NewSUBARU/SPring-8, Laboratory of Advanced Science and Technology for Industry (LASTI), Hyogo
A multi-element octupole-base corrector magnet is designed and fabricated. The new corrector magnet will be installed in the electron storage ring NewSUBARU in place of vertical steering (skew dipole) magnets. It has coil windings to produce skew quadrupole, skew sextupole, normal octupole, and the skew dipole field. The skew dipole element is used to achieve vertical steering. The skew quadrupole and the skew sextupole elements are for the resonance correction. The normal octupole element is used to control the higher order dispersion function and the higher order momentum compaction factor. In this design the main coil is wound around the return yoke instead of the pole. We expect improvement of the beam lifetime and injection efficiency during normal operation as well as improved isochronism during extreme quasi-isochronous operation. In designing the magnet, careful consideration is given to field interference caused by a neighboring magnet, set close to the corrector magnet of comparable yoke and bore diameter dimensions. The magnetic field with field interference is calculated using OPERA-3D.
JASRI/SPring-8, Hyogo-ken
NewSUBARU/SPring-8, Laboratory of Advanced Science and Technology for Industry (LASTI), Hyogo
RIKEN Nishina Center, Wako
Transverse beam instabilities were suppressed with chromaticity modulation (CM)* in the electron storage ring, New SUBARU. The horizontal and vertical betatron tune spread inside a bunch were introduced by CM with synchrotron oscillation frequency driven by an AC sextuple magnet**, to obtain Landau damping of the coherent bunch motion. The tune spread in a bunch is usually introduced by octupole field, however, its high nonlinearity reduces the dynamic aperture. And usual feedback against instabilities work only on m=0 mode and it is not easy to be applied to hadron synchrotrons because of their varying revolution period. The CM scheme has not such disadvantages. The damping time of coherent motion excited by external kick was measured and was found as less than 1ms, one order faster than that without CM. To observe the effect on instabilities, we intentionally tuned an HOM in a cavity to excite a horizontal multi-bunch instability. The instability peak in the spectrum of the beam motion was vanished with CM turned on and the instability was suppressed. We also observed the increase of the threshold current of the vertical single-bunch mode-coupling instability by factor 3 with CM.
* T. Nakamura, Proc. of PAC'95, p.3100 (1995).
** T. Nakamura, et al., Appl. Superconduct., IEEE Trans. Vol. 18, p.326 (2008).
NewSUBARU/SPring-8, Laboratory of Advanced Science and Technology for Industry (LASTI), Hyogo
A simple analytical formula for the transient bunch lengthening by betatron motion along bending sections is explained. The formula describes a longitudinal and transverse coupling for a single-pass line, which is obtained as an extension of the formula for a storage ring. The bunch lengthening is expressed by a product of three factors: the square root of horizontal betatron emittance, a betatron phase factor, and the square root of the H-function, in other words, dispersion action. That effect had been calculated in many reports concerning with sub-ps electron bunch generation, such as the laser-bunch slicing, the vertical bunch deflection by a crab cavity, and the beam transport along a quasi-isochronous bending arcs. In these works the transfer matrix elements, R15 and R25, had been calculated for each of various conditions. On the contrary, our simple and general analytical formula gives a good foresight to understand the observed phenomena and for an easy optimization of parameters of bending arcs.