IR Beam Position Monitor for
KEKB

Feb 10 2003 by M.TejimaIntroduction• To maintain the optimum collision condition of the twobeams, two special Beam Position Monitors(BPMs)
with 8-button electrodes, called OCTOPOS, wereinstalled inside the super-conducting quadrupole
magnets (QCSs)• Four normal BPMs with 4-button electrodes wereincorporated outside the QCSs.
• A finite orbit separation method between the twobeams was proposed to detect the beam positions ofeach ring with a common BPM having many electrodes.

QCS-L e/p
ø72

OCTPOS-L OCTPOS-R
8 1
8 1
72 72
ø52

ø55

63 6
54e+ LER

54HER e-

QCS-R

QCS-L
552

ø783IP
QC1-L

QCS-R e/p

QC1-R773

Schematic view of the KEKB IR for BPMs

OCTOPOS• Near the IP, the orbit separation is so small that bothbeams pass through a common chamber.
• Due to space problems inside the QCSs, we could
not adopt directional couplers to measure thepositions of the two beams separately• We elected to install two special BPM heads eachhaving eight button electrodes at the front end of theQCSs around the IP.
• Since the OCTOPOS is placed inside the inner boreof the QCS cryostat, the button electrode adopted
was an SMA-type connector.

Signal measurement of OCTOPOS• The output signal was detected with a narrow banddetector, the same electronics employed formeasuring the closed orbit at KEKB
• The signal processor detects the combined output
signal of both beams, and the pickup frequency is1019MHz .

Schematic layout of control system for the Octopos

Proccessing method• The data process is done by non linear fittinganalysis based on finite orbit separation method.
• The position response function used a simple BPM
model having very small electrodes.

Equation for the non-linear fitting• The output signal can be represented with a phaser,
and the peak value (Vi) of the phaser is measured by
the detector, as follow

where g, F(X,Y) andare the gain, the response
function of the i-th electrode and the phase difference
between two beams.
(Qe, Xe, Ye ), (Qp, Xp, Yp ) are the charge andposition for positron and electron beams.

Assumption for the non-linear fitting analysis
1. Response function (Fi(X,Y)) was calculated
from the BPM geometry as following figure.

Assumption for the non-linear fitting analysis
2.

The phase difference ( ) was defined to beconstant and can be calculated from the distance
between the OCTOPOS and the IP.

Correction for the non-linear fitting analysis
3.

Gain coefficients (gi) for each electrode werecalibrated with beam based position mapping data
under the condition of single-beam operation.

2003.1.15
Electrode# 0CTOPOS-L OCTOPOS-R
11.0000
1.00002
1.04470.97103
1.06291.0143
4
0.97700.90805
1.04331.0096
6
1.0786
1.05237
0.99670.960580.9515
0.9626

Improvement by g and correction
baca.
b.
c.

Variation of electron beam positions
without gain correction at OCTOPOS-L
Variation of electron beam positions after
correction of gain at OCTOPOS-L
Variation of electron beam positions after
correction of gains and phase difference
between electron and positron bunch atOCTOPOS-L.

Correction of RF phase change-19

1400
Positron-21-22Electron

-23

600
400

-24

RF phase200
0
15:00:00

15:30:00

16:00:00-25
16:30:00-26
17:00:00

Timeb

Xe of OCTOPOS_r stepped up.

c.

The phase difference( ) was corrected
in response to 2xRF phase.

c

OCTOPOS_R
RF phase change

OCTPOS_R

8.8

2

8.8

2

8.7

1.9

8.7

1.9

1.8

8.6
Ye

1.7

8.5

1.6

8.4

Ye[mm]

Xe[mm]b.

Xe[mm]

Beam[mA]

800

Example of RF phase change to adjust
collision point

-20

1200
1000

a.

1.8

8.6
Ye

1.7

8.5

1.6

8.4
XeXe
8.3
15:00:00

15:30:0016:00:00
Time

16:30:00

1.5
17:00:00

8.3
15:00:00

15:30:00

16:00:00
Time

16:30:00

1.5
17:00:00

Ye[mm]

Beam current and RF phase

RF phase[deg]aPerformance @ KEKB optics correction

Correlation error at IR was calculated from the difference in the singlekick orbits in HER by the three- BPM correlation method (upper:electron beam; lower: both beams)

Feedback Performance
Horizontal feedback

red: LER current
blue: HER current
purple: H target
pink: measured H canonical kickNormal BPMs for IR• Four normal BPMs with 4 electrodes at the exit of theQCS were also incorporated into this IR BPM system
as a back-up to the OCTOPOS BPMs.
• These BPMs are not supported firmly at the end of amagnet; as a result, heating of the vacuum chambercauses mechanical movement of the BPMs.
• These movements are measured by two displacement
meters, and are compensated in the beam position databy these displacement measurements.

Movement of the Normal BPM chamber due to heat up
by beam intensity

Position Resolution of OCTOPOS andQCS bpms
STDEV.(10)OCTOPOS_R
xp_r
yp_r0.003
0.005

STDEV.(10)OCTOPOS_L
xp_l
yp_l
0.007
0.002

xe_r
0.006

[mm]
ye_r
0.004

xe_l0.008

[mm]
ye_l
0.004

STDEV[mm]QCS BPMs_R
QCSLE:X QCSLE:Y QCSRE:X QCSRE:Y
0.002
0.005
0.003
0.007

STDEV[mm]QCS BPMs_L
QCSLP:X QCSLP:Y QCSRP:X QCSRP:Y
0.002
0.005
0.002
0.009

(Actual measurement data)Summary• The OCTOPOS system for the IR has been used forthe first time to optimize the collisions under twobeam operation for physics experiments since 2001November.
• There was a disagreement in the results of themeasurement between the OCTOPOS BPM and thefour normal BPMs during two-beam operation in thecase of high-current beam.
• The reliability of positions measured by OCTOPOS
has been improved after corrections for gains andphase difference.