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Flat beam generation from angular momentum dominated beam with a phase-space rotation technique is an unique method to manipulate the phase-space distribution of beam. As an application, the asymmetric emittance beam generation for linear colliders is considered to compensate the Beamstrahlung effect at Interaction point. By using this technique, the asymmetric beam can be generated directly with the injector, instead of radiation damping with a huge damping ring. We present the result of a proof-of-principle experiment at KEK-STF., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

KEK electron positron injector linac initiated the injection operation into Photon Factory (PF) light source in 1982. Since then for 39 years, it has served for multiple projects, namely, TRISTAN, PF-AR, KEKB, and SuperKEKB. Its total operation time has accumulated 200 thousand hours on May 7, 2020. We are extremely proud of the achievement following continuous efforts by our seniors. The construction of the injector linac started in 1978, and it was commissioned for PF with 2.5 GeV electron in 1982. In parallel, the positron generator linac was constructed for the TRISTAN collider project. The slow positron facility was also commissioned in 1992. After the KEKB asymmetric-energy collider project was commissioned in 1998 with direct energy injections, the techniques such as two-bunch acceleration and simultaneous injection were developed. As the soft structure design of the linac was too weak against the great east Japan earthquake, it took three years to recover. Then the construction and commissioning for the SuperKEKB project went on, and the simultaneous top-up injection into four storage rings contributes to the both elementary particle physics and photon science., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

ILC is an electron-positron linear collider based on Superconducting linear accelerator. Linear collider is an only solution to realinze high energy electron-positron collision beyond the limit of synchrotron radiation energy loss by ring colliders. Beam current of injector of linear colliders is much larger than that of ring colliders because the beam is not reusable. Providing an enough amount of particles, especially positron is a technical issue. In this article, we present a design of electron driven positron source for ILC. After optimizations, the system design is established with an enough technical margin, e.g. avoiding potential damage on the production target., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

The KEK injector Linac feeds beams to four rings. It is called J-Linac. SuperKEKB Main Rings are High-Energy-Ring (HER) and Low-Energy-Ring (LER). The Linac injects a 7.0 GeV electron beam to HER and a 4.0 GeV positron beam to LER. It also injects electron beams into two light source rings. We have successfully performed this simultaneous four-ring injection. We achieve this complex simultaneous injection by using two electron guns, a positron source with a flux concentrator, and pulsed magnets. In SuperKEKB phase 3 operation, 2.0 nC electron and 3.0 nC positron beam injections were achieved., Proceedings of the 65th ICFA Advanced Beam Dynamics Workshop on High Luminosity Circular e+e- Colliders, eeFACT2022, Frascati, Italy

The SuperKEKB broke the world record of the luminosity in June 2020 in the Phase 3 operation. The luminosity has been increasing since then and the present highest luminosity is 4.65 x 10³⁴ cm⁻²s⁻¹ with β_{y}^{*} of 1 mm. The increase of the luminosity was brought with an application of crab waist, by increasing beam currents and by other improvements in the specific luminosity. In this paper, we describe what we have achieved and what we are struggling with. Finally, we mention a future plan briefly., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

In E-Driven positron source of ILC, the generated positron is captured by a standing wave cavity. Because the deceleration capture method is employed, the positron is off-crest over the linac. Because the beam-loading is expected to be more than 1A in a multi-bunch format, the compensation is essential to obtain uniform intensity over the pulse. A conventional method for the compensation controlling the timing doesn’t work because RF and Beam induced field are in different phase. In this manuscript, we discuss the compensation with the off-crest acceleration case. A simple phase modulation on the input RF is a solution., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

