BPM displacement monitor
19th Mar. 2007, 2007 KEKB review committee
M. Arinaga, H. Fukuma, S. Hiramatsu, H. Ishii, M. Tejima and M. Tobiyama

KEKB beam monitor group
1. Introduction
2. Gap detector and fixing arm
3. Installation plan
4. Summary

1. Introduction
1) Movement of beam position monitors•The movement of beam position monitors(BPMs) due to the thermal
stress of vacuum chambers has been observed both in LER and HER
even though they are fixed to quadrupoles.

LERBPM

HER

BPMsupportsupporta) Movement of arc BPMs relative to nearby sextupoles at a beam abort
in HERmm

HER HorizontalmAmm

HER VerticalmAupout

beam current
indown1 hourtime

x 10 sec

beam current
time•BPMs move 0.9mm (max.) horizontally and 0.1mm (max.) vertically.
•It takes 30 to 40 min. to reach steady state after the following injection.
The BPMs move 0.3mm (max.) horizontally and 0.06mm (max.) verticallyduring transient.
•Movement of the BPM depends on beam current.

x 10 secb) Startup after 8 hours-maintenance-timemm

HER HorizontalmAmm

HER VerticalmAbeam current
beam current
time

x 10 sectime

x 10 sec

The BPMs move about 0.1mm (max.) relative to sextupoles horizontally and vertically.

c) Tune change by the orbit deviation at 4 sextupoles for local
chromaticity correction in LER
Horizontal movement

mmmA

beam current

time"& y =x 10 sec

1$ y ,i # K 2,i # "xi!
4% i

βy=460 m, K2=2.4 m-2

Δνy=0.0035 for Δx = 10µm, if 4 sextupoles shiftsame direction.

2) Effect of BPM movement on the optics•As the beam orbit is continuously corrected based on the beam position
measured by the BPMs, the displacement of the BPMs changes the beam
position relative to magnets.
•The movement of BPMs relative to sextupoles deforms a linear optics, thencould cause a difficulty of the machine tuning especially after recovery fromthe beam abort or startup after machine maintenance.
•The effect of the BPM movement on the optics was estimated by SAD.
◊Positions of sextupoles were shifted randomly by 0.1mm rms
horizontally or vertically.
◊Random distribution may not be a good assumption because we expect acorrelation of movement which is not much known at present.
◊However, the simulation will give an estimate of the effect of themovement.

Two numbers show the results fromdifferent random numbers.

•Simulation result

HER

Hor. disp. : 0.1mm (rms)Ver. disp.: 0.1mm (rms)"! y-0.0001/-0.003 *0.0029/0.010

0.0/-0.0003
0.0/0.0"! x* / ! x* (%)

34/-15-0.1/-0.7

2.6/2.9-0.1/0.1

0.85/-1.4
0.0/0.0-0.019/-0.13
0.024/-0.008"R1* (10 !3 )-0.02/-0.03-3.77/-0.93 (2.7/0.67 u)"R *2 (10 !3 m)

0.011/0.014

1.46/2.10 (1.9/2.7 u)"R3* (10 !3 m !1 )1.45/2.75

6.87/-712 (0.06/6.7 u)"R4* (10 !3 )-2.49/-5.33< !" x / " x > ring (%)

20/13.8

0.12/2.0< !" y / " y > ring (%)

1.8/8.2

0.05/0.2! y / ! x (%)

0.0/0.0

0.2/0.5"! x**"! y / ! y (%)"! *x (mm)"! *y (mm)-20.3/-537 (0.3/8.4 u)
U is an unit used inmachine tuning.
A value of several units
is changed to maintain
good luminosity.

