Yosuke Honda

Measurement and Control of Beam Losses Under High Average-current Operation of the Compact ERL at KEK

The compact ERL (cERL) [1, 2] at KEK is a superconducting accelerator aimed at demonstrating ERL technologies for the future light source. In cERL, low-emittance and high-average-current electron beams of up to 10 mA will be recirculated in future. Toward this goal, we studied high-average-current operations where the beam losses should be controlled to very-small fractions. We have so far succeeded in recirculating beams of up to 0.9 mA with very-small beam losses. We report our accelerator tuning method for high-average-current operation, and present measured radiation data showing very-small beam losses. INTRODUCTION In cERL, production and transportation of lowemittance (< 1 mmuf0d7mrad) and high-average-current (uf0b3 10 mA) electron beams are primarily important. In highintensity linacs such as cERL, reduction of beam loss is essential in order to reduce the risk of radiation hazard as well as to avoid damages in accelerator components. Till June of 2015, electron beam having an average current of 80 uf06dA was successfully transported to the beam dump in cERL. Due to careful accelerator tuning and the use of beam collimators, beam losses along a recirculation loop were reduced to small amounts. At this time, we conducted radiation measurements with several methods, and estimated amounts of beam losses [3]. Based on these data, we installed some additional radiation shields, and applied an increase in our authorized beam current, that is, from 100 uf06dA to 1 mA. This application was approved by the government in January, 2016. Until March of 2016, we established high averagecurrent operations of cERL up to a maximum beam current of 1 mA. Typical operational parameters are given in Table 1. We can choose one of two repetition rates of bunches, 1.3 GHz or 162.5 MHz, by selecting one of the laser oscillators of a photocathode DC gun. First, we tuned the machine at a higher bunch-repetition rate (1.3 GHz) with lower bunch charge (0.7 pC/bunch). After this Table 1: Typical Operational Parameters of cERL Beam energy 19.9 MeV Injection energy 2.9 MeV Bunch repetition rate (usual) (for laser-Compton scattering) 1.3 GHz

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