beam-beam sensitivity to parasitic crossings & xing angle

W. Kozanecki MCC AP meeting, 29 July 04

Goal:Goal: measure measure the luminosity the luminosity degradationdegradation associated with associated with parasitic crossings

horizontal crossing angle

PrinciplePrinciple

by-2 pattern: compare Lsp at minimum, nominal & maximum parasitic-xing separation ( = e- x-angle) with full L optimization at each setting sensitivity to Xing angle + parasitic crossings

by-4 pattern: compare Lsp at minimum, 0, & maximum (achievable) Xing angles ( = e- x-angle) with full L optimization at each setting sensitivity to Xing angle only

HEB only: measure impact (if any) of e- x-angle on e- beam properties

W. Kozanecki, Y. Cai, W. Colocho, J. Seeman, M. Sullivan, J. Turner (with special thanks to Nate Lipkovitz & Cliff Blanchette)

Beam-beam sensitivity Beam-beam sensitivity to parasitic crossings & Xing angleto parasitic crossings & Xing angle


Experimental aspects (I)Experimental aspects (I)

Horizontal separation @ parasitic crossingsHorizontal separation @ parasitic crossings XP(e-) more +ve X(PC) nominal: X(PC) = 3.22 mm @ z = +/- 63 cm

for XPmax(e-) = - 0.60 / + 0.85 mrad, X 3.6 mm (+ 12%) / 2.7 mm (-17%)

+ x


Experimental aspects (II)Experimental aspects (II)

Quality/reproducibility of measurementsQuality/reproducibility of measurements thermal / beam-beam effects keep currents constant (total / per bunch)

sparsified by-2: 836 bunches, 1201/751 mA, 1.44/0.90 mA/b

by-4: 851 bunches, 1221/758 mA, 1.43/0.89 mA/b

trickle both beams

re-optimize Lsp at each XP(e-) setting

tunes

local & global skews (both rings)

PR02 LER sext bumps (HER always, LER most of the time)

y-angle, collision phase (most of the time)


I. Measure LI. Measure Lspsp degradation associated with degradation associated with

parasitic Xings + Xing angleparasitic Xings + Xing angle

Sparsified by-2 pattern,Sparsified by-2 pattern, LER/HER = 1.4/0.9 mA/b LER/HER = 1.4/0.9 mA/b Setup

Set LER/HER YANG, SLM/interferometer light levels

In both LER & HER, optimize all local & global skews, PR02 SEXT bumps, SD2 bumps in LER Arcs 5 & 11, collision phase

Mini scan of XP(e -) (+- 0.3 mrad) to locate optimum e- angle (XPopt = 0 )


LLspsp degradation with parasitic Xings + Xing angle degradation with parasitic Xings + Xing angle (cont’d)

At XP = 0, + 850, - 600, - 300, + 300 rad Optimize LER+HER local & global skews, PR02 SEXT bumps

Optimize collision phase

Record tune spectra, gated camera data,

Lsp & Ib+,- patterns along the train

20% degradation at + 850 rad

Lsp vs. e- angle

3.0

3.2

3.4

3.6

3.8

4.0

4.2

-1000 -500 0 500 1000

XP [e-] (murad)

Lsp

(1E

30)

sp. by-2

After L sp optimization

at each XP setting

Investigated correlated variations in tunes & e+/e- spot sizes:

• no clear trend in LER/HER tune tracker readings (too few points compared to fluctuation size)

• no clear trend in LER SLM/interferometer sizes (fluctuations)

• definite trend in HER spot sizes


LLspsp degradation with parasitic Xings + Xing angle degradation with parasitic Xings + Xing angle (cont’d)

At XP = 0, +850, - 600, -300, + 300 rad Optimize LER+HER local & global skews, PR02 SEXT bumps

Optimize collision phase

Record tune spectra, gated camera data,

Lsp & Ib+,- patterns along the train


Lsp vs. e- angle

3.0

3.2

3.4

3.6

3.8

4.0

4.2

-1000 -500 0 500 1000

XP [e-] (murad)

Lsp

(1E

30)

sp. by-2


at each XP setting

HER y spot size

0.27

0.28

0.29

0.30

0.31

0.32

-1000 -500 0 500 1000

XP[e-] (murad)

Ver

tica

l sp

ot

size

(m

m)

sp. by-2After L sp

optimization at each XP setting

+ 12%

HER x spot size

0.56

0.57

0.58

0.59

0.60

0.61

-1000 -500 0 500 1000

XP[e-] (murad)

Ho

rizo

nta

l sp

ot

size

(m

m)

sp. by-2After L sp


+ 6%


II. Measure LII. Measure Lspsp degradation associated with Xing angle only degradation associated with Xing angle only

by-4 patternby-4 pattern, same LER/HER bunch currents, same LER/HER bunch currents Setup

Skew quads/sext bumps already restored to XP=0 settings found in step I

Optimize tunes, collision phase (in case RF-transient is pattern-dependent)

Mini scan of XP(e -) to check optimum e- angle (before further optimiation)

optimum XP very different (more +ve!)


