Abstract
In the past, a longitudinal dipole-coupled bunch instability had limited high current operation at the Cornell Electron-Positron Storage Ring (CESR) and resulted in a degradation of luminosity performance. A longitudinal feedback system successfully damps this instability, and the exchange of superconducting rf cavities for normal conducting rf cavities in CESR has further reduced the instability's strength. A description of the longitudinal dynamics with the instability present are described in this paper along with detailed measurements of the instability using a dual-axis synchroscan streak camera. The measurements were made on single trains of bunches, multiple trains, and colliding beams. These measurements give a characterization of the instability's degradation of luminosity, modes of oscillation, and bunch distribution changes.
Highlights
To achieve high luminosity, the Cornell ElectronPositron Storage Ring (CESR) is operated with multiple bunches
The measurements made for this paper do not provide enough information to make a complete theoretical model of multiple bunch longitudinal dynamics in CESR, but, instead, several aspects of longitudinal dynamics observed in the presence of the longitudinal dipole-coupled bunch instability (LDCBI) will be presented
The following experiments were performed with the dual-sweep streak camera: (i) measurement of a single train consisting of four bunches with the longitudinal dipole-coupled bunch instability present, (ii) measurement of multiple trains with the instability present, and (iii) oscillation of bunches during collisions to quantify the luminosity degradation due to the instability
Summary
The Cornell ElectronPositron Storage Ring (CESR) is operated with multiple bunches. One limitation to high luminosity is a longitudinal dipole-coupled bunch instability (LDCBI) which occurs when multiple bunches are present. The growth of amplitude for the spectral lines depends upon the spacing of the trains and bunches and it is the indication for when the instability threshold is exceeded. The threshold of the LDCBI depends on the spacing between bunches and the number of bunches in each train. To combat the LDCBI, two modifications were made: (i) a low Q, 1.1 GHz, accelerating cavity is used as a longitudinal feedback cavity to damp the LDCBI [1], and (ii) to increase the current threshold and reduce the effect of the LDCBI, four single cell niobium superconducting rf cavities have been installed in CESR replacing 20 cells of normal conducting rf cavities. A previous characterization of the LDCBI was performed with a single-sweep streak camera with the normal conducting rf cavities. A singlesweep streak camera can measure only single bunch dynamics; detailed information of bunch oscillations has not been previously measured [3]
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