CERN Accelerating science

Article
Title	Control Loops for the J-PARC RCS Digital Low-Level RF Control
Author(s)	Schnase, Alexander ; Anami, Shozo ; Ezura, Eizi ; Hara, Keigo ; Nomura, Masahiro ; Ohmori, Chihiro ; Takagi, Akira ; Tamura, Fumihiko ; Yamamoto, Masanobu ; Yoshii, Masahito
Affiliation	(JAERI/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken) ; (KEK, Ibaraki)
Publication	2005
In:	21st IEEE Particle Accelerator Conference, Knoxville, TN, USA, 16 - 20 May 2005, pp.1063
Subject category	Accelerators and Storage Rings
Abstract	The low-level radiofrequency control for the Rapic Cycling Synchrotron of J-PARC is based on digital signal processing. This system controls the acceleration voltages of 12 magnetic alloy loaded cavities. To achive a short overall delay, mandatory for stable loop operation, the data-processing is based on distributed arithmetics in FPGA. Due to the broadband characteristic of the acceleration cavities, no tuning loop is needed. To handle the large beam current, the RF system operates simultaneously with dual harmonics (h=2) and (h=4). The stability of the amplitude loops is limited by the delay of the FIR filters used after downconversion. The phase loop offers several operation modes to define the phase relation of (h=2) and (h=4) between the longitudinal beam signal and the vector-sum of the cavity voltages. Besides the FIR filters, we provide cascaded CIC filters with smoothly varying coefficients. Such a filter tracks the revolution frequency and has a substantially shorter delay, thereby increasing the stable operating region of the phase loops. The adaptive radial loop accumulates the orbit variation over several machine cycles to reduce the effects of measurement errors on the effective acceleration frequency program.