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A superconducting Compact Energy Recovery Linac (cERL) for electrons was constructed in 2013 at KEK to demonstrate energy recovery concept with low emittance, high-current CW beams of more than 10 mA for future multi-GeV ERL. Recently this cERL was operated not only to demonstrate energy recovery linac high current beam operation but also to promote and conduct a variety of industrial applications such as FEL, THz operation and Rare Isotope Production and irradiation for some materials. In this talk, I will present the status of the studies to realize the stable high-current low emittance CW beam and some applications with this beam., Proceedings of the 20th International Conference on RF Superconductivity, SRF2021, East Lansing, MI, USA

The compact Energy Recovery Linac (cERL) has been in operation at KEK since 2013 to demonstrate ERL performance and develop ERL technology. Recently KEK has launched an infrared FEL project with a competitive funding. The purpose of this project is to build a mid-infrared FEL at the cERL, and to use that FEL as a light source for construction of the processing database required for industrial lasers. The FEL system is composed of two 3-m undulators and a matching section between them, and generates light with a maximum pulse energy of 0.1 micro-J at the wavelength of 20 microns with an 81.25 MHz repetition rate. The FEL is also expected to become a proof-of-concept machine for ERL base FELs for future EUV lithography., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

The Compact Energy Recovery Linac (cERL) at KEK is a test accelerator in order to develop key components to realize remarkable ERL performance as a future light source. After the beam commissioning in December 2013, the legal current limit has been increased step-by-step like 1 uA, 10 uA, and 100 uA. Survey for the source of beam losses has been conducted in each step, and the study on beam dynamics and tuning has also been carried out. As a next step, 1 mA operation is scheduled in February 2016. In parallel to the increase in beam current, a laser Compton scattering (LCS) system which can provide high-flux X-ray to a beamline has been successfully commissioned. We report recent progress in various kinds of beam tuning: improvement of electron gun performance, high bunch charge operation, mitigation of beam losses, LCS optics tuning and bunch compression for THz radiation., The 7th International Particle Accelerator Conference (IPAC2016)

Four In-Vacuum Undulators (IVU) were recently installed to Photon Factory (PF) at KEK. The estimate of their impedance and kick factors is a very important issue, because they could considerably increase the total impedance of PF. Moreover, the coupling impedance of the IVUs could lead to the beam energy loss, changes in the bunch shape, betatron tune shifts and, finally, to the various beam instabilities. Using the simulation tool (CST Particle Studio), longitudinal and transverse impedances of the IVUs were evaluated and compared to analytical formulas and measurement results. The study provides guidelines for mitigation of unwanted impedance, for an accurate estimate of its effects on the beam quality and beam instabilities and for the impedance budget of a newly designed next-generation machine which has many IVUs and small-aperture beam pipes., Proceedings of the 60th ICFA Advanced Beam Dynamics Workshop on Future Light Sources, FLS2018, Shanghai, China

A low-emittance 3-GeV KEK-LS* ring has been designed at KEK. KEK-LS's undulators can produce extremely high brightness light ranging from VUV to X-ray. Brightness of undulator light strongly depends on the phase error of its periodic magnetic field. Then a precise magnetic field adjustment is required in order to prevent the reduction of the brightness performance. Generally, the adjustment is performed by the conventional field measurement system equipped with hole-probes on a huge stone table. But, for the in-vacuum undulator, the measurement must be performed without the vacuum chamber. The additional phase error caused by reattaching the chamber is not negligible for the low emittance rings. Therefore, some groups have developed measurement systems available for the direct field measurement inside the chamber**,***. We have started to develop a compact measurement system. Our system is compacted and stabilized by utilizing the rigid metal beam of the undulator frame instead of the stone table. In the conference, we will report the detail of the system and the present status of the development., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The Compact Energy Recovery Linac (cERL) project is ongoing at KEK in order to demonstrate excellent ERL performance as a future light source. The cERL injector was already constructed with its diagnostic beamline and successfully commissioned from April to June in 2013. In the next step, the cERL recirculation loop with a main superconducting linac and merger and dump sections has been constructed and its commissioning is scheduled to start in December 2013. Significant progress is expected by the IPAC14 conference date. In this presentation, we will describe the present status of the cERL including future developments., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

