R. Siemann

Beam Diagnostic Instrumentation at CESR

We discribe various beam diagnostic devices in use at CESR, an 8 GeV electron-positron storage ring operating primarily in the 4.7 to 5.5 GeV beam energy range. Getting the last 20% of performance depends to some extent on empirical tuning and appropriate presentation of various parameters is very important. Several devices are most useful in machine studies and we describe their operation. The individually regulated quadrupoles in CESR provide unique opportunities for lattice measurements and calibration of beam position monitors.

Coherent Normal Modes of Colliding Beams

The coherent normal modes of the nonlinear colliding beam system are calculated using averaging methods to find an equivalent infinite system of linear coupled oscillators. The condition of a consistent, stationary state leads to an eigenvalue equation for the characteristic frequencies and transverse exitation distributions of the modes. Results are presented for modes along the narrow axis of a Gaussian ribbon beam. The tune split is related to L/¿(I+I-) for different values of I+/I- and of ¿+/¿- .

Single Bunch Current Dependent Phenomena in CESR

Single bunch current dependent phenomena have been examined in CESR, the Cornell Electron Storage Ring. These measurements are described and their results compared with predictions using the broad band resonator model of vacuum chamber impedance. A transient anti-damping effect in the vertical plane has been observed. The influence of various machine parameters on this effect will be described and a possible mechanism suggested.

Instability Growth Rate Calculations for High Energy Storage Rings

Tolerances in the manufacture of rf cavities for high energy electron-positron storage rings lead to a statistical distribution of higher mode resonant frequencies. This distribution and its associated fluctuations are considered in calculations of the growth rate of instabilities. It is found that in some cases the fluctuations can lead to large growth rates and must be accounted for when designing higher mode damping probes. Calculations for the Cornell 50 GeV electron-positron storage ring are used as illustrations.

Computer Simulation Studies of Single Beam Stability

Single beam stability in large e+e- storage rings has been studied by computer simulation techniques. The inputs to the study are single particle equations of motion (including wakefield effects) and longitudinal and transverse wakefields. These wakefields are obtained by numerical integration of Maxwells equations in the time domain. Qualitative and quantitative comparisons are made between the results of the simulation and data from PEP and PETRA, and the agreement is reasonable. Since the only numerical data needed for the simulation are lattice parameters and RF structure dimensions, it is now possible to predict beam stability in future accelerators.

A Computer Simulation Study of e+ e- Storage Ring Performance as a Function of Sextupole Distribution

Operational experience at CESR has shown that sextupoles are important factors in the observed beam-beam behavior. A computer program to simulate colliding beam dynamics in e+ e- Storage rings has been written. The first version of the program which does not incorporate sextupoles shows some of the characteristics measured in various machines in the past. The focus or the present work is the understanding of the effects of sextupoles on these results. To do this thin sextupoles are added in two ways. The first employs a linear-transfer/nonlinearkick algorithm for each lattice cell. The second method is to create a symplectic second-order transfer map for the entire machine. While the first method is exact, it is slow for machine lattices with many sextupoles. The luminosities, beam sizes, and tune shifts from these programs are compared. In addition, the shapes of the time-averaged transverse distributions are studied.

A Single Beam Multibunch Instability at CESR

A transverse coupled bunch instability has been observed in the Cornell Electron Storage Ring CESR for both positrons and electrons. This instability shows strong horizontal or vertical coherent signals but very weak longitudinal signals. Positron injection, which requires sixty-one uniformly spaced bunches in CESR, was originally restricted by the enlarged effective horizontal beam size resulting from this instability. Although several cures have been discovered, only external octupoles provide a sufficient increase in the threshold and are compatible with injection. The theory for coupled bunch motion correctly predicts the instability threshold assuming that the driving mechanism is a 1140 MHz parasitic resonance in the RF cavity. Changes in the threshold with tune and octupole field strength are also correctly predicted.

Observation of the Beam-Beam Limit in CESR

CESR has produced e+ e- collisions for high energy physics in the very productive T region (4.7 to 5.7 GeV per beam) since the fall of 1979. The peak luminosity recorded during physics data taking over that period is shown in Fig. 1. The dramatic increase in the luminosity has resulted from the reduction of ßy* from 11 cm to 3 cm, an increase in ¿x *, and increases in the vertical aperture. Furthermore, observations of the beam-beam interaction show that the luminosity increases as the square of the beam current at low currents and linearly with current at high currents. These observations are consistent with a vertical beam size which is constant at low currents and increases linearly at high currents. A linearly increasing vertical beam size implies a constant vertical tune shift. The luminosity and the beam lifetime are limited by nongaussian tails which reach the physical aperture of CESR.

Beam Modulator for an Electron Linac

A system has been developed for modulating the beam of the Cornell Electron Storage Ring injection linac. It employs a sub-modulated, high frequency Lissajous raster of the electron beam on a collimator, followed by a subharmonic prebunching. In conjunction with a two amp electron gun, a variable pattern of single, one nanocoulomb, S-band bunches is produced. The Lissajous raster is generated by two orthogonal pairs of deflection plates attached to high frequency resonant coaxial lines. Submodulation of the raster is accomplished by a single pair of upstream of deflection plates driven by a broad band miniature tetrode. Details of the design calculations, hardware, and performance are given.