The FCC-ee is a high-luminosity, high-precision circular collider to be constructed in a new 100 km tunnel in the Geneva area. The physics case is well established and the FCC-ee operation is foreseen at 91 GeV (Z-pole), 160 GeV (W pair production threshold), 240 GeV (Higgs resonance) and 365 GeV (t-tbar threshold). Due to the large 6D production emittance and important thermal load in the production target, the positron injector, in particular the positron source, is one of the key elements of the FCC-ee, requiring special attention. To ensure high reliability of the positron source, conventional and hybrid targets are currently under study. The final choice of the positron target will be made based on the estimated performances. In this framework, we present a preliminary design of the FCC-ee positron source, with detailed simulation studies of positron production, capture and primary acceleration., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The SuperKEKB injector linac is required to deliver low-emittance electron and positron beams. Wire scanners are employed to measure Twiss parameters and to adjust beam optics conditions. Screen monitors also play important roles for single-shot measurements. However, the beam size became more than 10-times smaller compared with that of the KEKB injection. Beam tests have been performed in order to evaluate materials for high-precision transverse beam-size measurements at the injector. The main purpose of the beam tests is to quantitatively investigate the saturation effect of each screen material for generating the scintillation light, which is strongly depending on the beam-charge density. Several scintillating screen materials including YAG:Ce, LYSO:Ce, BGO and aluminum ceramic have been tested with high energy and high charge-density electron beams. The results are compared with that obtained by the OTR measurement. The saturation of the luminescence was confirmed for all crystals and evaluated in the charge density of 0.5-1.5 nC/mm2. The cause of the saturation and the effect to the measurement are discussed., Proceedings of the 8th International Beam Instrumentation Conference, IBIC2019, Malmö, Sweden

Linear collider is a concept to realize e⁺e⁻ collision beyond the limitation of the ring colliders by the synchrotron radiation. To obtain an enough luminosity, eg. 1.0·10⁺³⁴ cm⁻²sec⁻¹, the beam is focused down to nano-meter size with a high aspect ratio. This super flat beam is useful to improve the luminosity and to compensate the beam-beam effect, eg. Beamstrahlung. In a conventional design, the super-flat beam is produced by radiation damping in a storage ring. We propose to produce this super-flat beam with phase-space rotation techniques. We employ both Round to Flat Beam Transformation and Transverse to Longitudinal Emittance eXchange, the super flat beam can be generated by controlling the space-charge effect which spoiled the performance. We present the RFBT performance with respect to the initial conditions, i.e. beam size, initial emittance, solenoid field (strength and profile), etc., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

SuperKEKB injector linac delivers four different beam modes modulated pulse by pulse at 50 Hz, which have 100-times different beam charges, and a pulse may contain two bunches only 96-ns apart. Required low-emittance beams for SuperKEKB rings would need precise beam orbit controls in order to suppress the transverse wakefield in the accelerating structures. A new detection electronics with a wide dynamic range of 40 dB with a high resolution based on a 180-MHz narrow-band detection technique for stripline-type beam position monitors (BPMs) has been developed for the SuperKEKB injector linac. While such measurement condition is challenging, a position resolution of 3 micrometer in one standard deviation was successfully achieved with beam-based tests. The self-calibration system is also installed in order to compensate gain drifts for each input channel with an accuracy down to 0.1%, by using test pulses going through stripline heads between 50-Hz beam pulses. The design concept of the new detection electronics is described in detail, as well as operational performance of synchronized measurement with 100 BPMs for injection beams to four electron/positron storage rings., Proceedings of the 8th International Beam Instrumentation Conference, IBIC2019, Malmö, Sweden

The Positron Damping Ring (DR) for SuperKEKB successfully started its operation in February 2018, and the commissioning was continued until the end of SuperKEKB Phase 2 in July without serious troubles. This paper describes achievements of the beam commissioning of injection and extraction lines (LTR and RTL) between the LINAC and DR. In the LTR commissioning, the positron beam with high emittance, wide energy spread, and high charge were transported and injected into the DR. In the RTL commissioning, special cares were necessary to preserve the low emittance. The observed emittance growth in the RTL was not a problem for Phase 2, but it should be resolved in the coming Phase 3. In this paper, brief results of the commissioning of the DR is also reported., Proceedings of the 62th ICFA ABDW on High Luminosity Circular e$^+$e$^-$ Colliders, eeFACT2018, Hong Kong, China

To linear colliders, huge amount of positron has to be provided comparing to ring colliders, because the beam is dumped after the collision. Electron Driven ILC Positron source has been designed as a technical backup of the undulator position source including the beam loading effect, etc. The design of the detail will be presented. To linear colliders, huge amount of positron has to be provided comparing to ring colliders, because the beam is dumped after the collision. Electron Driven ILC Positron source has been designed as a technical backup of the undulator position source including the beam loading effect, etc. The design of the detail will be presented., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

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)

