of ultimate compactness.