BPM displacement monitor
4th Dec. 2007, KEKB review committee
M. Arinaga, H. Fukuma, S. Hiramatsu, H. Ishii, K. Mori,
M. Tejima and M. Tobiyama

1. Introduction
2. Monitor system
3. Displacement data
4. Effect on the optics
5. Summary

1. Introduction•As reported at the previous KEKB review committee, 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.
•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, then changes a linear optics. Especially the orbit
error at a sextupole generates a dipole, quadrupole and skew-quadrupole
component. Expected problems are,1) optics deviation in physics run from the corrected optics, because theoptics is corrected at small current of 30 mA in order to avoid thehardware damage due to the beam loss during the correction,2) fluctuation of the optics in physics run if the saw-tooth pattern of thebeam current is enhanced,3) change of the optics after recovery from the beam abort or startup
after machine maintenance until the operation moves to the steadystate.

•In order to measure the displacement of the BPMs and apply the correction to theBPM data, 232 gap detectors (118 in LER, 114 in HER) are in operation in thisautumn run. Most of the detectors were installed in this summer shutdown.

measured displacement
Sextupole to BPM*
QCS to BPM
Quad in Nikko straight section to BPM( Vacuum duct to floor

the number of detectorsHER1042LER10824480 )*All sextupoles are equipped with the gap detector.

•Main purpose is to correct the BPM movement near the sextupoles in order tomaintain relative position between the origin of a BPM and a nearby sextupole.

2. Monitor systemsensor1) Gap detectorxa) Specifications
methodCx
metalelectrostatic (capacitive)

Cx = ! 0S,xchannels
range (mm)20.5 - 2.5

V=resolution (µm)< 0.2

Cx = 0.89 - 0.18 pF

nonlinearity (%)< ±0.3

S: Area of sensor

frequency response (Hz)temperature
coefficient (µm/deg.)

0 - 100< 0.2

Gap detector developed at KEK.

C0C E
E= 0 xCx!0S2) Fixing arm•The gap sensors are fixed to a magnet with an aluminum arm of type A or B.
•The arm of type B, which was developed in cooperation with the magnet group, isfixed to a top surface of a quadrupole magnet to avoid the interference with partsof the magnet.

Type A

Type B3) Data acquisition•Signals from the gap detectors are A/D-converted, then transferred to anIOC by 12 network loggers.
•A problem is that some loggers stop the data taking with frequency ofroughly once per two weeks.
The problem might be resolved by modifying the firmware of the loggers
supplied by a company. The firmware was replaced last week.

pc (IOC)groundEthernet
network loggers

tunnelgap detectors

4) Alarm•A simple alarm system has been implemented in order to detect a faultof the gap detectors. The system detects a sudden change of the data or
no change of the consecutive data in an interval.
•As the system is not redundant, namely one sensor for one direction, adecision whether "abnormal" data is due to the fault or the real movement isnot easy as shown in an example below.
•Operation experience might be necessary to have a sophisticated alarm system.
mm

QF4E9 horizontal

targetsensor B
sensor A

sensor B

The data look showing a fault of adetector, but a target really moves
with respect to sensors.

sensor A

x 10 sec

5) Measurement error by temperature changea) Deformation of the fixing arm•Temperature dependence of the gap data was measured
immediately after the stop of the beam operation. No beams
were in the rings.

Verticaldeg.

mm4µm/deg
QS4OE2䟺type B䟻

Temperature dependence will beroughly same (4µm/deg.) in type
A and B.

QF4E31䟺type A䟻

Tunnel temperature
QD3E32䟺type A䟻

time

x 18 sec

Horizontal

mmdeg.

QF4E31(type A)
QS4OE2(type B)

Tunnel temperature

Change of the gap was notobserved in the temperaturechange of 0.7 deg..

QD3E32(type A)

time

x 18 sec•Deformation of the fixing arm by temperature change
Horizontal : < 1µm/deg., Vertical : 4µm/deg.

SF4NRE2 QF4E9 QD3E9b) Thermal expansion of the target

B2E24

Horizontal displacement vs. target temperature
deg.

BPM

beam
8.0m

aluminum target

9.6degsensor holder
850mA
HER currenttargetx 10 secmm!

xA

xB

shift
d=xA " xB
2sensor A L

sensor B"L = 2"# = 22µm from data!

11µm

expansion Estimated thermal expansion= 26 µm
x + xB"= A! 2

Horizontal measurement error by the expansion of the target = δ ≈ 10 µm (1.1µm/deg.)
Estimated vertical measurement error by the expansion of the target ≈ 8 µm (0.8µm/deg.)!

c) Summary of the temperature effecti)Fixing arm•Temperature of the arm is not much affected by the beam current.
It would be determined by the tunnel temperature.

•The deformation of the arm is estimated to be < 1µm/deg. horizontally,4µm/deg. vertically.
•As the stability of the tunnel temperature in operation is less than 1 deg., theerror of the measurement would be < 1µm horizontally, < 4µm vertically.

ii)Target•Thermal expansion of the target is estimated to be 1µm/deg.
horizontally and 0.8µm/deg. vertically.
•It brings the measurement error of about 10 µm during re-fill of the beam
because the temperature of the target depends on the beam current,
probably due to the temperature change of the cooling water of vacuum
ducts.
•The error depends on the distribution of SR power, the pipe network ofthe cooling water and the beam current. We need more data to understandthe error.