The International Linear Collider (ILC) is a next-generation accelerator for high-energy physics to study the Higgs and top sector in the Standard Model, and new physics such as supersymmetry and dark matter. ILC positron source based on Electron-driven method has been proposed as a reliable technical backup. In this article, we report the design study of the positron source based on the off-the-shelf RF components. The positron is generated and accelerated in a multi-bunch format. To compensate the energy variation by the transient beam loading effect, we employ AM (Amplitude Modulation) technique and the results were 16.60 ± 0.14 MV (peak-to-peak) for L-band 2m cavity driven by 22.5 MW power and 25.76 ± 0.19 MV (peak-to-peak) for S-band 2m ac-celerator driven by 36 MW power with 0.78 A beam load-ing., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

The Phase I beam commissioning of SuperKEKB has been conducted from February to June in the last year. The injector linac has successfully delivered the electron and positron beams to the SuperKEKB main ring. The linac beam studies and subsystem developments are also intensively going on together with the daily normal beam injection to both rings of the SuperKEKB and two light sources. Towards Phase II and III beam commissioning of SuperKEKB, one of key issues is a fine beam control with the new beam position monitor readout system, a positron capture system based on the flux concentrator, a pulsed quadrupole and steering magnets, and a low emittance photo-cathode rf electron source. In this paper, we report the control system status of SuperKEKB injector linac together with the commissioning result of Phase I. In addition, the improvement plant of injector control system is also mentioned., Proceedings of the 16th Int. Conf. on Accelerator and Large Experimental Control Systems, ICALEPCS2017, Barcelona, Spain

E-Driven method is a technical backup for positron source for ILC. In the positron source, the positron is generated and accelerated in a multi-bunch format with gaps in a macro-pulse. We employ AM (Amplitude Modulation) to suppress the transient beam-loading, but a small fluctuation is still expected, depending on the compensation accuracy. In this article, the positron yield which is ratio of numbers of positrons over electrons, is evaluated as a function of the compensation accuracy. With this result and the detail investigation of the beam loading compensation accuracy by AM, the positron yield of E-Driven Positron source for ILC is evaluated., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

KEK injector linac has delivered electrons and positrons for particle physics and photon science experiments for more than 30 years. It is being upgraded for the SuperKEKB project, which aims at a 40-fold increase in luminosity over the previous project of KEKB, in order to increase our understanding of flavour physics. This project requires ten-times smaller emittance and five-times larger current in injection beam from the injector. And many hardware components are being tested and installed. Even during the 6-year upgrade, it was requested to inject beams into light sources storage rings of PF and PF-AR. Furthermore, the beam demanding approaches from those storage rings are different. SuperKEKB would demand highest performance, and unscheduled interruption may be acceptable if the performance would be improved. However, light sources expect a stable operation without any unscheduled break, mainly because most users run experiments for a short period. In order to deal with the both requirements several measures are taken for operation, construction and maintenance strategy including simultaneous top-up injections., Proceedings of the 16th Int. Conf. on Accelerator and Large Experimental Control Systems, ICALEPCS2017, Barcelona, Spain

KEK injector linac has delivered electrons and positrons for particle physics and photon science experiments for more than 30 years. It was upgraded for the SuperKEKB project, which aims at a 40-fold increase in luminosity over the previous project KEKB, in order to increase our understanding of flavor physics beyond the standard model of elementary particle physics. SuperKEKB energy-asymmetric electron-positron collider with its extremely high luminosity requires a high current, low emittance and low energy spread injection beam from the injector. The electron beam is generated by a new type of RF gun, that provides a much higher beam current to correspond to a large stored beam current and a short lifetime in the ring. The positron source is another major challenge that enhances the positron bunch intensity from 1 to 4 nC by increasing the positron capture efficiency, and the positron beam emittance is reduced from 2000 μm to 10 μm in the vertical plane by introducing a damping ring, followed by the bunch compressor and energy compressor. The summary of the rejuvenation is reported., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Linear colliders is the only way to realize e⁺ e⁻ collision at higher energy beyond the limit of ring colliders by the huge synchrotron radiation energy loss. In the linear collider, the beam current should be much smaller comparing to the ring collider to save the required electricity. A way to realize an enough luminosity with the small beam current and less energy spread by Beamstrahlung, is collision in flat beam. This high aspect ratio beam can be made by phase-space rotation technique instead of the conventional way with DR (Damping Ring). We present a simulation of this technique and pilot experiments at KEK-STF and ANL WFA., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

