Collision feedback

OKI, Toshiyuki
on behalf of the working group2014/MAR/3rd
19th KEKB Accelerator Review Committee

Introduction
SuperKEKBKEKBey~8.6pm (LER)

150pm

by*

0.27mm(LER)

5.9mm

sy*

48nm

940nm

The beam size is 20 times smaller than one of KEKB.
How to collide beams successfully is much more difficult than KEKB.
High luminosity machine can be spoiled by the small vibration of QCS(QC1, QC2) caused by ground motion, even if it didn’t affect KEKB.

Estimation of QCS vibration and luminosity degradation(vertical)

100
10Measured
ground motion(input data)
125 nm@25 Hz

Phase (deg.)

Integral amplitude (nm)1000

H. Yamaoka

0.1

QC1RP and QC1RE vibrate with same phase,but the amplitude difference still arises: 25 nm at 25 Hz…
Luminosity degradation (based on beam-beam simulation by K. Ohmi)
Frequency (Hz)24.85 38.93 69.34 99.60
DyIP*rms (nm)L/L0average (%)
in case of model-A (and F)

18.63

1.72

8.29

3.14

95.4
99.8
99.7
99.7
(96.1) (99.8) (99.8) (99.8)

Y. FunakoshiBeam-Beam simulation results
K. OhmiJust 20 nm offset leads to 6 % degradation of luminosity.

Reinforcement of bridge structure

H. Yamaoka
2013 Dec.

Feedback scheme in vertical direction (y and y’)
The orbital feedback system was used successfully for KEKB,
which is based on beam-beam kick calculated from the BPM
readouts at both sides of the IP.
Offset at IP
Dy= ~5nm (0.1 sy*)
beams are deflectedby beam-beam kickDifference at BPM
Dy = ~1.3mm
4 BPMs(0.5m from IP)

For SuperKEKB, It will be expected to work well.
The target resolution of BPM is the order of micrometer.
Prototype BPM was tested by using sinusoidal input signal.
0.1mm resolution has been obtained, successfully. H. Fukuma

Rejection gain evaluation of the feedback systemdelay
3 clocks

reference+

PID−GaD

PID tuning
Ziegler-Nichols

ps & magnet
1st order lag(delay = 0.1ms)

Rejection gain (dB)monitor LPF(FIR)
100− 10− 20− 30− 40− 50− 60

plant
1st order lag(delay = 0.21ms)+GeM+

H. Fukuma

monitor outdisturbanceFs = 32kHzGroup delay of LPF =0.4ms

Result shows−16.7dB@25Hz is expected.
Amplitude of DyIP*rms @25Hz will be
reduced from 19 nm to 2.8 nm.
020 40 60 80 100 120 140 160

Frequency (Hz)Feedback scheme in horizontal direction (x)Beam-beam kick will not be a sensitive parameter for monitoring
collision, as beam-beam parameter ξx is so small:ξx ~ 0.0028(e+), 0.0012(e-)(↔ ξy ~ 0.0881(e+), 0.0807(e-))
Beams are not so much deflected in horizontal direction.
⇒The Luminosity dither is being considered, which was used for PEP-II
successfully.
When two beams overlap, the luminosity drops oneither side of the peak, giving modulation at 2ωdith.
Off center, there is additional modulation at thefundamental.

Dx

Dithering simulation by U. Wienands and S.Uehara:
Max. dithering amplitude, which corresponds to
10 % drop of the peak luminosity should beprepared.

Luminosity dither system for horizontal feedback(+ vertical offset and angle)(1) Fast luminosity monitor (ZDLM): S. Uehara and P. Bambade.
(2) Electronics to discriminate dither signals:
Lock-in amplifiers and/or digital technic are considered.
(3) Beam pipes: made of stainless steel for magnetic field penetration.
(4) Air-core dithering coils:- Dithering coils will be installed into 8 locations.
- Dither frequency: 60~100 Hz is chosen to avoid power line interference.
- Kick angle of dither coils: to generate ±50 mm horiz. bump.
- Design work, production and field measurement are under way at SLAC.
(5) Power supplies for actuating coils: commercial bipoler PS (± 65 V, ± 5 A).
(6) Iron-core steering magnets at HER for orbit correction.

Lock-in amplifier: Stanford research systems model SR830
SLAC loaned us the Lock-in amp. last summer.
We have tried to use it with EPICS for a remote control.

Locations for the dithering coils
8 locations for the dithering coils in LER around IP.