Superconducting(SC)-linac-based light sources, which can produce ultra-brilliant photon beams in CW operation, are attracting worldwide attention. In KEK, we have been conducting R&D; efforts towards the energy-recovery-linac(ERL)-based light source* since 2006. To demonstrate the key technologies for the ERL, we constructed the Compact ERL (cERL)** from 2009 to 2013. In the cERL, high-brightness CW electron beams are produced using a 500-kV photocathode DC gun. The beams are accelerated using SC cavities, transported through a recirculation loop, decelerated in the SC cavities, and dumped. In the February of 2014, we succeeded in accelerating and recirculating the CW beams of 4.5 micro-amperes in the cERL; the beams were successfully transported from the gun to the beam dump under energy recovery operation in the main linac. Then, precise tuning of beam optics and diagnostics of beam properties are under way. We report our experience on the beam commissioning, as well as the results of initial measurements of beam properties.

We have been developing a Terahertz free electron laser (FEL) based on the 40 MeV, 1.3 GHz L-band electron linac at the Institute of Scientific and Industrial Research (ISIR), Osaka University. After the FEL lasing at the wavelength of 70 um (4.3 THz)*, next targets of the FEL development are to extend the available laser wavelength, to increase the FEL power, and to evaluate characteristics of FEL. Since the lowest energy of the linac was restricted by a fixed-ratio power divider between the acceleration tube and the buncher, we have prepared the new one with a different ratio to extend the wavelength longer side. As a result, the wavelength region is able to be extended to 25 - 147 um (12.5 - 2 THz). The maximum output energy of the FEL macropulse so far obtained is 3.6 mJ at 66 um. The peak macropulse power available to user experiments is estimated to be 1 kW or less, given that the pulse duration is 3 us. Three users groups have begun experiments using the FEL. We will report these recent activities on the Terahertz FEL., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

We have been developing a rapid-polarization-switching source at the B15-16 straight section in the PF 2.5GeV ring. The source consists of tandem two APPLE-II type elliptically polarizing undulators (EPU), namely U#16-1 and U#16-2, and a fast kicker system. These two undulators are designed to obtain the soft x-ray at the energy region from 200eV to 1keV with various polarization states. We have constructed U#16-1 and installed in the PF ring in March 2008. The operation of U#16-1 for the user experiments has been started successfully since April 2008. The construction of the second undulator U#16-2 is underway. U#16-2 will be installed in the PF ring at this summer. We report the result of the magnetic field adjustment of the U#16-2., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

To supply the electrical power for the superconducting correctors in the interaction region of the proposed SuperKEKB, a kind of vapor cooled current leads is designed, which consists of 8 brass leads and can transport currents to 4 correctors simultaneously. The design current of the leads is about 50 A. The thermal and electrical behaviors have studied by the finite elements method and the cryogenic experiment is also planed to validate the performance. In this paper the design will be presented and the finite element model will be compared with the experimental data., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

The edge-focusing (EF) wiggler has been fabricated to evaluate its performance rigorously with the magnetic field measurement. It is a 5-period planar wiggler with an edge angle of 2° and a period length of 60 mm. The magnetic field is measured using Hall probes at four different wiggler gaps. It is experimentally confirmed that a high field gradient of 1.0 T/m is realized, as designed, along the beam axis. The magnetic field gradient of the EF wiggler is derived as a function of the magnetic gap. The field gradient decreases with increasing magnet gap more slowly than the peak magnetic field does for the present experimental model. An analytic formula for the field gradient of the EF wiggler is derived and it is shown that the slope of the field gradient with the magnet gap can be changed by varying the magnet width of the EF wiggler. We analyzed the relation between the orientation errors of the measurement system and the measured magnetic field or field gradient using a model magnetic field of the EF wiggle. We corrected the measurement magnetic field based on this analysis and evaluated the performance of the EF wiggler., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