\rm CsK₂Sb is a high performance cathode which can be driven with a green laser. The cathode is generated by evaporation on a substrate in a high vacuum environment. The cathode was evaporated on various material and surface condition to evaluate the dependence of the cathode performance. GaAs (100), Si(100), and Si(111) were examined as samples of the substrate. For each materials, the cathode on the cleaned and as-received substrates were examined and those on the cleaned showed better performance than the as-received for all materials. The detail of the experimental results are presented., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

\rm CsK₂Sb is a high performance photo-cathode for accelerators requiring the high brightness electron beam. It can be driven by a green laser generated as SHG of a solid state laser. The quantum efficiency is as high as 10\%. In this article, the robustness of the cathode was studied experimentally. We found that 1/e lifetime of the cathode was inversely proportional to the vacuum pressure. The normalized temporal life was \rm (4.72± 0.08)× 10⁻⁵~Pa.hour for 532 nm laser. The lifetime regarding to the extracted charge density was also inversely proportional to the vacuum pressure. The normalized charge life was \rm (1.19± 0.03± 0.04)× 10⁻⁴ Pa.C/mm². The cathode is robust enough for a high brightness electron accelerator., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

CsK2Sb photo-cathode is one of the ideal cathode for accelerators requiring the high brightness electron beam. It can be driven with a green laser which can be generated as SHG from solid state laser. The QE (Quantum Efficiency) of photo-electron emission is as high as more than 10% with 532nm light. In this article, the robustness of the cathode is studied. Two indexes of lifetime regarding to time and extracted charge density were evaluated experimentally. The result shows that the cathode is robust enough for a high brightness accelerator., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA

Inorganic scintillating screens are very useful tool to measure transverse profile of charged particle beams. The cerium-doped yttrium:aluminum:garnet (YAG:Ce) crystal scintillator is used in many accelerating facilities. The scintillating screen shows good resolution comparable to that of OTR screen. However, response to high charge density electron beam, more than 10 nC per square millimeter, has not been clarified. In KEK e⁺/e⁻ injector linac, the charge areal density (σ) will exceed 25 nC per square mm. Thus, beam tests has been performed on YAG:Ce, LYSO, and BGO crystals for 1.5 GeV, 1 nC/bunch electron beam at the linac. Saturation of the luminescence which causes degradation of the resolution has been observed above 1 nC per square mm in those crystals. We will report the response of the scintillating screens to high charge density electron beam and discuss the degradation of the resolution due to the saturation of the light yield., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

CsK2Sb photo-cathode is one of the ideal cathode for accelerators requiring the high brightness electron beam. It can be driven with a green laser which can be generated as SHG from solid state laser. The QE (Quantum Efficiency) of photo-electron emission is as high as more than 10% with 532nm light. The material is robust and the typical operational lifetime is more than several months. It is also vital against the high intensity beam extraction. The photo-cathode is generated as a thin film in-situ and the material property and optimized condition for the cathode formation is not understood well. In this article, we present UPS analysis of CsK2Sb cathode for deeper understanding., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA

ILC (International Linear Collider) is e⁺ e⁻ linear collider in the next high energy program promoted by ICFA. In ILC, an intense positron pulse in a multi-bunch format is generated with gamma ray from Undulator radiation. As a technical backup, the electron driven positron source has been studied. By employing a standing wave L-band accelerator for the capture linac, an enough amount of positron can be captured due to the large aperture, even with a limited accelerator gradient. However, the heavy beam loading up to 2 A perturbs the field gradient and profile along the longitudinal position. We present the capture performance of the ILC positron source including the heavy beam loading effect., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA

