Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan (August 3-5, 2013, Nagoya, Japan)

BEAM LOSS INVESTIGATION NEAR THE PULSE BENDING MAGNET
OF KEKB INJECTOR LINAC
Yoshiharu Yano#, Shinichiro Michizono, Shigeki Fukuda, Toshiya Sanami
High Energy Accelerator Research Organization
1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan
Abstract
A KEKB Linac delivers an electron beam of 8 GeV to a
KEKB high-energy ring (HER) and a positron beam of
3.5 GeV to a KEKB low-energy ring (LER). Furthermore,electron beams of 2.5 GeV and 3 GeV are supplied to thePF and the PF-AR, respectively. The KEKB Linac isoperated to deliver the beams to HER, LER, and PF every
20 msec. Beam loss is investigated by optical fiber andthermoluminescent dosimeter (TLD).

INTRODUCTION
The pulse magnet (BP_58_1) installed downstream
from the linac controls the beam distribution to 4 storagerings. The measured data of beam position monitor(BPM) show that the charge quantity of the PF beam
decreases before and behind this magnet. The beam loss
was investigated using an optical fiber, and the results
show that the beam loss occurs in the straight section of
the BP_58_1 lower stream. In order to prevent radiation
damage to the ceramic chamber inside the magnet, acollimator is located just before the magnet. A branchduct is also present immediately after the magnet. We
investigated the beam loss inside the magnet and measure
the magnet circumference. Under present operating
conditions, we found almost no beam loss in thecollimator but very large beam loss in the branch duct ofthe magnet.
The doses of radiation in various places underoperation are measured in TLDs, and these are comparedwith the optical fiber loss monitor's data [1]. Considerable
beam loss occurs near the branch duct. The duct of the~100-m PF beam line at lower streams of a pulse bending
magnet and the residual radioactivity of the flange areobserved. Under present operating conditions, it appearedthat the beam loss had occurred in the past although there
was almost no beam loss in the collimator.
We found that the optical fiber loss monitor is very
useful to detect beam loss. Since an optical fiber is anonmagnetic material, it can observe the beam loss eveninside a magnet. Since the information on a beam loss isacquired for every pulse in real time, the beam loss
occurring in a single shot can be observed. If one opticalfiber is laid to the beam line, the extent and location of
the beam loss can be observed with an oscilloscope.

BEAM LOSS MEASUREMENT BYOPTICAL FIBER
The electron/positron beams are distributed to thevarious rings at the end of the linac (the so-called “thirdswitch yard”), as shown in Fig.1. Concerning the PF-ringbeam injection, the beam line from BP_58_1 to apartition door (the boundary between the linac and the PFring) is 95 m in length. The beam charge decrease isobserved from the BPM located downstream of BP_58_1.
Since the beam loss monitors are not available, it is notwell understood whether this beam charge decrease
comes from actual beam loss or calibration error in theBPMs. In order to investigate these problems, the opticalfiber with a 400-µm-diameter core was installed in thissection. Two optical fibers fixed to both the right and leftof the duct were laid from the upper stream of BP_58_1
to a partition. Fig. 2 shows the results of beam loss

Figure 1: Layout of PF beam line in the third switching yard. The blue square represents the pulse bending magnet(BP_58_1); green, the bending magnet; yellow, the quadrupole magnet; light blue, the steering magnet; pink, thebeam position monitor; light green, the screen monitor; red, the charge limit; and black, the beam stopper.
Photographs of BP_58_1 and slit are also shown.
___________________________________________#yoshiharu.yano@kek.jp- 403 -Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan (August 3-5, 2013, Nagoya, Japan)

measurement at the third switch yard, where the beam isswitched from the linac to the PF ring.

The TLD dynamic range is 10 µSv to 10 Sv. Two or moremeasurements were performed with each beam condition
at about 0.3 nC/pulse at the 58_BP_1. The measured
doses by TLDs are shown in Fig. 4. TLD-1, TLD-2, andTLD-3 were irradiated for 5 minutes, And TLDs 4-20
were irradiated for 20 minutes. Measurement data werecalibrated to 1 minute. As shown in Figs. 3, and 4, theresults obtained using the optical fiber show correlation
with the results of TLD .

