H. Katagiri

The KEKB injector linac

Abstract An 8-GeV electron/3.5-GeV positron injector for KEKB was completed in 1998 by upgrading the existing 2.5-GeV electron/positron linac. The main goals were to upgrade its accelerating energy from 2.5 to 8 GeV and to increase the positron intensity by about 20 times. This article describes not only the composition and features of the upgraded linac, but also how these goals were achieved, by focusing on an optics design and commissioning issues concerning especially high-intensity single-bunch acceleration to produce positron beams.

Status of the low-level RF system at KEK-STF

RF field stabilities less than 0.3%, 0.3deg are required at the STF low-level rf (LLRF) system. In order to satisfy these requirements, a digital FB system using an FPGA is adopted. A total of eight cavities will be installed in STF- phase 1 and the vector sum control of eight cavity signals will be achieved. The performance of the FB system is examined using electrical cavity simulators prior to the rf operation. Other R&D projects such as the development of a simplified interlock system with an FPGA are also summarized.

Development of digital low-level RF control system using multi-intermediate frequencies

In a superconducting accelerator, an FPGA-based low-level RF system is adopted and a digital feedback control system is utilized to satisfy the requirement of stability in the accelerating field. A digital low-level RF system using multi-intermediate frequencies has been developed and the stability of the feedback operation is estimated using a cavity simulator based on an FPGA board. In this study, the RF system is examined and the results of estimations that are obtained using the cavity simulator are reported.

Progress of 7-GeV SuperKEKB Injector Linac Upgrade and Commissioning

KEK injector linac is being upgraded for the SuperKEKB project, which aims at a 40-fold increase in luminosity over the previous project KEKB. SuperKEKB asymmetric electron and positron collider with its extremely high luminosity requires a high current, low emittance and low energy spread injection beam from the injector. Electron beams will be generated by a new type of RF gun, that will inject a much higher beam current to correspond to a large stored beam current and a short lifetime in the storage 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 by introducing a damping ring, followed by the bunch compressor and energy compressor. The recent status of the upgrade and beam commissioning is reported.

Performance of the Digital LLRF Systems at KEK cERL

A compact energy recovery linac (cERL), which is a test machine for the next generation synchrotron light source 3-GeV ERL, was constructed at KEK. In the cERL, a normal conducting (NC) buncher cavity and three superconducting (SC) two-cell cavities were installed for the injector, and two nine-cell SC cavities were installed for the main linac (ML). The radiofrequency (RF) fluctuations for each cavity are required to be maintained at less than 0.1% rms in amplitude and 0.1° in phase. These requirements are fulfilled by applying digital low-level radio-frequency (LLRF) systems. During the beam-commissioning, the LLRF systems were evaluated and validated. A measured beam momentum jitter of 0.006% shows that the target of the LLRF systems is achieved. To further improve the system performance, an adaptive feedforward (FF) control-based approach was proposed and demonstrated in the beamcommissioning. The current status of LLRF system and the adaptive FF approach for LLRF control in the cERL are presented in this paper. INTRODUCTION At KEK, a compact energy recovery linac (cERL), as a test facility for future 3-GeV ERL project, was constructed, and the first beam-commissioning was carried out at June, 2013 [1, 2]. The cERL is a 1.3 GHz superconducting radio-frequency (SCRF) machine that is operated in continuous-wave (CW) mode. As shown in Fig. 1, the cERL consists of an injector part and a main linac (ML) part. A normal conducting (NC) cavity (buncher) and three two-cell superconducting (SC) cavities (Inj. 1, Inj. 2, and Inj. 3), were installed in the injector, and two main nine-cell SC cavities (ML1 and ML2) were installed in the main linac (ML). For lowemittance beam, the requirements of the RF field stabilities are 0.1% rms in amplitude and 0.1° in phase in the cERL. This requirements are fulfilled by applying digital low-level radio-frequency (LLRF) systems. The LLRF system in the cERL is disturbed by various disturbances include the 50-Hz microphonics, the 300-Hz high-voltage power supply (HVPS) ripples and the burst mode beam-loading [3-4]. The current LLRF system is not sufficient to reject all of these disturbances. In view of this situation, we have proposed a disturbance observer (DOB)-based approach for suppress the main disturbances in the cERL [3]. Based on this approach, the disturbances can be reconstructed by the cavity pickup signal and then removed from the feedforward (FF) table in real-time. Therefore, in terms of function, this approach is just like an adaptive FF control. In this paper, we first introduce the LLRF system in the cERL, and then present the measured LLRF stability and beam momentum jitter during the cERL beamcommissioning. In the next stage, we describe the basic idea of the proposed adaptive FF approach for disturbances rejection. Finally, we present the preliminary result of this adaptive FF approach for microphonics rejection in the cERL commissioning. Main linac 2 8 kW SSA Nine-cell SC 8 kW SSA Main linac 1 Two-cell SC SC SC 300 kW Kly. 25 kW Kly. 8 kW SSA Vector-sum Controlling ~8.5 MV/m for main linac Cavities ~3 MV/m for Injector Cavities ~ 20 MeV Dump 16 kW SSA Figure 1: Layout of the cavities in the cERL. The marked values of beam energy and accelerating field indicate the current state in the cERL beam-commissioning. HLRF SYSTEM RF power sources including 25 kW klystron, 300 kW klystron, 8 kW solid state amplifier (SSA) and 16 kW SSA were employed in the cERL. Figure 1 shows the layout of the cavities and corresponding power sources in the cERL. Table 1 gives the loaded Q value, required RF power, and RF sources for each cavity. It should be mentioned that, in the Inj .2 and Inj .3, a vector-sum control method is applied. All of these RF sources are stable and reliable in the beam commissioning. Table 1: Cavity Parameters of the cERL Cav. QL f1/2 [Hz] RF power [kW] RF source Bun. 1.1×10 57000 3 8 kW SSA Inj. 1 1.2×10 540 0.53 25 kW Kly. Inj. 2 5.8×1