The positron source under investigation is using channeling radiation of multi-GeV electrons in a tungsten crystal. The radiated photons are impinging on the amorphous targets creating e⁺e⁻ pairs. A dipole magnet between the crystal-radiator and the amorphous-converter allows the charged particles to be swept off and only emitted photons to generate e⁺e⁻ pairs in the converter. Granular targets of different thicknesses, made of small tungsten spheres, have been recently investigated as a target-converter. This paper is describing the experimental studies conducted at the KEKB linac with such device. After the description of the experimental set-up and beam parameters, the measurement methods and preliminary results are presented., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The SuperKEKB is e⁺/e⁻ circular collider for high luminosity, 8Â¥times10³⁵ as a target value. For the high luminosity, the injector linac is required to transport low emittance high-charged electron beam and positron beam to the ring. A charged beam with an offset from a center of cavity is affected by the wakefield depending on both the offset size in the cavity and longitudinal particle position in the beam. The wakefield causes emittance growth. This growth can be suppressed by appropriate orbit control so as to cancel the wakefield effect of the cavities in total. On the other hands, the beam variation in 6-dimensional phase space also induces the emittance growth. Emittance growth by both misalignments and 6-dimensional beam jitter was evaluated by particle tracking simulation. Investigation of beam jitter and drift was also performed by correlation analysis between beam position and measured parameter, charge or temperature., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The SuperKEKB B-Factory at KEK (Japan), after few years of shutdown for the construction and renovation, has finally come to the Phase-1 commissioning of the LER and HER rings, without the final focus system and the Belle II detector. Vacuum scrubbing, optics tuning and beam related background measurements were performed in this phase. Low emittance tuning techniques have also been applied in order to set up the rings for Phase-2 with colliding beams next year. An update of the final focus system construction, as well as the status of the injection system with the new positron damping ring and high current/low emittance electron gun is also presented., Proceedings of the North American Particle Accelerator Conf., NAPAC2016, Chicago, IL, USA

Recent progress on the accelerator science requires optimized phase space distributions of the beam for each applications. A classical approach to satisfy the requirements is minimizing the beam emittance with a bunch charge as much as possible. This classical approach is not efficient and not compatible to the beam dynamics nature. 6D phase-space rotation, e.g. z-x and x-y, gives a way to optimize the phase space distribution for various applications. In this article, we discus possible applications of the 6D phase space rotation. The x-y rotation generates the high aspect ratio beam for linear colliders directly without DR (Damping Ring). Combination of bunch clipping with a mechanical slit and x-z rotation can generate micro-bunch structure which is applicable for FEL enhancement and drive beam for dielectric acceleration. We present our theoretical and simulation study on these applications., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

Photocathode is one of the most important components in the next-generation accelerators, especially based on linear accelerators. Photocathode performance depends not only on electronic state in its bulk material but also on the surface condition. CsK2Sb multi-alkali photocathode is a candidate for the high brightness electron source because of its high quantum efficiency by green laser and its high robustness. We have carried out an UPS (UV Photoelectron Spectroscopy) experiment at UVSOR facility, synchrotron radiation light source in Aichi Japan. We have compared the UPS spectra among several samples, each one has a different quantum efficiency, and try to find physics which decide photocathode's performance. In this case, we focused some characters correlated to the quantum efficiency. I'm going to present a result of this analysis., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

The KEK e⁺/e⁻ injector linac will be operated in multiple modes for the electron beam injection to 3 independent storage rings, SuperKEKB HER, Photon Factory (PF) Ring and PF-AR, and the positron beam injection into the damping ring and the SuperKEKB LER. The operation modes can be switched every 20 milliseconds. The injector linac is under upgrade for the SuperKEKB, where the required resolution of beam position measurement is less than 10 micrometer. However, the current system based on oscilloscopes for stripline beam position monitors (BPMs) has the position resolution of 50 micrometers approximately. Thus, we have developed a new BPM readout system with narrow band pass filter, 16-bit, 250 MSa/s ADCs and calibration pulse generators. The system is based on VME standard and the beam position is calculated by FPGA on board. The new system has the position resolution of 3 micrometer and wide dynamic range from 0.1 to 10 nC/bunch. The calibration pulse follows every position measurement. The calibration pulse is used for the gain correction and the integrity monitor of the cable connection. We will report details of the system., Proceedings of the 4th International Beam Instrumentation Conference, IBIC2015, Melbourne, Australia

A high performance electron beam generated with a laser photo-cathode is one of the most important pieces in the advanced accelerator. Because the CsK2Sb photo-cathode is robust with more than 10 % quantum efficiency (QE) by green laser (532nm), it is considered to be the best candidates of the cathode for Energy Recovery Lin-ac (ERL) and Free Electron Laser (FEL) requiring a high brightness beam. We developed a system to evaporate the cathode as a thin film in vacuum to study the cathode performance. The cathode operational lifetime regarding not only on time, but also extracted charge density was studied. We found the lifetime is long enough for practical use in an accelerator., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

