Longitudinal feedback at CESR

Abstract

Total stored electron and positron beam currents at the Cornell Electron Storage Ring (CESR) have been limited by the presence of a dipole multibunch longitudinal instability having a threshold at approximately 250mA of total current. A longitudinal feedback system has been under constant development, test, operation and upgrade over the past several years. The result has been an increase in the High Energy Physics (HEP) operating current to over 500mA total. This paper describes the overall design of the present system, using a horizontal stripline kicker to produce combined horizontal and longitudinal bunch by bunch beam stabilization. Some details on specific subsystems, including: receiver design, individual bunch phase DC offset correction, digital signal filtering, and the RF modulator will be given, as well as an outline of plans for further development.

Betatron phase and coupling measurements at the Cornell Electron/Positron Storage Ring

“…The digitizing front ends are similar to those used in the CESR feedback system [4]. They remove the dc signal component at the analog level and issue a 10 bit digital signal with average value zero.…”

“…Consequently, the sign of the measured u needs to be changed before applying Eq. (4). Equation (4) shows an advantage of a digital system: In an analog measurement the delay time to the signal analyzer affects the measured phase as discussed in the Appendix.…”

Streak camera measurements of the longitudinal distribution with multiple bunches in CESR

“…The general conclusions from the measurements are that there is no difference between the positrons and electrons in CESR, in terms of longitudinal dynamics. The coupled-bunch longitudinal instability is a dipole mode instability, and a strong feedback system has allowed the instability to be damped, and increased the current thresholds [4]. The presence of the longitudinal instability was noticed in the beam spectra.…”

Performance of the beam stabilizing feedback systems at CESR

“…CESR multiple bunch feedback has recently been upgraded with the digital signal processor (DSP) using a direct sampling ADC (analog to digital converter) that operates at 71.4 MHz, capable of processing the 14 nanosecond spaced bunches in a single channel [2]. The single channel approach eliminates the need for signal stretchers and demultiplexed channels for each bunch.…”