SLAC Damping Rings in the 21st Century
April 6-7, 1998 at the Boulevard, Woodside
Summary for the Accelerator Physics and Beam Dynamics Session
The goal of the session on Accelerator Physics and Beam Dynamics was to summarize current thinking on ways to improve the beam properties attainable in the damping rings. The discussion focussed mainly on techniques for improving the transverse emittance, with some eye on instabilities. The boundary conditions for any improvements are that they are consistent with future operating modes for the damping rings. During the PEP II era the rings can operate at a lower repetition rate and furthermore the ring energy is no longer constrained by the polarization spin preservation requirement. The motivation for improving the beam parameters is twofold. First, there may be an opportunity to run some components at a lower rating and still obtain the same beam quality, thereby improving reliability. Secondly, there is an ongoing interest in understanding performance limitations for future damping rings in linear colliders.
T. Raubenheimer reviewed an earlier proposal for lowering the operating energy of the rings. Since the emittance is proportional to E-3 while the damping time is proportional to E3, lowering the energy from 1.19 GeV to 750 MeV and changing to 30 Hz repetition rate gives 4 times longer store time and 4 times lower emittance. This is compatible with 30 Hz PEP II injection, but also with higher injection rates, when the beam would not be fully damped, since PEP II can tolerate ~3x SLC emittances. The PEP injection rate can also be doubled by adding a NIT line kicker, since electrons and positrons can then be produced on the same machine pulse.
Low emittance beams produced this way are very interesting for NLC studies in order to study emittance limiting effects. The high phase space density of the bunches may reveal hitherto unobserved instabilities. Shorter bunches resulting from a lower energy relative to a fixed RF amplitude allow longitudinal instabilities at higher single bunch currents to be probed.
Intra beam scattering limits can be explored in the low emittance beams at the lower energy and can be compared to the predicted behavior versus energy.
Operating the rings at lower energy allows critical components such as the RF, kickers and septum magnets to be run at lower power, improving their reliability.
M. Woodley reviewed the practical requirements for implementing the low energy lattice. Although the dc magnets, kickers and RF can all run lower, more correctors are needed to compensate for saturation in bends. The bends in the transport lines are effected by saturation and will require the installation of several new trim windings to steer correctly at the lower energy. The B1 and B2 magnets that are powered in series with the injection and extraction septum magnets are also effected and will require changes to their shunt resistors to trim them at the new energy.
The next two talks looked at ways of changing the transverse damping partition numbers to improve the horizontal emittance.
J. Spencer reported on an earlier proposal to modify the magnet pole pieces of the ring dipoles to add focussing to the bends which changes the horizontal partition number. The modification is an adjustable pole piece that is bolted to the existing magnet.
The second technique for modifying the partition was reported by F. Zimmerman. A collaborative experiment was described where the frequency is shifted during the store. The partition function in this case is changed by moving the central orbit off-axis in the quadrupoles by shifting fRF mid-store. Some encouraging results from early tests were shown.
In a general discussion session we talked about how it was still relevant to solve some of our unresolved problems in the damping rings, since they are likely to arise in future damping rings. For example, model fudges to make measured and predicted tunes agree are not completely understood. Neither are the sextupole strengths vs. bend standardize cycle well understood. There also remain other problems like SRTL anomalous ex growth, etc.
Some interesting lessons were learnt from SPEAR machine studies. In particular, how they actually schedule studies on a regular basis. Their procedure for quadrupole shunting to align quadrupoles and BPMS would be relevant for the damping rings. It was pointed out that we could retrieve the SLC FF shunts for this purpose.