  The low level rf (LLRF) control system for the rapid cycling synchrotron
(RCS) of the Japan Proton Accelerator Research Complex (J-PARC) started its
operation in 2007. The key functions of the LLRF control system are the dual
harmonic auto voltage control, which generates superposed voltages of the
fundamental accelerating harmonic and the second harmonic in a single wideband
magnetic alloy (MA) cavity, and the multiharmonic rf feedforward to compensate
the beam loading in the MA cavity caused by high intensity beams. These
functions are necessary to accelerate high intensity proton beams. The system
has been working well without major problems for more than ten years. However,
the old FPGAs (Xilinx Virtex-II pro etc.) are discontinued and not supported by
the current development environment. It will be difficult to maintain the
system in near future. Thus, we are planning to replace the existing VME-based
LLRF control system with a new MicroTCA.4 based system. The system controls
twelve cavities independently and calculates vector sum of the cavity voltages
in real time for phase feedback. Signal and data transfer between the modules
is a key to realize the functions. In the existing system, the transfer is
implemented not only the backplane but also serial link via cables between the
VME modules. It is much more simplified in the new system thanks to the high
speed communication capability of the MicroTCA.4 backplane. In this article, we
describe the configuration of the system under development, the implemented
functions, and preliminary test results.