ZDS1LPZDS2LPZDS4LPZDS3LPZDS1RPZDS2RPZDS3RPZDS4RP

Designs for 3 type of beam pipes
ZDS1{L, R}P
Inner diameter: 80 mm
Thickness: 2 mm

Y. Suetsugu

ZDS2{L, R}P
Inner diameter: 90 mm
Thickness: 3 mm
10 mm Cu coating inside.

ZDS{3, 4}{L, R}P
Inner diameter: 90 mmThickness: 5 mm10 mm Cu coating inside.

Pipes will be delivered to KEK by this March.
All pipes are made of SS316L.

Damping of magnetic field due to eddy current
Damping ratio
Bin12
Bout1+  

where:  

m0 bt,
2 SS316L
Copper(KEKBLER pipe)

b: outer radius (m),t : thickness (m),: resistivity (  m)
A.W. Chao and M. Tigner, Editors,
Handbook ofAccelerator Physics and Engineering,
World Scientific, Singapore (1999), p. 268

Ditheringfreq. range
1k 10k 100k

Resistive and thinner pipe suppresses the damping effect.

Coil mechanical design

M. Kosovsky, SLAC

Coil field evaluation
Calculations show that the field quality meets the requirements:
10 mm (15 sx + 2 mm) radius of good field region for dB/B < 10−3‒ verification of model using OPERA-3D was performed.
‒ Note: Effect of chamber eddy currents not included.

Example of horizontal coil
10−3

10−3

U. Wienands, SLAC

Coil parameter

Resistance, inductance and power are within acceptable limits(up to ~ 10 V, 13 W/coil (real peak power) at 5 A, 100 Hz)
U. Wienands, SLAC

Fabrication and field measurement

Fabrication and measurement of coils will be finished by this March.
U. Wienands, S.D. Anderson, MFD Metrology, SLAC

Summary— It is challenging to collide small beams successfully.
— For vertical offset and angle feedback:Orbital feedback system is under consideration. It was used for KEKB andis based on beam-beam kick calculated from the BPM readouts at bothsides of the IP.
— For horizontal offset feedback:Luminosity dither is being considered as used for PEP-II, because beambeam kick will not be a sensitive parameter for monitoring collision due tosmall beam-beam parameter.

Appendix

Model-A

L側

R側

Oscillation phases of
QC1RP and QC1RE.

25nm@20Hz
14nm@50Hz

H. Yamaoka

山岡Experience at KEKB
(1) Finding good collision
1. Calculate beam-beam kick (b-b deflection)using BPMs.
2. Change bb kick by changing setting ofsteering magnets located on either side of the IP.
3. Luminosity scan: Lumi. vs bb kick.
M. Masuzawa

Luminosity scan

Bump height created bysteering magnets @IP tomaintain bb-kickVertical bump height @IPBeam-beam kick(2) Maintaining good collisionDamping and phase delay of a magnetic field
Bout  B0 sin t Bin 

11+  

2B0 sin t  arctan   Damping ratio

Phase delaywhere,m0 bt

2 : ramping time for step response,b: outer radius (m),1/√2p /4
 =1/cutofft : thickness (m),

 : resistivity (  m)
A.W. Chao and M. Tigner, Editors,
Handbook ofAccelerator Physics and Engineering,
World Scientific, Singapore (1999), p. 268

Test setup

KEKB LER beam pipe made of copper:53 mm radius, 6 mm thickness.

— I (mA)— B (mT)— B/I (T/A)— Phase difference (B−I) (deg)Test results

Without beam pipe

With beam pipe

1000

1000

100

100

1010

1

1

0.1

0.1

0.01

0.01

0.001

0.001

0.00010.0001
1000
0.1

0.1

1

10100

frequency (Hz)

1

10100
frequency (Hz)1000

Test result summary

1k 10k 100k

Material: Copper
Inner diameter: 94 mm
Thickness: 6 mm

1k 10k 100k

Good agreement with calculation.
Strongly damped at the target frequency range: Dithering (60~100 Hz)

Conceptual drawing of beam pipes
Material: SS316L
Inner diameter: 90 mm
Thickness: 3 mm for round type, 6 mm for antechamber
10 mm Cu coating inside.

Suetsugu
Material: SS316L
Inner diameter: 80 mm
Thickness: 2 mm
Kanazawa

Calculations for the round SS beam pipes

 =0.26, 0.12, 0.07 ms

1k 10k 100k

1k 10k 100k

Resistive and thinner pipe suppresses the damping effect.
The Cu coating is effective above 1 kHz.