J. R. Rees

Beam Energy Loss to Parasitic Modes in Spear II

The energy loss due to the excitation of parasitic modes in the SPEAR II rf cavities and vacuum chamber components has been measured by observing the shift in synchronous phase angle as a function of circulating beam current and accelerating cavity voltage. The resulting parasitic mode loss resistance is 5 M¿ at a bunch length of 6.5 cm. The loss resistance varies with bunch length ¿z approximately as exp(-0.3 ¿z). If the measured result is compared with reasonable theoretical predictions, we infer that the major portion of the parasitic loss takes place in ring vacuum components rather than in the rf cavities.

Automatic Control Program for Spear

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Spear II Performance

The single beam and colliding beam performance of the SLAC electron-positron storage ring SPEAR II is described. The sevenfold increase in harmonic number in SPEAR II in comparison to SPEAR I has made significant changes in single beam behavior. Strong synchrobetatron resonances and a new transverse instability are observed and our first studies of these phenomena are described, Measurements on current dependent bunch lengthening are presented.

Operating Results from SPEAR

Initial operating experience with the SLAC electron-positron storage ring SPEAR is described. A luminosity of 1.2 × 1031 cm-2 sec-1 has been achieved and two-beam interaction effects are described. A single-beam coherent instability, which can be suppressed either by control of the ring chromaticity or by feedback, has been observed. Current-dependent bunch lengthening and widening have been observed, and experiments indicate that these phenomena may be associated with an increase in the energy spread of the beam. Procedures to increase the luminosity to the design value are discussed. Plans to increase the maximum beam energy of SPEAR to 4.5 GeV are described.

Recent Improvements in Luminosity at PEP

We will describe improvements which have led to new records for peak and average luminosity at PEP. Comparison of recent results with several earlier lattice and optical modifications shows rather good correlation with the predictions of a beam-beam simulation program.

Operation of the PEP Transverse Beam Feedback

The PEP Storage Ring has been equipped with a wide band beam feedback system capable of damping the vertical and horizontal motion of six bunches. The oscillation detection is done at a symmetry point on the Storage Ring and feedback is applied at the same location one orbital period later. The signal is synchronously gated and the system appears as twelve independent feedback loops, operating on the two coordinates of each of the six bunches. Two beam deflection electrodes are driven each by a low-Q push-pull amplifier which is tuned at the 72nd harmonic of the revolution frequency and suppressed-carrier modulation is generated by a sequence of the detected bunch oscillations. The design parameters are reviewed as well as the salient features of the hardware, and the impact of this system on the machine operation is evaluated in the light of experimental results.

Measurements on Beam-Beam Interaction at SPEAR

Measurements of luminosity at SPEAR, which were performed at different operational conditions of the storage ring are shown and discussed. The parameters varied are: the current in both beams, the minimum betatron amplitude functions at the collision point, the energy and the vertical betatron frequencies. As a result of these measurements we found that the maximum achievable luminosity is much higher than predicted by the incoherent beam-beam limit using ?v = 0.025. We also found that the maximum achievable luminosity is a strong function of the betatron frequencies. After computing the largest linear tuneshifts, we found that the quarter resonance seems to be the limiting effect for beam-beam collision.

PEP Lattice Design

Details of the current lattice design for PEP, the proton-electron-positron colliding beam system will be described. This system allows collisions of protons up to 150 GeV with electrons or positrons up to 15 GeV, by storing the proton beams in a superconducting storage ring and the electrons or positrons in a concentric conventional ring, and allows collisions between electrons and positrons in the same ring up to 15 GeV.