Beam Position Monitor issues
M. Tejima
KEKB beam monitor group
2005.2.21

Contents
1.
2.

Diagnosis of BPM data
Check the BPM consistency during the measurement––3.
4.

Imperfect contact of connector
BPM signal cable breaks

Diurnal variation of BPM data
Movement of BPM chamber.
–
BPM of HER arc section–
Reinforcement of support–
BPMs of LER local correction
5.
SummaryDiagnosis of BPM data
Consistency check among four positions of three electrodes
Beam positions are calculated from the four electrode outputs.
" X = FX (H,V )(A % B % C + D)
(A + B % C % D),then H =,V =#A+ B+C+ D
A+ B+C+ D$ Y = FY (H,V )!

The position can be obtained from only threeelectrode outputs also.
" X1 = FXABC (H1,V1)"Y1 = FYABC (H1,V1)$$$ X 2 = FXBCD (H2,V1) $Y 2 = FYBCD (H2,V1), ##ACD
ACD$ X 3 = FX (H2,V 2) $Y 3 = FY (H2,V 2)$ X 4 = F ABD (H1,V 2) $Y 4 = F ABD (H1,V 2)
%%X
Y
A&B
D&C
B&C
A&Dthen H1 =, H2 =, V1 =,V2 =
A+ B
D+ C
B+C
A+ D!

If all electrode signals are correct, the results from combinationsof (ABC), (BCD), (CDA), (DAB) should coincide with each other.
Here we defined the “consistency” by the standard deviations of four positions"Consistency _ X = STDDV (X1, X2, X 3, X 4)#
$Consistency _Y = STDDV (Y1, Y 2, Y 3, Y 4)

Check the BPM consistency during themeasurement
To reject incorrect data,
BPM software always check the BPM consistency
during the measurement
Usually, consistency of BPMs
expected to be less than 0.1mm (Foursignals are correct)
But the consistency of some BPMs
have increased by various causes.
1.
2.
3.
4.

Irregular connection of BPM cables
Imperfect contact of connectors
BPM signal cable breaks
Drift of cable impedance oftransmission line
Red line means an increase of consistency morethan 0.1mm from initial value.

Diagnosis of imperfect contactof connectors

X_position

Y_position

Output of C_electrode is unstable

BPM consistency change remarkably
during injection of LER beam.

LER beamBeam position also change in thesame time

LER beamConsistency_X

Irregular output of C electrodeLER beamConsistency_X
Red:A
Green:B
Blue:CBlack:D

After change cable of C electrode,
BPM consistency is stable

BPM signal cable breaks
Output of C electrode is unstable incomparison with other electrodes
Red:A
Green:B
Blue:CBlack:DBPM consistency scatters frequently

Consistency_X

Button electrode is here

OpenC

Semi-rigid cable

TDR check

Feed throughCentral pin was broken in N-type connector

Diurnal variation of BPM consistency
Increasing the temperature made impedance of coaxial cable drift.

••••

BPM signal cables were installed fromground level to tunnel.
Part of the total cable length are cabled inoutdoor area.
Effect of air temperature and sunshine caused
the diurnal variation of BPM consistency.
To avoid sunshine, cables are wrapped by theinsulating sheet.
Yellow & red line: temperature over the cable rack
Blue & purple line: temperature under the cable rack

After shield

Consistency_X

Temperature & consistency

LC4 local hutsunshine

Wrapping by using insulating sheet

Sunshine and air temperature shielding plan••

The other three local buildings havealso been cabling in outdoor.
To intercept from sunshine and airtemperature, we are planning onshielding completely this year.

D06 local building

D03 local building

D12 local buildingMovement of BPM chamber due to heat up
by beam intensity.
How to prevent mechanical movement of BPM is serious problem forSuper KEKB•

The movement of a BPM chamber is measured by displacement meter.

•BPM chambers were moved a several hundred µm from the setting
position due to the heating up of vacuum chamber.

HER beam

0.5mm

Movement of bending chamber

AbortDisplacement

Movement of BPM head

KEKB BPMs

Reinforcement of support• 21 BPMs of HER located just downstream at the twinbends, have been fixed more firmly with additional uppersupport on a each quadrupole magnet.
• The movements decreased less than 0.05mm

BendbeamMovement of BPM chambers at LER localcorrection section•LER V tune changes due to the movement of BPM chambers in sextupole magnets at thelocal chromaticity correction section.

•These movements are compensated in the beam position data by these
displacement measurements.

red:set tune-gated tune

green:tune change estimatedby using BPM offset fluctuationabort
9/17 15:00
Movement of BPMs offset

LER Vertical tune change

Development of displacement sensor for
super-KEKB•

Ver. sensor
Hor. sensor

Requirement– Free from magnetic field– Radiation resistance– Resolution less than 1µm•••

Such as sensor is electrostatic monitor.
Commercial displacement sensor is too
Quadrupole
expense. ($5,000/unit)
We started development of theinexpensive displacement sensor,
because Super-KEKB needs a lot ofsensors.

SextupoleBPM

Displacement sensor attached to sextupole magnet

Summary• BPM consistency check during the measurement isimportant to guarantee the COD data.
• BPM data are affected by the signal cable temperaturecaused by sunshine and air temperature variation.
• Movement of BPM chamber due to thermal stress at
Super-KEKB will be serious problem.

END