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An AC sextupole magnet system was developed and installed in the electron storage ring, NewSUBARU. This system is for a proof of principle experiment of a new suppression method of transverse beam instabilities. The magnet produces a modulation of the chromaticity with a synchrotron frequency, which creates a large tune spread and suppresses instabilities with Landau damping. The rating of the system is determined from the ring parameters and the yoke length is 150 mm, the bore radius is 80 mm, the maximum field is 36 T/m2 at 300 A and the operation frequency is 4-6 kHz. The coil conductor consists of an inner side half turn (closer to the magnet axis) and an outer side half turn for each pole. At the operation with peak current of 300 A, the power loss in the magnet was 900 W. The eddy current loss in the inner side coil conductor is the main part of this loss and is induced by the magnetic field excited by the outer coil current. It has no water-cooling system and the coil temperature was raised to higher than 70degC after 5 minutes operation and the interlock system is set to turn off the power at 80degC. However this is acceptable for the experiment in a short period. To drive the current of 300 A peak, an external capacitance is attached to the magnet, which forms the LC resonant circuit that is excited by a general-purpose AC power amplifier (DC-20 kHz, 170 V peak, 23 A peak). The Q-value of the circuit is 16.5 for 5 kHz. The resonant frequency is variable by remote switching of the several capacitances. With this magnet system, we successfully observed the suppression of beam instabilities.