BPM and BSM Tune Measurements August 2, 2007 B. Cerio, R. Holtzapple

Introduction During CESR-c operation, with 26 bunches in the machine (9x3, and empty bunch in train 1 bunch 3), BPM tune measurements, as well as BSM beam size measurements, were made. The aim of the first BSM measurements was to calibrate the timing of the e- and e+ PMT arrays. To determine the timing setting that maximized the signal, time sweeps were done at PMT HVs of 380 V to 500 V in increments of 20 V for the positron system, and 400 V to 560 V in increments of 20 V for the electron system. Turn-by-turn BSM measurements over 1000 turns for two bunches were then made at each PMT HV with the timing that we determined in the calibration. In past CHESS tune measurements, a tune shift due to the electron cloud was observed. To test for similar dynamics in low energy operation, tune was measured with the BPM for all 26 bunches over 1024 turns. The beam was bumped in the vertical plane during BPM data acquisition to maximize the tune signal in the frequency domain. Beam size was measured with the BSM over 2048 turns immediately before the tune measurement. This procedure was done before and after three fills.

BSM West (e-) System fill - At this current (~2mA/bunch), the peak signal channel does not become saturated in this HV range. - The machine was filled between the 480 V and 500 V measurements.

BSM East (e+) System saturation Intensity fluctuations decrease due to saturation. - At this current (~2.3mA/bunch), the peak signal channel saturates at ~450 V. These points are not accurate, due to saturation.

BPM Tune Measurements: VAX vs. MATLAB In the past, tune was calculated from turn-by-turn BPM signals with VAX signal processing software. MATLAB software was recently written to calculate tune; however, if the tune calculated by the VAX software was compared to that calculated by MATLAB, there was a difference of between 200 and 400 Hz, which was unacceptably large compared to the resolution. Including more significant figures in the period of revolution appears to alleviate the discrepancy, as illustrated below. - The difference in vertical tune ranges from -60 Hz to 60 Hz - The difference in horizontal tune ranges from -80 Hz to 80 Hz. - These differences are much less than the resolution of ~381 Hz. - For the sake of consistency and convenience, tune calculations will henceforth be done with the MATLAB software.

BPM Tune Measurement: Fill 1 i) Electrons experience a negative shift of ~0.6 kHz in vertical tune along the train, which may result from the electron cloud effect or beam-beam interaction at IP. The decrease in tune along the train is not as predictable at low current, which is consistent with both electron cloud and beam-beam, warranting further experimentation to distinguish the dominant effect during CESR-c operation. Interestingly, the tune shift in CESR-c is comparable to the shift measured during CHESS. ii) Positron vertical tune first decreases and then increases along the train, perhaps a combination of parasitic crossing and electron cloud effects. iii) There is a positive shift of ~2.5kHz in positron vertical tune between high and low currents, which is probably a result of the beam-beam interaction at IP. iv) Vertical and horizontal tunes are nearly identical for electrons and positrons, due to transverse coupling. The beam was kicked in the vertical plane, so the measured horizontal tune is in fact the vertical tune.

BPM Tune Measurement: Fill 2 i) Results are consistent with measurements from the first fill. ii) Again positron tune is greater at high current, and electrons show similar behavior for this fill (tune increases by ~0.8 kHz). iii) Although the currents are comparable, positron tune shifts by ~0.8 kHz for this fill, whereas for the last fill it shifted by ~2.5 kHz. The operator must have adjusted the tune in the first fill to mitigate beam loss. iv) There was not a strong e+ horizontal tune signal, suggesting that transverse coupling is weaker in the positron beam (interesting because optics are the same for the two. Must be IP effect?).

BSM Beam Size Measurement: Fill 1 low high e-e+ beam loss i) There is a positive shift in beam size of electrons along the train. ii) Positron beam size typically increases and then decreases along the train. iii) During the BPM tune measurements, we experienced e+ beam loss due to high vertical pinger amplitude. iv) A negative tune shift corresponds to a positive beam size shift along the train, suggesting that we are near a high order resonance in tune space.

BSM Beam Size Measurement: Fill 2 low high e-e+ i) Results are similar to those from the first fill.