A Disturbance-Observer-based Controller for LLRF Systems

Digital low-level radio frequency (LLRFs) systems have been developed and evaluated in the compact energy recovery linac (cERL) at KEK. The required RF stabilities are 0.1% rms in amplitude and 0.1° rms in phase. These requirements are satisfied by applying digital LLRF systems. To further enhance the control system and make it robust to disturbances such as large power supply (PS) ripples and high-intensity beams, we have designed and developed a disturbance observer (DOB)-based control method. This method utilizes the RF system model, which can be acquired using modern system identification methods. Experiments show that the proposed DOB-based controller is more effective in the presence of high disturbances compared with the conventional proportional and integral (PI) controller. In this paper, we present the preliminary results based on the experiments with DOB-based controller. INTRODUCTION At KEK, a 3 GeV energy recover linac (ERL) light source is proposed. For the demonstration, a compact ERL (cERL) was constructed as a prototype machine for the 3-GeV ERL project [1,2]. The cERL, which is a 1.3 GHz superconducting (SC) project, consists of an injector part and a recirculating loop part. Three two-cell cavities, called Inj. 1, Inj. 2, and Inj. 3, were installed in the injector, and two main nine-cell cavities were installed in the recirculating loop. To fulfill the required beam quality, the RF field fluctuations should be maintained at less than 0.1% (in amplitude) and 0.1° (in phase) in the cERL. Field programmable gate array (FPGA)-based digital lowlevel ratio frequency (LLRF) systems were developed to implement the RF field control [3]. In the LLRF systems of the cERL, disturbance signals such as 50-Hz microphonics and 300 Hz high-voltage power supply (HVPS) ripples will severely limit the performance of the LLRF systems [3]. Furthermore, during beam commissioning, the beam loading can be seen as another disturbance. In principle, these disturbance signals can be rejected or suppressed by applying high proportional and integral (PI) gains in the feedback (FB) control; however, the PI gains are limited by the loop delay. In the cERL, during the beam commissioning, we found that the PI gain is not sufficient in the presence of large disturbances. In view of this situation, we present a disturbance observer (DOB)-based approach that aims to control an LLRF system subject to large disturbances [4-5]. In this paper, we first introduce the LLRF system in the cERL, and then describe the principle, design, and implementation of this DOB-based approach. Finally, preliminary results are used to compare the proposed DOB control and the previous PI control in the cERL beam commissioning. LLRF SYSTEM A simplified block diagram of the cERL LLRF system is shown in Fig. 1. The 1.3-GHz cavity probe signal is down-converted to a 10-MHz intermediate frequency (IF) signal. The IF signals are sampled at 80 MHz by 16-bit ADCs and then fed into the FPGA. The baseband and quadrature (I/Q) components are extracted from the IF signal with a non-IQ method. In the next stage, the I/Q signals are compared with their set values, and the errors are calculated. The errors are regulated with a PI controller and then added with a feedforward (FF) table. Finally, the combined signal is fed into the I/Q modulator via the 16-bit DACs to regenerate the 1.3-GHz RF signal. This regulated RF signal will be used to drive the highpower source, which drives the cavities [6,7].