VARIATION OF THE CIRCUMFERENCE OF THE KEKB RING
AFTER THE GREAT EAST JAPAN EARTHQUAKERyuhei Sugahara A), Mika Masuzawa A), Yasunobu Ohsawa A)A) High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba-shi, Ibaraki, 305-0801, JapanAbstract
At the time of The Great East Japan Earthquake, which took place on March 11, 2011, the alignment ofmagnets in the KEKB ring was deformed in the order of some millimeters. In the construction of theKEKB ring, the tolerance for the ring circumference was as tight as ±10 mm, 3 ppm of the KEKBcircumference. So the circumference of the ring was measured after the earthquake with a laser tracker
and a mekometer. Results of this measurement are reported.
laser tracker and a mekometer.

1. INTRODUCTION
The *KEKB ring [1] is a high luminosity
electron-positron two-ring collider constructedin 10m deep underground tunnel. The energy ofelectron and positron beams was 8 GeV (HER)
and 3.5 GeV (LER) respectively. This KEKB is
now reconstructed as a higher luminosity
B-factory, SuperKEKB [2]. The layout of theKEKB ring is shown in Figure1. The KEKBring is divided into four arc sections, and thesearc sections are connected with 200 m longstraight sections. Its circumference is about 3km.
The electron and positron beams circulate in theopposite direction. They collide at the Tsukuba
experimental hall in the north area, and crossover each other in the Fuji experimental hall inthe south area.
In the construction of the KEKB ring, thetolerance for the ring circumference was as tightas ±10 mm, 3 ppm of the KEKB circumference.
The error of the circumference was less than
5mm in the preceding project, TRISTAN andKEKB. But the Great East Japan Earthquake
took place on March 11, 2011, and thealignment of magnets in the KEKB ring waslost in the order of some millimeters [3]. Thecircumference of the ring was thought to be alsochanged and the error of the circumference
might have become larger than 10 mm. So thecircumference of the ring was evaluated with a*sugahara@post.kek.jp- 292 -Figure 1: Layout of the KEKB ring

2. INSTRUMENTS AND
MEASUREMENT
Two instruments were used, a laser tracker forthe arc sections and a mekometer for the 200 mstraight sections. The laser tracker is ION™ ofFARO, and the mekometer is ME5000 of Leica.
The precision and the range are as follows in themakers’ specifications:
Laser tracker, ION™
Precision:
8 µm + 0.4 ppm for distance (ADM mode)(10 + 2.5/m) µrad for angle

Range:0 – 30 m with 1/2” SMR

measured. Measurement was carried out
counter-clock wise starting from the Tsukuba
straight section. The edge magnet wasoverlapped in the next measurement. The
distance between the left and the right magnets
was calculated in the horizontal plane, andcompared with the designed value in the lattice.
dL is defined as follows:
dL = (measured distance) – (designed value)dL was accumulated from the starting point till
the final point. The final value of thisaccumulation provides an estimation of the error
for the circumference. Measurements werecarried out 5 times from July 2011 till June2012 as shown in Table 1. Only themeasurement #3b was performed with a total
station for an instrument test [4]. First threemeasurements were performed every about twomonths. And the last measurement wasperformed for two months, from April till June,
2012. A set of the circumference measurement
consists of 55 measurements in the arc sections
and 4 measurements in the straight sections.

Mekometer, ME5000
Precision:
0.2 mm + 0.2 ppm for distanceRange:
20 – 1,000 m in low range
The position of magnets mainly in thehigh-energy ring (HER) was measured at thespatial interval of 40 m in the arc section. The
position of magnets at both edges was measured.
At the 200 m straight section, the position ofmagnets at both ends of the straight section wasTable 1. Period of measurements
YearDuration

No.

Instruments

2011

7/28 - 8/24

#1LT+Mekometer

9/21 - 9/30

#2LT+Mekometer

12/2 - 12/7

#3aLT+Mekometer

12/7 - 12/19#3bTS

4/11 - 6/7#4LT+TSA c c u m_ # 1 LT+ M e ko
A c c u m_ # 2 LT+ M e ko
A c c u m_ # 3 a LT+ M e ko
A c c u m_ # 3 b TS
A c c u m_ # 4 LT+ ST( St r t )KEKB RI N G C ir c umf e r e nc e M e asur e me nt
wit h LT+ M e ko me t e r _ #1 - #4Oh oFu j i35

N i kko

25Ts uku ba

302015Ts uku ba

D i f f e r e n c e be t we e n M e a s. a nd Lat t ic e ( mm)2012

1050- 5
0500

1,000

1,500

2,000

2,500

3,000

Po sit io n alo ng t he C ir c umf e r e nc e ( m)Figure 2: Accumulation of dL (difference between measurement and designed value)- 293 -

4. COMPARISON WITH THE
ERROR DERIVED FROM THE RFFREQUENCYTheprecision
forthecircumference
measurement is estimated to be about 2 mmbased on the precision listed in the makers’specifications.

The error for the circumference of the ring
derived by A. Morita from the change of thering RF frequency is shown in Figure 3. The
error stayed between +8 and +10 mm sinceJanuary 2003 till the end of 2010, although theerror was reported to be less than +5 mm in theTRISTAN operation and the beginning of theKEKB operation.

3. MEASUREMENT RESULTS
Accumulation of dL is shown in Figure 2. The
error of the circumference is about +28 mm in
the first measurement performed in early August
2011. Then it became about +15mm in thesecond measurement. Since then the error
stayed around +15 mm.

Figure 3: The error for the circumference of the KEKB ring derived by A. Morita from the change of theRF frequency of the KEKB main ring.

SUMMARY+28 mm in August 2011, and about +15 mm in
the end of September. It looks settling downaround +15 mm since then. It is said that theextension of the circumference of the ring by+15 mm can be managed by lowering the RF
frequency in the linac and the ring as well. But
the circumference of the LER and the HER ringshould match within 3 mm which is the range ofthe beam passage adjusting chicane system.
It will be decided soon how to manage thiserror of the circumference in the discussion

The error of the circumference of the KEKB
ring was reported to be less than +5 mm in thebeginning of the KEKB operation. And it looks
shifted to around +9 mm on the way of theKEKB operation. This change is underinvestigation on when and how it took place.
Then the KEKB alignment was deformed in theGreatEastJapan
Earthquake.
Thecircumference was thought to be changed
largely at that time. And it returned to about- 294 -between KEKB hardware groups and the beam
optics group.

East Japan Earthquake”, MOPS151, Proceedingsof the 8th Annual Meeting of Particle Accelerator
Society of Japan, Tsukuba, Ibaraki-ken, Japan,REFERENCES

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TUPPR006,

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Miyasaka, “Performance Test of Various Laser

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Height Survey of the KEKB Magnets after the- 295 -