Takako Miura

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 and test of klystron linearization packages for FPGA-based low level RF control systems of ILC-like electron accelerators

We report the development and implementation of two FPGA-based predistortion-type klystron linearization algorithms at the Fermi National Accelerator Laboratory (FNAL), USA and the Deutsches Elektronen Synchrotron (DESY), Germany. With those the generation of correction factors on the FPGA was improved, avoiding quantization and decreasing memory requirements. At FNAL the linearization algorithm was tested at the Advanced Superconducting Test Accelerator (ASTA) demonstrating a successful implementation. The functionality of the algorithm implemented at DESY was demonstrated successfully in a simulation.

Observation of Emittance Growth at KEK PS

Emittance growth has been observed in the transverse direction at the injection period of the 12 GeV main ring of the KEK proton synchrotron. Measurement of the beam profiles using flying wires has revealed a characteristic temporal change of the beam profile within a few milliseconds after injection. Horizontal emittance growth was observed when the horizontal tune was close to the integer. The effect was more enhanced for higher beam intensity and could not be explained with the injection mismatch. Resonance created by the space charge field was the cause of the emittance growth. A multiparticle tracking simulation program, ACCSIM, taking account of space charge effects has qualitatively reproduced the beam profiles.

Beam position measurement using linac microstructure at the KEK booster synchrotron

The position information of the most recently injected beam from the linac is obtained by picking-up the signal of the harmonic component of the beam, if the bunch structure disappears during one turn due to the momentum spread. The experiment for 201 MHz pickup was performed in the KEK booster ring, and is compared with the simulation. The possible application of this method to the 3 GeV rapid cycling synchrotron in the Japan Proton Accelerator Research Complex (J-PARC) is also addressed.

Space charge effects during the injection period of the KEK PS main ring

Space charge effects during the injection period of the 12 GeV main ring of the KEK proton synchrotron have been studied. Measurement of the transverse beam profiles using flying wires has revealed a characteristic temporal change of the beam profile within a few milliseconds after injection. Horizontal emittance growth was observed when the horizontal tune was close to the integer. The effect was more enhanced for higher beam intensity and could not be explained with the injection mismatch. Resonance created by the space charge field was the cause of the emittance growth. A multiparticle tracking simulation program, ACCSIM, taking account of space charge effects has qualitatively reproduced the beam profiles.

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.

Long Term Cavity Performance in Compact-ERL Injector Cryomodule

After cryomodule assembly and first cool-down tests in 2012, cERL injector cryomodule including three 2-cell cavities has been stably operated with beam for 3.5 years. Remarkable increases of x-ray radiation levels due to field emission were observed during beam operation in June, 2015. High power pulsed RF conditioning as a cure method was applied in the successive cool-down period in 2016, so that degraded cavity performances have almost recovered up to the original levels.

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

Feedback optimization in MicroTCA-based LLRF systems

Low-level radio frequency (LLRF) control systems are used to achieve extremely precise field control of superconducting cavities in accelerators. Recently, a μTC-based LLRF system has been developed at compact energy recovery linac (cERL) at KEK. To improve the feedback control performance, we have developed some real-time digital signal processing (DSP) algorithms for applications with FPGA in the LLRF systems. We have surveyed the proportional and integral gains to optimized the feedback control. In the cERL, the existence of the parasitic mode in the nine-cell cavity limits the loop gain during the feedback operation. To solve this problem, a high-order IIR filter has been proposed and developed to suppress the parasitic modes. Here, we present these real-time DSP algorithms and FPGA works with preliminary results.

Targetry R&D for PRISM Project

A conceptual design of the targetry for the high intensity slow muon source is presented. Based on simulation studies, critical issues are discussed and a baseline design is determined. We have started R&D works on superconducting solenoids as well as conducting targets, which is an alternative option.