Recent Progress in the Energy Recovery Linac Project in Japan

Future synchrotron light source using a 5-GeV energy recovery linac (ERL) is under proposal by our Japanese collaboration team, and we are conducting R&D efforts for that. We are developing high-brightness DC photocathode guns, two types of cryomodules for both injector and main superconducting (SC) linacs, and 1.3 GHz high CW-power rf sources. We are also constructing the Compact ERL (cERL) for demonstrating the recirculation of low-emittance, high-current beams using above-mentioned critical technologies. ERL PROJECT IN JAPAN Energy recovery linacs are expected to bring innovation to the synchrotron-radiation (SR) science [1]. Our Japanese collaboration team is proposing to construct a 5GeV ERL at KEK as a future project of the KEK Photon Factory [2]. Goals of the 5-GeV ERL are shown in Table 1. We aim at achieving normalized beam emittances (both in horizontal and vertical planes) of 0.1 mm·mrad at an average beam current of 10 mA (high coherence mode), and those of 1 mm·mrad at a beam current of 100 mA (high flux mode). The ERL will also produce ultra-short beam pulses having pulse lengths of less than 100 fs (rms) with bunch charges of higher than 77 pC at a typical repetition frequency of about 1 MHz. The ERL is also expected to deliver high-brightness electron beams to an X-ray free-electron laser oscillator (X-FELO) [3] that is a very promising option [4] for our ERL project. In order to save both construction cost and space, we are currently considering a 2-loop ERL configuration. Figure 1 shows our idea which can make the ERL and XFELO compatible. Under an ERL mode, beams are accelerated twice through a 2.5-GeV SC linac, 5-GeV beams are used for the SR experiments, and they are decelerated twice and dumped. Under the other recirculating-linac mode, we change a path length of an outer loop by a half rf-wavelength by introducing an orbit bump. Then, beams are accelerated three times through the SC linac, yielding 7.5-GeV beams used for the XFELO. The higher beam energy is favorable for obtaining higher FEL gain. Since an average beam current required for the X-FELO is relatively low (typically 20 μA under 20 pC/bunch with a bunch repetition of about 1 MHz), 7.5-GeV beams are immediately dumped after they are utilized. In principle, a hybrid operation of these two modes is possible by switching the pulsed bump at a repetition frequency of about 1 MHz as shown in Fig. 2. Table 1: Goals of 5-GeV ERL and Compact ERL in Japan Parameter 5 GeV ERL Compact ERL Beam energy 5 GeV 35 245 MeV Average current 10 100 mA 10 100 mA Normalized emittance 0.1 1 mm·mrad 0.1 1 mm·mrad TUPE091 Proceedings of IPAC’10, Kyoto, Japan 2338 02 Synchrotron Light Sources and FELs A16 Energy Recovery Linacs Figure 1: A convertible configuration of a 2-loop ERL and a recirculating linac. Figure 2: Hybrid operation of ERL/recirculating-linac. R&D EFFORT FOR THE ERL PROJECT We are conducting active R&D efforts for the ERL project since 2006. The latest results are briefly reported. High-Brightness DC Photocathode Gun The ERL project requires a high-brightness electron gun which can produce beams of 100 mA (10 mA) with a normalized emittance of 1 mm·mrad (0.1 mm·mrad). For this purpose, we are developing a DC photocathode gun having a gun voltage of 500 kV with a beam current of 10 mA (100 mA in future). A schematic drawing of the 500kV gun is shown in Fig. 3. In order to prevent any damage to a ceramic insulator due to emitted electrons from a support-rod electrode, this gun is equipped with a segmented ceramic insulator having guard-ring electrodes. As we expected, this gun could successfully be conditioned up to a very-high voltage of 550 kV, followed by a long-time holding test for 8 h at a high voltage of 500 kV [5]. Detail status of this gun is reported in [6]. Following the above-mentioned development, we decided to construct another 500-kV gun at a KEK site. The purpose of the second gun is to provide an opportunity for further development of the gun technology after one of them has been installed in the Compact ERL. For the second gun, we also employed titanium chamber having low outgassing rate and segmented insulators. The second gun can also serve as a backup for the first gun, and to this end, the insulators were designed to be compatible with those of the first gun. The vacuum components of the second gun are under assemble. Basic research for the photocathode is also underway [7]. Figure 3: A schematic drawing of a 500-kV, 10-mA photocathode gun which was constructed at JAEA.