An energy recovery linac upgrade for the advanced photon source located in the storage ring infield

Recently physicists at the advanced photon source (APS) have begun to explore a revolutionary upgrade based on emerging energy recovery linac (ERL) technology. In an ERL, the energy of the 7-GeV, 100-mA beam is recovered after the beam passes through user beamlines by decelerating the beam back through the same superconducting linac cavities that accelerated it. The main constraint on this upgrade is that the existing APS beamlines must not be disturbed. This requires that the APS storage ring be used as a single-pass transport line in the overall ERL beamline layout. A natural place to locate the ERL is inside the existing APS storage ring "infield" area, which has unoccupied space south of the existing APS injector complex. Other important constraints include minimal disturbance of existing buildings and injector beamlines. The existing injector complex should be preserved so that stored beam operation can be continued throughout and beyond ERL construction and commissioning. In this paper we describe a layout that satisfies these constraints. We also estimate the amount of emittance increase the beam will experience before it is de livered to user beamlines.