  The betatron tune, which is defined as the number of transverse oscillations
in one turn of a ring accelerator, is one of the most important parameters. An
undesired betatron tune increases the amplitude of the transverse oscillation
so that many particles are lost from the ring sooner than designed. Since a
betatron tune is controlled by the magnetic fields in the ring, the ripple of
the magnet current directly displaces the betatron tune from its designated
value. We have developed a system that corrects the betatron tune displacement
using the measured magnet current at the J-PARC (Japan Proton Accelerator
Research Complex) Main Ring. We adopted Field Programmable Gated Arrays (FPGA)
to convert from the measured magnet current to the betatron tune in real time.
Using the system, we have decreased the fluctuation of the betatron tune from
5.2e-4 to 3.3e-4 at the frequencies less than 250 Hz.
