1. BB Studies at the Tevatron
Team: A. Valishev, Yu. Alexahin, G. Annala, B. Hanna, V. Lebedev, R.S. Moore, V. Shiltsev, D. Still, FNAL, R. Miyamoto, F. Schmidt, CERN, W. Fischer, X. Gu, S. White, BNL, J. Qiang, LBNL
Plan Evolution
Topics
Organization
Preliminary Results
General Comments
Sideshow
21. Sep 2011
BB at Tevatron

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BB at Tevatron

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BB at Tevatron

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BB at Tevatron

5. Beam Conditions
All experiments were performed at 980 GeV in LowBeta optics
Machine was loaded with 3 proton and 3 antiproton bunches
P1, P13, P25 and A1, A13, A25
Beams were on the helix – head-on collisions at B0 and D0, separated at A0, C0, E0 and F0 by 8-10s
Typical beam parameters
Parameter
Value
Protons / bunch
2.8×1011
Antiprotons / bunch
0.8×1011 (0.4x1011 *)
Transverse Emittance (P, A)
22, 8 mm (22, 14 *)
Bunch Length (P, A)
0.51, 0.45 m
Momentum Spread (P,A)
1.4, 1.2 ×10-4
Hourglass Factor
0.61
Beam-Beam Tune Shift (P, A)
0.013, 0.02
* Ecool off
21. Sep 2011
BB at Tevatron
10/6/2011
A.Valishev, APC Seminar

6. LHC Offset Beam-Beam Collision Studies (Ji Qiang)
The offset collision is unavoidable due to the different bunch collision schemes at LHC
Such offset collision might cause emittance growth that degrades luminosity lifetime and experimental conditions
Emittance Growth vs. Offset
We have made a systematic scan of the separation in the possible range (~2 s h, ~1.1 s v) which is consistent with the simulations
There is no emit growth which would be consistent with the worst case being at ~1.1 – 1.7 s
In the simulations (no losses!) “real” emittance blow-up not checked.
21. Sep 2011
BB at Tevatron

7. 21. Sep 2011
BB at Tevatron

8. Separation Scan #2. Horizontal
BB at Tevatron
10/6/2011
A.Valishev, APC Seminar

9. Separation Scan #2. Horizontal
BB at Tevatron
10/6/2011
A.Valishev, APC Seminar

10. Separation Scan #2. Horizontal
Specific luminosity after scan
0.92x before scan
consistent with natural decay
21. Sep 2011
BB at Tevatron
10/6/2011
A.Valishev, APC Seminar

11. Coherent BB studies
Due to lack of high intensity & low emittance for the Pbars till about Wednesday 24th we couldn’t quite do that part (also the “driving force” Simon left on Sunday)
I am sure that several of those studies would be highly desirable and quite possible to be done at the Tevatron!
However, “the other half”, i.e. the chromaticity threshold study in conjunction of BB was very conclusive:
Whenever BB is present any chromaticity value can be dialed in without causing an instability! Some minor Schottky activity for Q’ [0, ~-1]
This remains true independent of the chosen working point.
In case BB is weak but the emittance is large there is also no effect.
For a nominal 3x0 the instability was very fast slightly above 0, causing a quench – sorry!
In the first attempt of 3x0 keeping the tunes very closed was beneficial
21. Sep 2011
BB at Tevatron

12. AC-dipole “Proof of Principle”
The goal has been to excite the “weak” beam through the strong beam using the AC-dipole
However, “without” excitation the strong beam
We need to record the TBT BPM data around the ring
Unfortunately, we had to reverse the weak-strong set-up since the pbar TBT BPM system was not quite ready for these studies  use lowest possible proton intensity against nominal low emittance pbars
Changes to the linear lattice function due to BB hopefully to be derived from a reference measurement with protons only
We have a couple of valid shots.There is good hope that the linear analysis is possible
Finding non-linear terms difficult since noise level high
We have now a better understanding of the AC-dipole which seems to have un-adiabatic behavior at times (very destructive!!!)  one must wait a minute after turning it on
The measurement is tricky and once one eats into the beam distribution it is over.
Tunes of the weak protons difficult to measure but all procedures are well in check by now!
21. Sep 2011
BB at Tevatron

13. ~0.85 s
21. Sep 2011
BB at Tevatron

14. AC-Dipole excitation too close to beam distribution
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BB at Tevatron

