Beam Intensity Upgrade of a Storage Ring by Curing Instabilities by Chromaticity Modulation and its Usage Test

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Transverse instabilities of beams are the one of the most serious factor that limits the beam current of storage rings. Chromaticity modulation is a scheme to suppress transverse instabilities and is invented by the head investigator.The chromaticity modulated by synchrotron frequency couples the synchrotron motion of a beam and introduces the frequency spread of the transverse oscillation, or the spread of betatron tune, inside of a bunch. This spread damps the coherent oscillation of the bunch to overcome the excitation of the coherent oscillation driven by instabilities. Octupole magnets and feedback systems are devices usually used to suppress the instabilities but injection of a beam under the octupole magnet is intrinsically difficult and feedback systems can not suppress the single-bunch instabilities that occur inside of a bunch and requires a large amount of work for tuning. However chromaticity modulation has no such shortage.We applied this scheme to NewSUBRU storage ring. A 5 kHz AC sextupole magnet, a LC resonator type power supply with capacitance bank which enables continuous tuning of the drive frequency, the beam pipe vacuum chamber for the AC magnetic field and a lattice to introduce dispersion at the AC sextupole magnet section were developed.The beam experiment showed the strong enhancement of damping of coherent oscillation and we obtained the damping time less than 1ms which is one order shorter than that of the radiation damping. The beam experiment also showed the suppression of the horizontal multi-bunch instability driven by a higher order mode of an acceleration cavity and suppression of the vertical single-bunch mode-coupling instability driven by broad-band impedance of a beam pipe.We proved the effectiveness of the chromaticity modulation to suppress the transverse instabilities and this scheme leads to further increase of the current of rings.