M. A. Allen

Superconducting Niobium Cavity Measurements at SLAC

The program of measurements at SLAC on superconducting niobium cavities is described. Results for TE and TM mode X-band cavities are presented. An RF magnetic breakdown field of 960 gauss and Q values greater than 1011 were measured for an electron beam welded TE011 mode cavity at 10.5 GHz. The best result for a TM mode cavity was a Q of 1.4 × 109 and a breakdown field of 360 gauss, limited probably by the quality of the electron beam weld. Data on the effect of exposure to air and to nitrogen are presented.

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.

RF System for the PEP Storage Ring

The main parameters of the rf system for the PEP storage ring are given. A total of 6 MW will be provided by twelve klystrons, each capable of delivering 500 kW of continuous rf power. Each klystron feeds two accelerating cavity sections, each 2.1 meters in length. The general layout is shown. The klystrons are housed in shelters above ground. The wave guides run through vertical penetrations to connect the klystrons to the accelerating cavities in the tunnel. The shelters are placed at three symmetrical regions around the ring. At each of regions 4, 8, and 12 there will be four klystrons and their eight cavities with the eight cavities all being grouped together in the long straight section on one side of an interaction region. The klystron power supplies are on pads outside the shelters. They are unregulated but their voltage may be slowly varied over a 40 percent range by means of variable voltage transformers in the 12 kV input line to each power supply. The output of each power supply passes into the shelter to a high voltage cabinet which contains filter capacitors, crowbars, interlock circuits, and control and metering circuits.

A Longitudinal Feedback System for PEP

Whether the wide bandwidth longitudinal feedback system described in this paper is made to act on the individual modes in frequency domain or on the individual bunches in time domain, it represents a clean and efficient way of damping the longitudinal oscillations without influencing other beam parameters such as bunch shape or synchrotron frequency distribution. The frequency domain feedback presents the advantage of providing information on which modes are unstable and on their risetimes, which may be helpful in locating dangerous resonators in the ring.

Damping Ring RF System for SLC

The linear collider project at SLAC contains two damping rings to reduce the emittance of short electron or positron bunches which contain 5 x 10/sup 10/ particles per bunch. Two of these bunches are stored at a time and then extracted for acceleration in the collider. The rf system is subject to strong transients in beam loading. A computer model is used to optimize capture while minimizing rf power. The introduction of phase jump in the rf drive at injection time together with offsets in the tuning loops of the rf cavities when no beam is stored allows optimum performance under heavy beam load conditions. The rf system (800 kV at 714 MHz) for the electron damping ring has been built, tested and installed, and is being tested with beam.

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.

The SLC Energy Upgrade Program at SLAC

The SLAC Linear Collider (SLC) must reach a nominal center-of-mass energy of 100 GeV to fulfill its high energy physics goals. This paper describes the energy upgrade program that is being implemented on the SLAC linear accelerator to meet these goals. It includes a discussion of the design requirements and available technical options, the rationale for the adopted solution, and the technical problems involved in the engineering and production of klystrons and modulators.

Accelerator Physics Measurements at the Damping Ring

Besides the optics measurements described elsewhere, machine experiments were done at the SLC damping ring to determine some of its parameters. The synchrotron radiation energy loss which gives the damping rates was measured by observing the RF-voltage dependence of the synchronous phase angle. The emittance was obtained from the synchrotron light monitor, scraper measurements and by extracting the beam through a doublet and measuring its size for different quadrupole settings. Current dependent effects such as parasitic mode losses, head tail instabilities, synchrotron and betatron frequency shifts were measured to estimate the impedance. RF-cavity beam loading and its compensation were also studied and ion collection was investigated. All results agree reasonably well with expectations and indicate no limitations to the design performance.

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.