LLspsp degradation with Xing angle only degradation with Xing angle only (cont’d)

Optimize Luminosity at XP = +550, +850, - 600, 0 rad note XP=0 is by definition the optimum e- angle found in the by-2 pattern

Even after optimization @ + 850 rad,

L is higher at somewhat smaller

XP(e-), and then drops again.

y- displays a corresponding trend.


Even after optimization @ - 600 rad,

Lsp is higher at larger XP(e-).

y- displays a corresponding trend.



Similar effect seen in previousXing-angle MD (by-4 pattern, 11 May 04)


Specific luminosity

3

3.2

3.4

3.6

3.8

4

4.2

-1 -0.5 0 0.5 1

XP - XP opt (mrad)

L_

sp

(1

0^

30

mA

^-2

b^

-2)

1.35 / 0.85 mA/b, optzd @ XP=700

1.35 / 0.85 mA/b

RFTunes, skews,

sext. bumps



Optimize specific luminosity at XP = +550, +850, - 600, 0 rad note XP = “0” is by definition the optimum e- angle found in the by-2 pattern

Lsp > 4.1 @ XP = “0”


Lsp vs. e- angle

3.0

3.2

3.4

3.6

3.8

4.0

4.2

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

(1E

30)

by-4


at each XP setting

HER x spot size

0.55

0.56

0.57

0.58

0.59

0.60

0.61

0.62

-1000 -500 0 500 1000

XP[e-] (murad)

Ho

rizo

nta

l sp

ot

size

(m

m)

by-4

After L sp


+ 6%

HER y spot size

0.27

0.28

0.29

0.30

0.31

0.32

-1000 -500 0 500 1000

XP[e-] (murad)

Ver

tica

l sp

ot

size

(m

m)

by-4

After L sp



Without parasiticWithout parasitic Xings Xings (by-4) (by-4) LLspsp exhibits a exhibits a parabolicparabolic

dependence on dependence on XP(e-)XP(e-)

With parasiticWith parasitic Xings Xings (by-2)(by-2)

the peak Lsp is ~ 5% lower (@ nominal PC separation) than in the by-4 pattern

the larger XP(e-), the steeper the Lsp degradation

The optimum eThe optimum e-- x angle is ~ x angle is ~ 0.2 mrad 0.2 mrad more -ve in the by-2more -ve in the by-2 pattern (pattern ( weaker PCweaker PC effects) effects)

This suggests that in the This suggests that in the presence of parasitic Xings, presence of parasitic Xings, the the optimum eoptimum e-- angle angle is a is a compromisecompromise between between Xing-angleXing-angle & & PC-inducedPC-induced luminosity degradationluminosity degradation

LLspsp dependence on Xing angle & PC separation: dependence on Xing angle & PC separation: experimental summaryexperimental summary

Lsp vs. e- angle

3.0

3.2

3.4

3.6

3.8

4.0

4.2

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

(1E

30)

sp. by-2

by-4


at each XP setting

Normalized Lsp vs. e- angle

0.75

0.80

0.85

0.90

0.95

1.00

1.05

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

/ L

sp(X

P=

0)

by-4, msrd

by-4, fit

sp. by-2, msrd

sp. by-2, fit


LLspsp dependence on Xing angle & PC separation: dependence on Xing angle & PC separation: datadata vs. vs. simulationssimulations

Lsp vs. e- angle

3.0

3.2

3.4

3.6

3.8

4.0

4.2

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

(1E

30)

sp. by-2

by-4


at each XP setting


0.75

0.80

0.85

0.90

0.95

1.00

1.05

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

/ L

sp(X

P=

0)

by-4, msrd

by-4, fit

sp. by-2, msrd

sp. by-2, fit

Normalized Lsp vs. e- angle (by-4, no PC)

0.75

0.80

0.85

0.90

0.95

1.00

1.05

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

Half crossing-angle (mrad)

Lsp

/ L

sp(X

P=

0)

SIm. (no PC)

by-4, msrd

by-4, fit

Lsp degradation vs. PC separation

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.5 2.0 2.5 3.0 3.5 4.0

Horizontal sepration at parastic crossing (mm)

Lu

min

osi

ty d

egra

dat

ion

fac

tor

msrd (1.43 / 0.89 mA/b)

sim. (1.56 / 0.96 mA/b)

Nominal PC separation

Simulation neglects Xing-angle effects


Related topics...Related topics...

ParasiticParasitic crossings crossings how do the Pacman bunches fare?

what is happening in the long minitrain?

Crossing angle (w/o PC)Crossing angle (w/o PC) why do the HER optics vary (or appear to vary) with electron x-angle,

even though there are non-linear elements inside the XP bump?


