1

Brief report of nonlinear
collimator study (The study isnot yet finished, so this is aprogress report.)
G. Broggi, Y. Funaoshi (Study leader), N. Iida, T. Ishibashi,
H. Koiso, Q. Liu, A. Morita, Y. Mulee, I. Nakamura, A. Natochii
K. Uno, K. Oide, Y. Ohnishi, H. Sugimoto, S. Tanaka,
S. Terui, B. Urbschat,
The 27th KEKB Accelerator Review Committee
24th Mar. 2024

Outline2• Introduction• Report of nonlinear collimator(NLC) study• Summary
Photo of nonlinear collimator section at OHO

D05V1

By T. Ishibashi
SNAP.1(Skew sextupole)

Introduction (Motivation of NLC installation)• As the beam background noise (BG) is reaching its upper limit, further beam current increase
requires a BG reduction.
• To increase the bunch current while avoiding LER vertical beam size blow-up, theimpedance needs to be reduced.
• Use NLC as a countermeasure to sudden beam loss ( not included in this presentation).

2022ab run

Without NLC, the design
bunch current cannot bereached while keeping theLER vertical emittance low.
By*1mm3

One of the causes of thisbeam size blow-up isimpedance.
More information on thisbeam size blow-up can befound in Ref (Study for -1
Mode Instability inSuperKEKB Low Energy
Ring by Ohmi-san).

Introduction (Motivation of NLC installation)
2021c physics run

4(βy*=1 mm, βx*=80 mm)• It is estimated that the D05V1,
with a half-aperture of about 45 mm and βy =4.05 m, can havecomparable backgroundperformance to the D06V1.
→ If D05V1 could be used insteadof D06V1, the vertical impedanceof the ring could be reduced.

After LS1(βy*=1 mm, βx*=80 mm)

D06V1

By T. Ishibashi

D06V1D05V1• Σβyky decreases from 3.97×104 to 3.18×104 V/pC. The TMCI threshold increases from ~1.8 mA to ~2.1 mA.
• If it turns out that the D05V1 will completely replace the D06V1 in the next commissioning, there is a possibility that we willremove the D06V1 from the LER in the future, in which case further impedance reduction could be expected.
4Introduction (NLC concept)

5• A pair of skew sextupoles are used for betatron collimation in the vertical plane.
• The phase advance between the paired skew sextupoles is set to π in both planes.
• The vertical kick by a skew sextupole is expressed as
Image of Skew sextupole kick

By A. NatochiiReport of nonlinear collimator(NLC) study

6The studies performed on 8 Mar. and 14 Mar. checked whether D05V1 (NLC) would bea substitute for D06V1, which had been very much relied upon until previous runs.
By*1mm, crab waist=0
Assumption: Bunch length=6 mm, V emittance = 20 pm

Under the conditions described below,
the lifetime was almost the same.

Beam lifetime

This table shows the collimator gap at the time of measurement.
TimeBeamcurrent

21:11:50

50

94

2.375

8141.164394

128

71

D06V1D05V1-64σ 21:26:40

50

94

2.375

8

3.5

1.164

394

64

71

50

94

88

3.5

1.1

218

394

64

71

14 Mar.

Name

D06V1-64σ

D05V1-64σ

21:41:00# of D06V1 D06V2 D05V1 D02V1 D06V1 D06V2 D05V1 D02V1bunchgap
gap
gap
gap
σσσσ

Report of nonlinear collimator(NLC) study
Storage beam background:*D05V1 is more capable of storage BG
reduction than D06V1.
*The better collimation performance ofD05V1 compared to D06V1 is expected.
NLC is much closer to the IP thanD06V1, so scattered particles between
D06V1 and D05V1 can now be stopped
by D05V1 before they hit the IR.
*The storage BG was also observed todecrease when the D05V1 gap wasclosed.

7Storage beambackgroundReport of nonlinear collimator(NLC) study
Injection beam background:*The injection BG reduction capability of D05V1is significantly less than that of D06V1. Whenboth D06V1 and D05V1 are used, the injection
BG reduction capability is higher than that ofD06V1 alone.
Vertical kick =Injection beam
background8(𝑦 − 𝑥 )We expect that D05V1 is less effective forinjection BG reduction because of this term.

This countermeasure is presented in thesummary.

Even with a narrower gap in D05V1, the injection beam
background is greater than in D06V1.

Report of nonlinear collimator(NLC) study

9LER Pilot bunch
vertical spectrum

No vertical beam size blowup was observed in thedesign bunch current when
D05V1 was used in its
narrowest setting.

1.25 mA

LER vertical beam
size blow-up

0.82 mA

Mea_D06V1-65sigma(46E15 V/C)=0.0097 /mA
Cal_D06V1-65sigma(46E15 V/C)=0.0092 /mA
Mea_D05V1-65sigma(28E15 V/C)= 0.0059 /mA
Cal_D05V1-65sigma(28E15 V/C)=0.0055 /mA
Vertical tune shift was nearly consistent inmeasurement and calculation.

LER vertical tune shiftSummary• The motivation for the installation of the NLC and theconcept of the NLC were briefly explained.
• D05V1 is more capable of storage BG reduction thanD06V1.
• The injection BG reduction capability of D05V1 issignificantly less than that of D06V1.( Funakoshi-sansuggested to optics group that the βx at the SNAP
location could be smaller.)• Experiments confirmed that D05V1 has a smaller
impedance than D06V1.
*Currently, the Bx at SNAP is 7m. We can see that to cut the injection BG at this time,
the gap of cut injection BG needs to be smaller than the gap of cut of the storage BG.
*Due to the collimator head width constraints, at the next opportunity we plan to do astudy with optics with a Bx = 2m at SNAP.

10Cut storage BG

Bx at SNAP =7m
for cut injection BG

Back upY. Funakoshi

2Jxofinjectingbeam• Dipole oscillation

N. Iida• 2Jx = 7.84 x 10^-7 m

Y. Ohnishi• Beam emittance of injecting beam• gex = 222±35 mm-> ex = 2.83 x 10^-8 m• Ring acceptance• 2Jx = 1.62 x 10^-6 m (with 3sx,inj)-> 2Jx = 2 x 10^-6 m (with some margin)12

Y. Funakoshi

Collimation for storage and injecting
2Jx (injection acceptance) = 2 x 10^-6 mbeams
No x-offset (storage beam)bx,PSNAC.1 = 2m

bx,PSNAC.1 = 7.08m
3.47mm

3.47mm

1.88mm

3.01mmwith x-offset (injection beam)

13

Report of nonlinear collimator(NLC) study
Injection efficiency :*The injection efficiency is almost thesame when D06V1 is 64σ and when
D05V1 is 64σ.
* A slight decrease in injection
efficiency was observed when D05V1was lower than 57σ.

Injection
efficiency (%)

14

[A. Natochii]Report of nonlinear collimator(NLC) study

94 bunchFC on
1 Hz
1 bunch injection

16