LER

Hor. disp. : 0.1mm (rms)Ver. disp.: 0.1mm (rms)

"! x
"! y

-0.0017/0.0006
0.0011/-0.002-0.0001/0.0
0.0/0.0"! x* / ! x* (%)-11/16.5

0.2/1.0"! *y / ! *y (%)-3.9/4.8

0.0/0.0"! *x (mm)"! *y (mm)0.27/0.03
0.0/0.0-0.015/0.014-0.052/0.026"R1* (10 !3 )0.0/0.0

4.72/4.35 ( 1.4/1.3 u)"R *2 (10 !3 m)

0.0/0.0

3.14/-0.88 (8.7/-2.4 u)"R3* (10 !3 m !1 )0.0/0.0-529/431 (-3.2/2.6 u)"R4* (10 !3 )0.004/-0.005< !" x / " x > ring (%)< !" y / " y > ring (%)

10.3/17.3
3.1/4.8

1.2/0.63
0.04/0.02! y / ! x (%)

0.0/0.0

0.3/0.14-54.4/-15.1 (-1.7/-0.5 u)

Sextupoles for local chromaticity correction are not included in the calculation.

•According to the simulation, the movement of several 10 µm could give asizeable effect on the optics.
•As fixing BPMs firmly to magnets will give large stress to BPM
chambers and tight fixing may be difficult due to large thermal stress,
we decided to measure the movement of a BPM relative to a nearby
sextupole then apply a correction to beam position data.
•As commercial gap detectors are expensive, a gap detector which is
low cost but has enough performance, was developed.

2. Gap detector and fixing arm1) Gap detectora) Specifications
methodelectrostatic (capacitive)

channels
range (mm)20.5 - 2.5resolution (µm)

< 0.2nonlinearity (%)< ±0.3frequency response (Hz)temperature
coefficient (µm/deg.)

0 - 100< 0.2b) Principle
Electrostatic capacitive sensor

E(ω)
C0V
Cxsensorx

Cx
metal×1
AS
C0C0 EE=x
Cx = ! 0 , V =Cx!0Sx

S: Area of sensor

S = 50.3 mm2, x = 0.5 - 2.5 mm

Cx = 0.89 - 0.18 pF

Vc) Circuit (S. Hiramatsu, M. Arinaga)
C ! = Camp + Csensor + Ccable
Camp : input capacitance of amp.
Csensor : capacitance between sensor and groundE(ω)
C0Ccable : capacitance between signal wire and ground
V/AC0
1C # C0"1Cx
V = (1 +++(1 " A)) ! AE
j $C x R C x C xCx

AVCʼ

RC"
<< 1, #C x ! R >> 1, 1 - A << 1Cx
Csensorguard ring, Ccableguard shield, Camp , R

Camp=6.5 10-5 pF, R=-5.4 105 MΩ, 1−Α=1.5 10−5
oscillator 10 kHz

tri-axial cableguard ring
15:147pF
sensorelectrodereference

guard shieldbootstrap circuits in FET
input stage

isolation amp.

Voutabsolute cir.

amp.

LPF 100Hzd) Sensor headtri-axial cablelemo connector
stainless pipe
shield pipe
printed board•Shield pipe to reduce the leakage capacitance.
•Electrodes made of a printed board to reducesignal wire
guard
ring

electrodethe cost.
•Polyethylene cable against radiation.

e) Signal cable•Insulator : Polyethylene•Outer sheath : PEEK (Poly-ether-ether-ketones)Radiation resistancef) Temperature coefficient and reproducibility•Measurement in KEK
Heat cycle 25 deg.

35 deg.

25 deg.

15 deg.

25 deg.

Temperature coefficient < 0.15 µm/deg.
Reproducibility in a temperature cycle of 15 - 35 deg. < 1 µm.

•Measurement by the makerHeat cycle 25 deg.

35 deg.

25 deg.

15 deg.

25 deg.

24h

24h

1µm/dev

10 deg/dev
24h

24h

24h

Temperature coefficient < 0.15 µm/deg.
Reproducibility in a temperature cycle of 15 - 35 deg. < 1 µm.

g) Linearity
Deviation of output from a linearly fitted line for 20 sensors
Δ = δy/y±0.3%•Linearity Δ of most sensors is within ±0.3%.
•Some sensors have slightly larger linearity
than the range of ±0.3%.
•Systematic large deviation at -0.5 mm isunder investigation.

y (output)
δyy

Δ = δy/yx2) Fixing arm (M. Arinaga)•A fixing arm is made of aluminum
because of easy fabrication.
•Deformation of the fixing arm bytemperature change was estimated byANSYS.
•Result
Horizontal movement at a horizontal sensor

20µm/10 deg.
Vertical movement at a vertical sensor

5µm/10 deg.
•Stability of the tunnel temperature in
operation is less than 1 deg..
Horizontal and vertical measurement errors by the fixing arm
are estimated to be less than 2µm and 0.5µm respectively.
•The errors would be reduced by using materials which have a low linear
expansion coefficient.