Parasitic crossings: how do the Pacman bunches fare?

Sparsified by-2 pattern


Parasitic crossings: the drooooping minitrain


e- x-angle response of Lsp & HER beam sizes in collision

Collisions, by-4

No optimization during scan

Collisions, by-4 Collisions, by-4


HEB x size (e- only,

by-2)

HEB y size (e- only,

by-2)

e- x-angle response of HER beam sizes

Collisions, by-4 Collisions, by-4


HEB y tune (e- only,

by-2)

HEB x tune (e- only,

by-2)

e- x-angle response of HER tunes

Collisions, by-4

Collisions, by-4


Summary (in words...)Summary (in words...)

In the by-4 pattern (where parasitic-crossing ing effects are In the by-4 pattern (where parasitic-crossing ing effects are expected to be negligible)expected to be negligible)

The specific luminosity exhibits a roughly parabolic dependence on the horizontal e- angle (after reoptimization @ each angle). It degrades by ~ 6-7 % for an e- x-angle of ~ 650 rad above the optimum.

At the same angle, the simulation predicts a 3% degradation only. More generally, the crossing-angle dependence of the luminosity is significantly steeper in the data than in the simulation.

Systematic variations of the e- horizontal beam size and vertical tune, observed in e- x-angle scans recorded in collision, are also apparent, and of comparable magnitude, when varying the horizontal e- angle in single-beam mode. The large variations in vertical HEB spot size, observed in collision only, are strongly correlated with Lsp variations and clearly of beam-beam origin.

Whether the horizontal spot size variation could be associated to image motion on the SLM screen remaisn to be verified. But it is unlikely, because the x-angle bump is reasonably well closed.

Even though the e- horizontal-angle bump spans only linear optical elements (apart from the solenoid), the observed tune variation suggests the presence of significant non-linear fields in that region of the HER.


Summary (more words...)Summary (more words...)

In the presence of parasitic crossings (sparsified by-2 pattern)In the presence of parasitic crossings (sparsified by-2 pattern) The peak specific luminosity is ~ 5% lower (@ nominal PC separation)

than in the by-4 pattern, where parasitic crossings should be negligible; the more positive the e- x-angle, the steeper the additional luminosity degradation.

The optimum e- x-angle is ~ 200 rad more negative (i.e. weaker PC effects) in the by-2 pattern, than in the by-4 pattern. This suggests that in the presence of parasitic crossings, the optimum e- angle is a compromise between Xing-angle & PC-induced luminosity degradation.

The dependence of the PC-associated luminosity degradation on e- angle (i.e. on horizontal PC separation) is consistent with, and slightly weaker than, that predicted by beam-beam simulations.

“Pacman” bunches exhibit a luminosity degradation that varies from 20-25% (wrt to other minitrain bunches) near the optimum e- angle, to 10-15% at large positive angle (850 rad). This effect is not understood and requires further study.


Summary (in pictures)Summary (in pictures)

Lsp vs. e- angle

3.0

3.2

3.4

3.6

3.8

4.0

4.2

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

(1E

30)

sp. by-2

by-4


at each XP setting


0.75

0.80

0.85

0.90

0.95

1.00

1.05

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

/ L

sp(X

P=

0)

by-4, msrd

by-4, fit

sp. by-2, msrd

sp. by-2, fit

Normalized Lsp vs. e- angle (by-4, no PC)

0.75

0.80

0.85

0.90

0.95

1.00

1.05

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

Half crossing-angle (mrad)

Lsp

/ L

sp(X

P=

0)

SIm. (no PC)

by-4, msrd

by-4, fit

Lsp degradation vs. PC separation

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.5 2.0 2.5 3.0 3.5 4.0

Horizontal sepration at parastic crossing (mm)

Lu

min

osi

ty d

egra

dat

ion

fac

tor

msrd (1.43 / 0.89 mA/b)

sim. (1.56 / 0.96 mA/b)

Nominal PC separation


Appendix: documentation & data setsAppendix: documentation & data sets

PEP-II e-logPEP-II e-log collision data: dedicated MD, 1 Jul 04, day + swing + early owl shifts

HEB-only data: opportunistic MD, 14 Jul 04, swing shift

Data setsData sets collision data: PHYSICS4_DATA:[pep2.char.1Jul04]

L, currents, beam sizes, tunes, quads & bumps: lumtun_*_1Jul.dat

bunch-by-bunch data: XP*_BICDATA.MAT

gated camera: gacam_*_1Jul,dat

single-beam data: PHYSICS4_DATA:[pep2.ip.witold.smr04B]lumt_herxpcall_2_14Jul

orbit fit set to PR02 BPMS 7052-8012 (HIPP) throughout

beam-beam sensitivity to parasitic crossings & xing angle

Documents