Abstract

The distribution of reactive and resistive components of the transverse impedance around the circumference of LEP has been obtained by exciting the beam and measuring the variations of phase advance and of closed orbit displacements with beam current. Tune shifts with current and instability thresholds were determined for two machine lattices with different beta functions and momentum compaction for various values of bunch length and synchrotron frequency. These measurements are particularly interesting for LEP2, which will require larger bunch currents to achieve the design luminosity.

Highlights

  • In the present operation of LEP near 45 Gev/beam, the single bunch current is limited to less than about 0.6 mA both by the transverse mode coupling instability (TMCI) and by the beambeam effect. Due to the latter limitation, there is presently no advantage in increasing the single bunch current, and the way to reach higher luminosities is to use a larger number of bunches, such as is done in the ”Pretzel” and ”bunch-train” schemes

  • For LEP2, operating at higher energies by using superconducting cavities, the total beam current will be limited by synchrotron radiation losses which have to be compensated by the available RF power

  • Considering the uncertainties in bunch length and shape, the agreement is very satisfactory

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Summary

INTRODUCTION

In the present operation of LEP near 45 Gev/beam, the single bunch current is limited to less than about 0.6 mA both by the transverse mode coupling instability (TMCI) and by the beambeam effect. Due to the latter limitation, there is presently no advantage in increasing the single bunch current, and the way to reach higher luminosities is to use a larger number of bunches, such as is done in the ”Pretzel” and ”bunch-train” schemes. In addition to measurements of the total values of the impedances, it has become possible to obtain individual contributions of structures situated around the circumference of LEP

Introduction
Discussion
Comparison with Computation
The Second Experiment
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