The Compact Energy Recovery Linac (cERL) is a superconducting test accelerator aimed at establishing technologies for the ERL-based future light source. After its construction during 2009 to 2013, the first CW beams of 20 MeV were successfully transported through the recirculation loop in February 2014*. Then, initial tuning of beams and evaluations of beam properties were carried out. From September to December in 2014, we are constructing a Laser Compton Scattering (LCS) source** which aims at demonstrating technology for the future high-flux quasi-monochromatic gamma-ray source. In the next run of the cERL, which begins at the end of January 2015, we plan such works as an increase in the beam current (from 10 uA to 100 uA), commissioning of the LCS source, and sustained tuning of beams for lower emittance. We will report up-to-date results of these developments., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

Update of the first-generation undulators installed in 1980s is pushed forward at PF-Ring, a 2.5-GeV SR source of KEK, taking advantage of the expanded straight sections reconstructed in 2005. New undulators have been designed as elliptically polarizing undulators each has 6 magnetic arrays to obtain various polarization states, not only circular polarization but also linear (horizontal and vertical) polarization. Three undulators will be installed in FY2013 and FY2014 for BL02, BL13 and BL28. For BL02, the longest straight section of about 9 m, the new undulator will be installed in tandem with the existing planar undulator, in order to cover the wide photon energy range from 15 eV to 2 keV. At PF-AR, a 6.5-GeV SR source, a new direct beam transport (BT) line from the injector LINAC is under construction. Super KEKB which shares the injector LINAC with PF-Ring and PF-AR will be commissioned at the end of FY2014. The full-energy continuous injection of PF-AR will be available as a simultaneous injection with the 7-GeV HER, the 4-GeV LER and PF-Ring not so later than the commissioning of Super KEKB., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

Two synchrotron radiation sources of KEK, the PF-ring and the PF-AR, continue their user operation with various improvements. Scrap and build of the first generation undulators of 1980s at the PF-ring is pushed forward year by year. Five new elliptically polarized undulators have been installed in these five years, and we have also installed four very narrow-gap short-period undulators generating high brilliant X-ray. The new beam transport line that enables the 6.5-GeV full energy injection for PF-AR will be completed by the end of 2016 in order to make the top-up operation of the two SR sources compatible with the continuous injection for two main rings of the Super-KEKB. We have proposed a project of further upgrade of the 2.5-GeV PF-ring to improve its horizontal emittance as 8 nm rad using combined bending magnets at the arc sections. And we are also moving ahead on proposal of constructing a new KEK light source of an extremely low emittance as 0.3 nm rad. The progress and detail of our future project will be described in this paper., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

Future synchrotron light source using a 5-GeV-class energy recovery linac (ERL) is under proposal by our Japanese collaboration team, and we are conducting active R efforts for that. We are developing super-brilliant DC photocathode guns, two types of cryomodules for both injector and main superconducting linacs, 1.3 GHz high CW-power rf sources, and other important components. We are also constructing a compact ERL for demonstrating the recirculation of low-emittance, high-current beams using those key components. We present our recent progress in this project., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

KEK manages two synchrotron radiation sources, Photon Factory storage ring (PF-ring) of 2.5 GeV and Photon Factory advanced ring (PF-AR) of 6.5 GeV. These rings share an injector linac with the two main rings of KEK B-factory, 8-GeV HER and 3.5-GeV LER. Recently, the linac has succeeded in a pulse by pulse multi-energy acceleration. A top-up operation of PF-ring has been realized as the simultaneous continuous injection to the 3 rings, PF-ring, HER and LER. Development of new injection scheme using a pulsed sextupole magnet continues aiming at practical use in the top-up operation. A rapid-polarization-switching device consisting of tandem two APPLE-II type undulators has been developed at PF-ring. The first undulator was installed in 2008, and the second one will be installed in 2010 summer. PF-AR, operated in a single-bunch mode at all times, has been suffered from sudden lifetime drop phenomena attributed to dust trapping for many years. Using the movable electrodes installed for experiment, we confirmed that the discharge created by the electrode was followed by the dust trapping, and succeeded in a visual observation of luminous dust streaking in front of CCD cameras., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan