FEL oscillation with a high extraction efficiency at JAEA ERL FEL

One of challenges that high-power FEL oscillators driven by energy recovery linac (ERL) are facing is to increase the extraction FEL efficiency as high as possible. The high-efficiency oscillation relaxes the total beam current needed for high-power lasing and makes the optical micropulse length shorter, which is useful for various applications. Two triple bend achromatic arcs have been used in a recovery loop for an ERL FEL at Japan Atomic Energy Agency (JAEA). The return arc after an undulator has an energy acceptance of 15%, which is large enough to recover the electron beam used for high-efficiency FEL oscillation. Recently we have achieved the efficiency exceeding 2%, which accompanies large energy spread, by doubling the electronbunchrepetition rate. The FEL efficiency has been measured from the horizontal profiles of the exhausted electron beam with a wire scanner installed at a dispersive point in the return arc. The other arc placed upstream of the undulator has been found to work as a bunch compressor for the high-efficiency FEL oscillation.