Plans To Stabilize Collective Beam Motion in SPEAR

- R. Hettel

The Accelerator Group is embarking on a new program to monitor and stabilize single- and multi-bunch beam motion in the longitudinal and transverse planes that can degrade photon beam quality. Longitudinal motion, with oscillation periods given by the ring synchrotron frequency (~30 kHz) and its harmonics, is accompanied by energy oscillation that can broaden photon beam spectral line widths and also cause emittance-diluting horizontal beam motion with amplitudes given by the ring's dispersion function. Transverse motion occurs at the much higher betatron frequencies (a few hundred kHz) and also acts to blow up the beam size and reduce emittance. Both types of instabilities can result in beam loss, or limit the amount of beam that can be stored in the ring, and become more troublesome as beam current is increased.

Because it is possible for longitudinal motion to damp transverse instabilities by disrupting their coherence, reducing longitudinal motion can lead to a need for transverse stabilizing mechanisms as well. Stabilizing both types of motion can result in a lower beam lifetime because of increased intrabeam Coulomb scattering. Some facilities optimize ring performance by trading off beam stability for lifetime; SSRL may have to develop such optimized running conditions as well, especially as stored beam current is increased beyond 100 mA.

Bunch motion is driven by wakefields, electromagnetic fields arising from the interaction of beam fields, either from the bunch itself or from other bunches, with vacuum chamber and ring component structures. Cavity-like chamber shapes, aperture discontinuities, holes and slots used for vacuum pumping, bellows, and the finite resistance of even smooth chamber walls all extract or exchange energy with the beam and thus contribute to the "impedance" of the ring. Narrowband cavity impedances have a relatively high Q, trapping fields for a longer time and therefore capable of having a strong influence on many bunches.

This type of impedance is the dominant source of multi-bunch instabilities. Broadband, low Q impedances have a much shorter range of influence, which in some cases can still stimulate motion of closely spaced multiple bunches, but otherwise tend to be the cause of single-bunch internal motion as the fields from the front of a bunch act on trailing portions of the same bunch.

Several components are known or thought to contribute to the SPEAR ring impedance. These include: higher order mode structures in the rf cavities; cavity-like chambers for injection kickers, separating plates, electrostatic quadrupoles, and some beam position monitors; step discontinuities in insertion device and other chamber apertures; and pump-out ports, synchrotron light ports, bellows, and other chamber assemblies. The dominant induced beam motion at present is in the longitudinal direction: both single- and strong multi- bunch modes have been observed. The principal source of the longitudinal multi- bunch modes is thought to be in the rf cavities, but other components are likely contributors as well to these and other modes.

In the past we have measured large longitudinal coherent oscillation amplitudes that translate to hundreds of microns of horizontal motion in high dispersion ring regions that include insertion device sourcepoints. We have also observed a coupling between longitudinal and transverse modes, but have recently had some success in reducing this coupling by adjusting sextupole magnet strength and by powering octupole magnets that serve to decohere those modes.

In FY97 we will begin a program to specify new components and systems needed to further stabilize collective beam motion. This effort will begin by resuming previous work to identify and quantify single- and multi-bunch longitudinal oscillation modes and their sources using conventional spectral analysis methods and specialized instruments, such as those developed for the ALS and SLAC B-Factory longitudinal feedback systems.

Bunch-by-bunch longitudinal feedback system for the ALS and SLAC B- Factory

Future measures that might be taken to reduce this motion include removing unneeded high impedance components, damping modes in required components, stabilizing the rf cavity cooling-water temperature, installing narrowband feedback for suppressing a strong cavity mode or a higher harmonic cavity for reducing single-bunch motion, and/or implementing wideband bunch-bunch feedback for multiple mode disturbances. We will assess transverse stabilizing requirements once longitudinal motion is attenuated.

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December 2, 1996