3. Displacement data
Whole ring
HorizontalmmVertical
100 µmHER790mAx 10 sec

LER1530mA

South tunnelmmHorizontal

Vertical50 µm

HER800mA
beam current
x 10 sec

LER1530mA

•The data show the large horizontal movement in HER. The maximum
horizontal movement amounts to 500µm at a beam abort and 300µm in refilling after the abort.
The movement during the continuous injection is less than 10µm.

HERhor. ver.

LER
hor. ver.

max. δ at abort

500

20200

≈0max. δ in re-filling

300

≈0150

≈0100

100

100

100

10<55<5δ from 0 current
to steady state
δ in continuousinjection (p-p)δ : displacement (µm)
The beam current is 790 mA in HER and 1530 mA in LER.

4. Effect on the optics•As we know the displacement of the BPMs with respect to the sextupoles, theeffect of the BPM movement on the optics if the correction is not applied can be
estimated by a simulation.
•In the simulation the position of the sextupoles was shifted by measured displacement,
then optics parameters such as tunes, betas, etas and R parameters at IP were
calculated by SAD.
mALER

HER

Beam current

x 10 secmm

10 µm

r.m.s. displacementof the sextupoleshor.

ver.

Data used in the simulationHERtune changeΔβ∗/β∗

%

η∗mm

ver.
hor.

hor.
0.015

ver.

ver.

3%

0.4mm

hor.

x 10 sec
x 10-3R1∗

0.9u

x 10-3m-1

5.6u

x 10-3m

R2∗

1.6u

R3∗

x 10-35.9u%

εy/εx0.045%

R4∗"u" is an unit used in machine tuning.
Several units are changed by "scan" tomaintain good luminosity.
A star mark means IP.

LERtune changeΔβ∗/β∗
4%

hor.

ver.
0.01mm0.2mm

η∗hor.
ver.

ver.
hor.
x 10 sec

x 10-3R1∗

0.2u

x 10-3m-1
0.5u

x 10-3mR2∗

0.02%

2.7u

R3∗

x 10-3
1.9u

%

R4∗

εy/εx

Effect of the tune adjustment•In real operation the tunes are continuously adjusted to a target
values by the tune feedback.

•The effect of the tune adjustment on the optics was simulatedusing the measured displacement.

HER

Δβ∗/β∗xΔβ∗/β∗yη∗xR2∗

η∗yR4∗

εy/εxwith tune adjustment
without tune adjustment

R1∗

R3∗

x 10 sec•Optics parameters are not much improved by the tune adjustment.
*Vertical units are same as those in the previous figures.

LERΔβ∗/β∗x

R1∗

R3∗

x 10 secΔβ∗/β∗yη∗xR2∗η∗yR4∗

εy/εx

The effect of the tune adjustment is similar to the HER.

Summary of the simulation (tune adjustment "on")HER

max. change in re-fill"! xchange from 0 current
to steady statechange in continuous inj.

(p-p)

"! y

0
0

0
000

"! x* / ! x* (%)

73≈0"! *y / ! *y (%)

10

2.5

0.5

"! *y (mm)

1.4
0.0

0.4
0.0

0.1
0.0

"R1* (10 !3 )0.7(1.3u)

0.4 (0.7u)

0.1(0.2u)"R *2 (10 !3 m)0.6 (1.6u)

0.6 (1.6u)

0.1(0.3u)

250 (4.7u)20(0.4u)"R4* (10 !3 )250 (4.7u)
170 (5.3u)

170 (5.3u)20(0.6u)! y / ! x (%)

0.04

0.04

0.003

"! *x (mm)"R3* (10 !3 m !1 )R parameters considerably changes from 0 to steady beam current.
During the stable continuous injection, the change of the parameters is small.

LER

max. change in re-fill"! xmax. change from 0current to steady statechange in continuous inj.

(p-p)"! y000000"! x* / ! x* (%)

1.4

0.3

0.2

"! *y / ! *y (%)

1222"! *x (mm)

0.2

0.2

0.01

"! *y (mm)

0.0

0.0

0.0

"R1* (10 !3 )0.7 (0.2u)

0.7 (0.2u)

0.1 (0.0u)"R *2 (10 !3 m)

0.8 (3.6u)

0.8 (3.6u)

0.1 (0.5u)

40 (0.6u)

40 (0.6u)"R4* (10 !3 )40 (0.6u)
60 (2.2u)

50 (1.9u)

5 (0.2u)! y / ! x (%)

0.024

0.024

0.003

"R3* (10 !3 m !1 )Similar behavior of the parameters to HER is seen.

5. Summary•The gap detectors were installed at all sextupoles in order to measure the movementof the BPM with respect to the sextupole, then apply the correction to the BPMdata.
•Large systematic error of about 10 µm due to thermal deformation of the target andthe fixing arm exits during re-fill of the beam. We need more data such as thetemperature of the targets to understand the error.
•The data show that the maximum horizontal movement in HER amounts to 500µm
at a beam abort and 300µm in re-filling of the beam after the abort.
The movement during the continuous injection is less than 10µm.
•The simulation according to the measured displacement shows that, if thecorrection to the BPM data is not applied,1) from 0 current to steady current, vertical tunes change 0.01 - 0.015, beta
function changes several % and R parameters at IP change several operation
units which is in a range of parameter scan in the operation,2) the change of the beta functions, etas and R parameters would not be
cured by the tune feedback.

•Effect of the displacement correction on the luminosity is not clear though thesimulation shows that the R parameters considerably change between small andhigh current operation. Luminosity strongly depends on machine conditions,tuning and so on. A statistical analysis may be useful to clarify the effect of thecorrection.

11/23/2006

11/28/2007

How much does the recovery of the luminosity after the abort differ ?