CESR

Beam-Beam Effects at CESR

Mark A. Palmer

Cornell University

July 14, 2001

July 14, 2001 M. A. Palmer Snowmass 2001 2

CESR CESR Pretzel Operation• Single beampipe• 9 trains of 4 or 5 bunches

– Bunches spaced by 14 ns– Spacing between 1st bunches in

trains: 280 ns, 280 ns, 294 ns,….– 1281 RF buckets not divisible by 9

• Collisions at a single IP (CLEO)• Electrostatic Pretzel

– Provides horizontal separation in the arcs

• Vertical electrostatic separators– Separates the bunches at the 2nd

(unused) IP

• Crossing Angle at IP: ~2.5 mrad

July 14, 2001 M. A. Palmer Snowmass 2001 3

CESR Parasitic Crossings

• Horizontal separation of >except at 2nd IP

• Beam-beam tune shift is smallest for : h(s) - 0 ~ /2

• Primary limit on train length!

VerticalSeparation

Ibh

x2=

Ib

x02sin2(h(s)-0)

Qh~

Beam-beam tune shift:

Closed orbit:

~

x(s) ~ x0 h(s) sin(h(s)-0)

July 14, 2001 M. A. Palmer Snowmass 2001 4

CESR Parasitic Bunch-by-Bunch Effects

– Strong-strong simulation– Tunes:

• O(1kHz) Spread in horizontal and vertical tunes by bunch

• Size of this spread has been verified by direct measurement

– Vertical displacement of bunches at parasitic crossings

• Distorts vertical closed orbit• Occurs at both IPs• Bunch at the start (end) of train

experiences kick as it leaves (approaches) the IP

– Additional effects:• Variations in chromaticity• Variations in angles and beta

functions at IP

• Simulation of long range beam-beam interaction

July 14, 2001 M. A. Palmer Snowmass 2001 5

CESR Beam-Beam Tuneshift

• Beam-beam tune shift

– Observe v ~ 0.07 in 4-bunch running

– Decreases for 5-bunch operation although improved net luminosity performance

4-bunch 5-bunch

July 14, 2001 M. A. Palmer Snowmass 2001 6

CESR Multi-bunch Performance Issues

• Bunch Current Limits– Observations

• 11mA/bunch possible in 9x1 running• See decreasing bunch current limit as increase the number of bunches/train

Parasitic crossings limit the maximum bunch current NOT the main beam-beam interaction– Installation of superconducting IR (underway now) will significantly improve

the first parasitic crossing adjacent to the CLEO IP

• Tune Spread– Simulation indicates a bunch-by-bunch spread of O(1kHz) in both vertical and

horizontal– Observed tune spread is consistent in size with the simulation– Width of working point in the tune plane:

• ~100 Hz Horizontal• ~1 kHz Vertical

– Currently investigating the use of an RFQ to correct the bunch-dependent tune

July 14, 2001 M. A. Palmer Snowmass 2001 7

CESR Bunch-to-Bunch Luminosity• Monitor barrel calorimeter

bhabha rate in CLEO detector– Tracking information provides

bunch identification

– Specific Luminosity:

– Information integrated over run (~ 1hr) for statistics– Car location of bunch in train

• Observe significant variations in all quantities– ~25% degradation in luminosity

for worst bunch relative to best

L(bunch) dtI(bunch) dt

July 14, 2001 M. A. Palmer Snowmass 2001 8

CESR Bunch-to-Bunch Differential Orbits

• BBI Luminosity Monitor– Shake a particular bunch (or bunches) at a fixed frequency– Measure the BBI induced amplitude in

the opposing bunch– Provides much faster response than

CLEO luminosity measurement

• Adjust differential offset between e- and e+ bunches at IP (VCROSING 7 Knob)– Vary betatron phase advance in the

vertical separator bump at the 2nd IP– Optimize collisions for each car

• Observations– Car-to-car orbit differences at the 0.5

level (vm)– Strong dependence on beam current– Consistent with machine operators

having to actively tune VCROSING 7 through the course of a run

Increasing time decreasing current

July 14, 2001 M. A. Palmer Snowmass 2001 9

CESR Bunch Luminosity Optimization

• Verification of BBI Luminosity Monitor performance relative to the CLEO Luminosity Monitor– Optimize BBI signal for a particular car at the beginning of run

– Integrate luminosity for approximately 1/2 hour and analyzed CLEO bunch luminosity

Results appear consistent given strong current dependence of

differential orbits

Results appear consistent given strong current dependence of

differential orbits

July 14, 2001 M. A. Palmer Snowmass 2001 10

CESR Bunch-to-Bunch Orbit Correction

• DC pedestal of the vertical feedback system– Measures orbit of all bunches simultaneously

– Feedback monitor point located 1.16 wavelengths from IP

– Assuming no kicks between the monitor and the IP, obtain position at the IP by scaling measured positions with ipfm

– Complications:• Current dependence

• Bunch-to-bunch X-talk

• Feedback Kicker– Modifications to allow

bunch-by-bunch

deflections

– Present system capable

of ~0.5 m corrections

July 14, 2001 M. A. Palmer Snowmass 2001 11

CESR Bunch-to-Bunch Orbit Correction (2)

• Preliminary test of feed forward kicking during normal operations– Measured relative differential

displacements using BBI monitor technique

– Applied fixed feed forward kick using vertical feedback system

– Monitor CLEO bunch-by-bunch luminosity for one weekend of running

July 14, 2001 M. A. Palmer Snowmass 2001 12

CESR Bunch-to-Bunch Summary

• Have observed bunch-by-bunch specific luminosity variations at the 15-25% level– Direct measurements of differential e+-e- displacement suggests ~0.5v offsets – The luminosity degradation cannot be explained by simple displacement alone

(would require 0.8-1.1v offsets):

– This suggests that the poor specific luminosity of the worst bunch is probably due to a combination of effects such as blowup of the beam envelope in addition to a simple differential displacement of the electron and positron trajectories

• Work continues– Improvement of measurement techniques and simulation

– Further modifications to feedback system to increase the available kicking strength for corrections at the 1v level

L = L0 exp[-(y)2/4y2]

July 14, 2001 M. A. Palmer Snowmass 2001 13

CESR Summary• Long Range Beam-Beam Interaction at Parasitic Crossings

– Induces spread in horizontal and vertical tunes of O(1 kHz)– Distorts vertical closed orbit of individual bunches

• Ongoing Efforts– Improved monitoring of bunch-by-bunch effects– Modifications to vertical feedback system to allow bunch-by-bunch correction

of differential (e+-e-) vertical orbit displacements– Radiofrequency quadrupole for bunch-by-bunch tune correction– Installation of a superconducting IR which will provide better bunch separation

at the parasitic crossing point nearest the CLEO IR

• Beam-Beam Tuneshift– Have observed a beam-beam tuneshift of nearly 0.07 while running with 9

trains of 4 bunches– Poorer performance in 9x5 running is consistent with the poor performance of

the worst bunches