International Linear Collider is a future accelerator to find new physics behind the electroweak symmetry breaking by precise measurements of Higgs sector, Top quark, and so on. ILC has capacities to reveal new phenomena beyond Standard model, such as Supersymmetry particles and dark matters. In current design of positron source, undulator scheme is adapted as a baseline. In the scheme, positrons are generated from gamma rays through pair-creation process in Ti-alloy target. Generations of the gamma rays by the undulator radiation requires more than 130 GeV electrons. Therefore, a system demonstration of the scheme is practically difficult prior to the real construction. Consequently, it is desirable to prepare a technical backup of this undulator scheme. We study an optimization of positron source based on the conventional electron-driven scheme for ILC. In this scheme, positron beam is generated by several GeV electron beam impinging on W-Re target. Although heavy heat load and destruction of the target is a potential problem, it can be relaxed by stretching the effective pulse length to 60 ms instead of 1 ms, by a dedicated electron linac for the positron production. In this report, a start-to-end simulation of the electron-driven ILC positron source is performed. Beam-loading effect caused by multi-bunch acceleration in the standing wave RF cavity is also considered., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

Toward SuperKEKB project, the electron/positron injector linac upgrade is ongoing at KEK in order to deliver the low emittance electron/positron beams with high bunched charge intensity and small emittance. A large number of accelerator components and control devices will be newly installed before the autumn of 2014. Finally, we are aiming at the simultaneous top-up operation for the four independent storage rings including two light sources. The high availability and reliability of control system is strongly required for the long-term stable beam operation under such complex operation schemes. In this presentation, we will describe the control system upgrade plan and status., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

FEL is one of the ideal radiation source over the wide range of wavelength region with a high brightness and a high coherence. Many methods to improve FEL gain has been proposed by introducing an active modulation on the bunch charge distribution. The transverse-longitudinal phase-space rotation is one of the promising method to realize the density modulation as the micro-bunch structure. Initially, a beam density modulation in the transverse direction made by a mechanical slit, is properly transformed into the density modulation in the longitudinal direction by the phase-space rotation. The micro-bunch structure made with this method has a large tunability by changing the slit geometry, the beam line design, and the beam dynamics tuning. For FEL, enegy chirp made by the emittance exchange and chromaticity made by this chirp should be properly corrected. Simulation results and possible applications are discussed., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

Advanced electron linear accelerator such as Energy Recovery Linac and Free Electron Laser needs high brightness electron source. Photocathode is suitable for the high brightness requirement because some of them has low emittance and high quantum efficiency. In the photocathode, CsKSb multi-alkali photocathode has excellent features: high quantum efficiency, long lifetime, and driven by visible light, for example green laser. Therefore, the multi-alkali photocathode is considered to be one of the best candidates for the high brightness electron source of the advanced electron accelerator. We report developments of our evaporation system and results of quantum efficiency and lifetime measurement in Hiroshima University. Multi-alkali surface analyzation has being measured by ultra-violet photoemission spectroscopy to study conditions between the multi-alkali performances and the surface condition in Institute Molecular Science. We also report the status of the progress abort the study., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

GaAs is an unique and advanced photocathode which can generate highly polarized and extremely low emittance electron beam. The photo-emission is possible up to 900nm wavelength. These advantages are due to NEA (Negative Electron Affinity) surface where the conduction band minimum is higher than the vacuum energy state. The NEA surface is artificially made with Cs-O/F evaporation on the cleaned GaAs surface, but the NEA surface is fragile, so that the emission is easily lost by poor vacuum environment and high emission density. NEA activation with any vital material is desirable. We found that the GaAs can be activated by CsTe thin film which is known as a vital photo-cathode material. The photo-electron emission spectrum extends up to 900 nm wavelength which corresponds to the band-gap energy of GaAs. The result strongly suggests that the surface becomes effectively NEA state by the CsTe thin film., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

Toward SuperKEKB project, the injector linac upgrade is ongoing for aiming at the stable electron/positron beam operation with low emittance and high intensity bunch charge. To obtain such high quality beam, we have being commissioning many newly developed subsystems including a low emittance photocathode rf gun since October of 2013. Eventually, we will perform the simultaneous top-up for the four independent storage rings including two light sources. The stable beam operation as long as possible is desired since the prospective physics results strongly depends on the reliability and availability of accelerator operation. Since the middle stage of KEKB project, the injector linac control system has been gradually transferred to the EPICS based one from the in-house system based on RPC. We are expanding the existing control system for the newly installed devices like a network attached power supply, timing jitter monitoring system, and so on. In addition, many commissioning tools are now under development to accelerate the high quality beam development. In this paper, we will describe the present status of injector linac control system and future plan in detail., Proceedings of the 15th Int. Conf. on Accelerator and Large Experimental Physics Control Systems, ICALEPCS2015, Melbourne, Australia