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16. Topic 4. Phase Averaging
The goal was to collide bunches at different bunch length/beta* ratios
This was achieved by cogging (moving antiproton bunches longitudinally wrt protons, thus colliding off beta minimum)
Produced excellent data, in qualitative agreement with expectations! Good for benchmarking future simulations.
proton (left) and pbar (right) decay rate vs b*/s
21. Sep 2011
BB at Tevatron
10/6/2011
A.Valishev, APC Seminar

17. General Remarks
Excellent spirit for these studies in the control room. ( I could not detect any gloominess!!!)
Machine Coordinators went out of their way to help us!
The Tevatron team did a great job and in particular Sasha Valishev who spent almost 2 full weeks to do these studies with us!
Availability of Tevatron was not optimal
TBT BPM system and AC-dipole work albeit we have encountered many limits
Despite the problems it seems that all studies can (sadly could have been!) be done at the Tevatron, although we simply lacked time to do them all
21. Sep 2011
BB at Tevatron

18. Side-Show Lots of expertise concerning space charge:
Valery Kapin, collaborator of Giuliano, suggests to try out simple models for space charge with PTCORBIT the ultimate check in the end. He also has included space charge directly in MAD-X and has an improved fringe field formalism in his version.
Leonid G. Vorobiev is developing his own version of ORBIT. Dislikes the “supercode” input, but fears the promised Python interface might not come. Reflects on going to 
Synergia, developed at Fermilab, 3D PIC code
Dmitry Shatilov is upgrading Lifetrac:
Tunefoot print work now so that the code comparison, started at BNL, should be finalized shortly
Amazingly enough Sasha and Dmitry have decided to integrate SixTrack into Lifetrac for a better treatment of single particle part. Good progress has been made but more effort is needed
Sasha has added a new element: a nonlinear lens that creates a nonlinear machine that is integrable at the same time it is also in PTC.
Eliana Gianfelice has discovered a bug in the arbitrary MATRIX element in MAD-X. I found and fixed it as one of my last contributions to this code.
21. Sep 2011
BB at Tevatron

19. Reserve
21. Sep 2011
BB at Tevatron

20. Tevatron Machine Parameters
Value
Beam Energy
0.98 TeV
Circumference
6.28 km
Bending Radius
754 m
RF Harmonic
1113
RF Voltage
1.1 MV
Synchrotron Tune
0.0074
Momentum Compaction
0.0029 b*
0.3 m
Betatron Tunes, Protons (Qx,Qy)
20.584,
Betatron Tunes, Antiptorons
20.572,
Betatron Coupling (P, A)
<0.003, 0.01
21. Sep 2011
BB at Tevatron

21. Tevatron Beam Parameters
Value
Number of Bunches 36
Protons / bunch
3.0×1011
Antiprotons / bunch
0.9×1011
Transverse Emittance (P, A)
2.8, 1.3 mm
Bunch Length (P, A)
0.51, 0.45 m
Momentum Spread (P,A)
1.4, 1.2 ×10-4
Hourglass Factor
0.61
Number of IPs (Long-Range)
2 (70)
Beam-Beam Tune Shift (P, A)
0.018, 0.026
Initial Luminosity
4.3 ×1032 cm-2 s-1
Initial Luminosity Life Time
5 h
Store Duration
12-16 h
21. Sep 2011
BB at Tevatron

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BB at Tevatron

23. Cogging history and B4 separator issue
BB at Tevatron

24. Lifetime as a function of beta*/bunch-length
21. Sep 2011
BB at Tevatron

25. Possible Explanation of A-Asymmetry
Strong local coupling in D0 IR leads to asymmetry in x and y beta minima.
This effect is different on proton and antiproton helix due to sextupoles
Coupled with a small crossing angle this can lead to asymmetry in generated losses
4D b-functions in B0 (left) and D0 (right) IR
21. Sep 2011
BB at Tevatron
10/6/2011
A.Valishev, APC Seminar

26. Summary
Despite numerous technical issues and machine failures, the studies were successful in many aspects:
Measured the effect of transverse beam-beam separation on intensity and emittance
No rapid emittance growth was observed
Maximum of losses / beam decay at ~1.5s, consistent with expectations
Data provide input for benchmarking weak-strong simulations
Demonstrated Landau damping of coherent instability by beam-beam interaction
The effect does not depend on the tune working point
Data is limited to one beam intensity/emittance configuration
Did not measure the unstable mode growth rate
Proof-of-principle experiment with AC dipole acting on colliding bunches
Pending data analysis
Measured the effect of “phase averaging” – lifetime vs. bunch length / beta*
Essential for confirmation of theory and benchmarking simulations
21. Sep 2011
BB at Tevatron
10/6/2011
A.Valishev, APC Seminar