  A light source based on an Energy Recovered Linac (ERL) [1] consist of a
superconducting linac and a transfer line that includes wigglers and undulators
to produce the synchrotron light. The transfer line brings the electrons
bunches back to the beginning of the linac so that their energy can be
recovered when they traverse the linac a second time, lambda/2 out of phase.
There is another interesting condition when the length of the transfer line is
(n+/- 1/4) lambda. In this case, the electrons drift through on the zero RF
crossing, and make a further pass around the transfer line, effectively
doubling the circulating current in the wigglers and undulators. On the third
pass through the linac, they will be decelerated and their energy recovered.
The longitudinal focusing at the zero crossing is a problem, but it can be
canceled if the drifting beam sees a positive energy gradient for the first
half of the linac and a negative gradient for the second half (or vice versa).
This paper presents a proposal to use a double chicane at the center of the
linac to provide this focusing inversion fo rthe drifting beam while leaving
the accelerating and decelerating beams on crest.