Multi-alkali photocathode has excellent features: high quantum efficiency, long lifetime, and excitation by visible light, for example green laser. The multi-alkali cathode is considered to be one of the best candidate of the high brightness electron source of the advanced electron accelerator such as ERL and FEL. We study conditions of multi-alkali evaporations, such as thicknesses, substrate temperature, and evaporation rate, and examine the cathode performances, such as quantum efficiency and extractable current density. Antimony (Sb), potassium (K), and cesium (Cs) are used in our evaporation system., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

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

ILC (International Linear Collider) is a future project of high energy physics. In the current baseline design, positron generation by gamma rays from undulator radiation is assumed. However, this approach is totally new and it is very difficult to demonstrate the system prior to the construction because it requires more than 100 GeV beam as the driver. A conventional positron generation (e-driven) has been proposed as a technical backup option. In this method, the technology is well established, but the issue is to obtain an enough amount of positron with a manageable energy deposition on target. We present a result of a systematic study of capture efficiency defined by DR (Damping Ring) acceptance where the beam emittance is reduced by radiation damping. We performed a start-to-end simulation of the positron source of ILC and found that an enough amount of the positron per bunch is obtained with a manageable energy deposition on the production target., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

Electron source is one of the most important component in the advanced linac. There is a strong demand on the high performance cathode, such as small emittance, high brightness, and short pulse generation. NEA-GaAs photo-cathode is a unique technology which is capable for generating highly polarized and extremely low emittance beam. Quantum efficiency (QE) of the cathode is high in near IR region, so it is favor to generate a high current density beam. These advantages are originated to the Negative Electron Affinity (NEA) surface, but it is fragile so the operational lifetime is limited. A study on a robust NEA surface cathode is reported. According to the hetero-junction model, Cs-Te thin film deposited on GaAs forms a robust NEA surface. We performed the Cs-Te evaporation experiment on a clean GaAs cathode and measured QE spectra. We found that some sample showed a high quantum efficiency up to 900nm wavelength which strongly suggested a NEA surface formation., 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.

Using a weak-strong beam-beam code, in which the symplectic maps for the linear coupling and chromatic aberrations were implemented, the luminosity degradation caused by the linear and chromatic X-Y couplings at the interaction point (IP) were evaluated for the SuperKEKB project under design. The linear and chromatic X-Y couplings were estimated through modeling the machine errors using random seeds, based on a baseline design of the SuperKEKB rings. It was found that the linear and chromatic X-Y couplings can potentially degrade the luminosity performance., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Synchro-beta resonances enhance beam sizes dynamically. For accelerators aimed for high luminosity, the effect can be more serious since a difference between vertical emittance and longitudinal emittance tends to be larger. Therefore, it is necessary to estimate a tune spread of the synchro-beta resonances properly. Synchro-beta effect is caused by chromatic aberrations, which characterize how optics parameters, including tune, Twiss parameter, X-Y coupling parameter, and other parameters, depend on the momentum deviation. The chromatic aberrations are actually defined by coefficients of an optics parameter in is expansion in terms of momentum deviation. The synchro-beta resonances caused by chromatic aberrations are discussed in this conference. We use 6-dimensional symplectic map which is obtained from measured optics parameters in order to simulate beam motion precisely*., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Optics parameters at the interaction point, beta, x-y coupling, dispersion and their chromatic aberrations, seriously affect the beam-beam performance as is shown in experiments and simulations. The control of the optics parameters is essential to maintain the high luminosity in KEKB. They drift day by day, or before and after the beam abort. They were often monitored at intervals of the operation with taking the study time. They are recently measured during the physics run using a pilot bunch without collision. We show the measured the optics parameters and their variations and discuss the relation to the luminosity., 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

The SuperKEKB main ring is currently being constructed for aiming at the peak luminosity of 8 x 10³⁵ cm⁻²s^{−1}. The electron/positron injector linac upgrade is also going on for increasing the intensity of bunch charge with keeping the small emittance. The key upgrade issues are the construction of positron damping ring, a new positron capture system, and a low emittance photo-cathode rf electron source. The injector linac beam commissioning started in the October of 2013. In this paper, we report the present status and future plan of SuperKEKB injector commissioning., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea