Status of ERL and cERL Projects in Japan

Aiming at constructing a future synchrotron light source based on a 5-GeV energy recovery linac (ERL), we are developing high-brightness DC photocathode guns, gun-drive lasers, superconducting cryomodules, and 1.3 GHz high-power rf sources. We are also constructing the Compact ERL (cERL) at KEK for demonstrating the recirculation of high-brightness beams using these components. We report up-to-date status of the Japanese ERL project. ERL PROJECT IN JAPAN Synchrotron light sources based on ERLs are expected to bring innovation to the synchrotron radiation (SR) science [1]. We are proposing to construct a 5-GeV ERL at KEK as a future project of the KEK Photon Factory. The 5-GeV ERL is expected to deliver high-brightness electron beams having normalized beam emittances of 0.1-1 mm·mrad at beam currents of 10-100 mA, as well as to produce ultra-short electron bunches having rms lengths of about 100 fs with bunch charges of higher than 77 pC/bunch. The ERL is also expected to produce highquality electron beams for a proposed X-ray free-electron laser oscillator (X-FELO) [2]. Typical beam parameters [3] which are required for the X-FELO at a typical photon energy of 12 keV are: the electron beam energy of 7 GeV, the bunch charge of 50 pC, rms bunch length of 1 ps, the normalized beam emittance of 0.2 mm·mrad, the bunch repetition frequency of approximately 1 MHz, and rms energy spread of 2×10. We aim at attaining the abovementioned parameters by operating the same ERL as a recirculating linac [4]. The ERLs are also expected to produce both highbrightness and quasi-monochromatic gamma-rays using the laser Compton scattering. The JAEA is proposing an application of ERLs for detecting radioactive isotopes using such gamma-rays [5]. Both the ERL-based SR source and the gamma-ray source share the common ERL technologies. We are conducting R&D effort for the ERL technologies since 2006. HIGHLIGNTS OF R&D EFFORT High-Brightness DC Photocathode Guns To produce high-brightness electron beams having a repetition frequency of 1.3 GHz, we are developing two 500-kV DC photocathode guns. Design of the first 500kV gun started in FY2008. Schematic drawing of the first 500-kV gun is shown in Fig. 1. In order to protect a ceramic insulator against electrons emitted from a support rod, the ceramic insulator was divided into several pieces and each piece was covered by a guard ring. Under highvoltage tests, the ceramic insulator with the support rod was successfully conditioned up to the maximum voltage of 550 kV [6]. After above-mentioned high voltage test, cathode and anode electrodes, NEG pumps, and electrostatic shielding meshes, were installed in the vacuum chamber of the gun. After the chamber was baked, vacuum pressure in the chamber reached down to 2×10 Pa. Then, high voltage conditioning of the assembled gun was carried out. In the summer of 2010, the gun could be conditioned up to the maximum voltage of 380 kV. The maximum voltage was currently limited by local X-ray radiation of approximately 30 μSv/h. As a next step, we first plan to extract beams under the current setup at a lower voltage of 300 kV, and then, we will investigate the source of the radiation. The latest status of the first 500-kV gun is reported in [7]. We also started to develop the second 500-kV gun [8]. The purpose of the second gun is to continue the gun development after one of them has been installed in the Compact ERL. One of the guns can also serve as a backup for the other one when there happens a serious damage in its component such as a ceramic insulator. Both a titanium vacuum chamber and a ceramic insulator have been produced, and vacuum tests [9] of them are in progress. Figure 1: The first 500-kV DC photocathode gun [7].