3) Recent data from the displacement monitors•The 100 sensors were delivered in this fiscal year and 54 and 11sensors have been installed in HER and LER, respectively.
•KEKB is being operated with low beam currents since thisFebruary, thus the displacement of BPMs is small.
mAmm

HER horizontal

HER horizontal

beam current

beam current

0.5 hourtimex 10 sec

A small oscillation of a period about 90 sec
is observed.

3. Installation plan
•The sensors will be installed at all sextupole magnets both in HER and LER.
•The number of the sextupoles is 108 in LER and 104 in HER.

1) Problem•The "standard" fixing arm can not be adapted for many sextupoles
especially in LER.

Interference with a name plate
Obstacles•We are now discussing with the magnet group to use reference planes forthe alignment of the sextupole.
•The magnet group needs time until this summer to investigate the damageof the reference planes and judge whether the reference planes can beused or not.
•Recently we started a discussion to find other ways to fix the sensor to thesextupole which do not use the reference planes.
•The design of a new fixing arm which hasless thermal deformation is in progress.

Fixing arm under test.

Reference planes

2) Installation schedule•The sensors will be installed according to the following priority list
from the optics group.
i) D-sextupoles in HER,ii) F-sextupoles at energy analyzers in HER and LER,iii) the remainder in order of the magnetic strength.
•The 100 sensors were delivered in this fiscal year and 54 and 11sensors have been installed in HER and LER, respectively.
•The 120 sensors will be ordered in next fiscal year and deliveredat the end of this September.
•If the problem to attach the fixing arm is resolved,
the 35 sensors will be installed in this summer shut down andthe remaining about 100 sensors will be installed in machine
maintenance time in this autumn.

4. Summary•Movement of BPMs has been observed in KEKB rings.
•The observed maximum movement is 0.9mm horizontally and0.1mm vertically at a beam abort.
•The movement affects to the optics. We suspect that it disturbs the operation.
•A low cost electrostatic gap detector was developed in order to measure themovement of the BPM relative to the nearby sextupole and apply acorrection to the BPM data.
•The gap detector has a positional resolution of 0.2µm and temperature
coefficient less than 0.2µm/deg. At present, temperature dependence of thegap measurement is about 2µm/deg. due to the thermal expansion of thefixing arm. The systematic error by the nonlinearity of the gap detector is ±2µm if a correction by a calibration curve is not applied.
•The gap detectors will be installed at all BPMs near sextupoles in bothrings, hopefully in next fiscal year.

Supplement
to "BPM displacement monitor"

Movement of BPMs in the region of local chromaticity
correction in LER
The horizontal movement of two BPMs was measured relative
to nearby sextupoles and quadrupoles simultaneously.
A) Location of BPMs
1) BPM QC5LP
Tsukuba straight section (left side)e+ beamquad QC5LP

bend BL2LP.1

sextupole SL0TLP.1

BPM QC5LP

bend BL2LP.2

2) BPM QC5RP
Tsukuba straight section (right side)e+ beamquad QC5RP

bend BL2RP.4

sextupole SL0TRP.2

bend BL2RP.3
BPM QC5RP

B) Pictures of the sensors

QC5LP

QC5RP

80mm
70mm

the sensor fixed to the sextupole
the sensor fixed to the quad

the sensor fixed to the quad
the sensor fixed to the sextupole

BPMs are fixed to quads and free from sextupoles.

C) Data (10/31/2004)
BPM QC5LPmm

beam currentBPM QC5RPmm

mA

movement (mm)movement (mm)0.05 mmmA

movement from sextupole
movement from quad

movement from sextupole

movement from the sextupole= 1.2 x movement from the quad

movement from quad

movement from the sextupole= 1.4 x movement from the quad