Figure 2: Oscilloscope waveform of 2 optical fibers laidon the duct in parallel, superposed with the layoutof the end of the linac.
In Fig. 2 the large beam loss near the downstream ofthe pulse bending magnet can be observed. Both fibers,
left- and right-laid, detect the large beam loss. From thisexperiment, we found that the beam charge decrease is
not the result of BPM miscalibration but real beam loss.
Since we laid the fibers in parallel, we can obtain the
difference between the left and right sides of the beameffectively. The left-hand side seen from the beam
entrance detects a larger signal, which is shown as the
red-colored signal in Fig. 3. Our interpretation is thatsince the low energy electrons in the beam bunch are bent
strongly (even in the same bending magnetic field), larger
beam losses occur here than on the other side.

Figure 4: Dose measured by TLDs.
Although measurement of the place of a collimator wasalso performed, there is no beam loss now. Since thebranch duct contains a flange, the optical fiber isseparated from the duct by about 2–3 cm in the flangeneighborhood.
A photograph showing the TLD in the branch duct isgiven in Fig. 5.

BEAM LOSS MEASUREMENT BY TLD
As is known, beam loss is very large in the branch ductof the magnet [1]. However, since the signal of the opticalfiber is not calibrated, we cannot estimate the absoluteextent of the beam loss. Thus, TLDs were installed in theduct, and the dose of radiation under beam operation wasmeasured. The setting positions of the TLDs and thesignal of the optical fiber are shown on top in Fig. 3 [2].

Figure 5: Photograph of optical fiber and TLD in abranch duct.
TLD-2 is in contact with the duct, while TLD-1 andTLD-3 are separated from the duct by about 2–3 cm.
Here, TLD-1' and TLD-3' are in contact with the duct.
Those comparison tables are shown in Table 1. A large
dose decay is observed with the distance from the beam
duct. The most effective approach to identifying thebeam loss location clearly is to locate the TLDs (using anoptical fiber) on the beam duct.
Figure 3: Setting positions of TLDs, and the signal of theoptical fiber on top.

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Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan (August 3-5, 2013, Nagoya, Japan)

Table 1: Radiation dose measured by TLDsPosition

Etotal (300 shots)

Eshot (mSv/shot)

1

1891

6.30

2

18228

60.76

3

1578

5.29

1’

36050

120.2

3’

12560

41.9

Measurement data are standardized in one shot. If theTLD is separated from the duct, the radiation dose will
become about 1/10.

RESIDUAL RADIOACTIVITY
MEASUREMENT NEAR THE PULSE
BENDING MAGNET

Figure 7: Measurement result on the surface of branchduct of the 58_BP_1 lower stream.

Next, we investigated the relation between the beamloss and residual radioactivity. We measured the residual
radioactivity of the duct and the flange about 1 hour after
stopping the beam. The measurement results on thesurface of a collimator of the 58_BP_1 upper and lower
streams are shown in in Figs. 6 and 7, respectively.

SUMMARY
The observations using the optical fiber were correlatedwith the results of the TLDs. The radiation dose at theduct surface was considerably different from that at theposition separated from the duct. To evaluate the beamloss distribution from the waveform of the observed beamloss, the laying state of the optical fiber has to beconsidered. It is possible to estimate beam loss fromresidual radioactivity. However, record of the past beamloss is also included in the measurement result of residual
radioactivity.

ACKNOWLEDGMENT
We appreciate the staff of the Mitsubishi systemservice, which laid the optical fiber. I am thankful to Mr.
Nakamura of the radiation group for the measurement ofthe TLD.

Figure 6: Measurement result on the surface ofcollimator of the 58_BP_1 upper stream.
We also measured the residual radioactivity fromBP_58_1 to a partition door. Almost all the positions of
the duct parts from BP_58_1 to BM_61_F1 were 1–3µSv/h. The flange of the gate valve was 10 µSv/h, andthat of the Position-13 neighborhood was 7 µSv/h. Almost
all the positions of the duct parts from BM_61_F1 to thepartition door were 1–3 µSv/h. The highest dose was 20µSv/h at the main part in the downstream slit. The flangepart before and after the slit was 3 µSv/h, and the ductthat was 1 m near the slit lower streams was 1 µSv/h. The
duct support stand (aluminum block) of the place was 3µSv/h.

REFERENCES[1] Y. Yano et al., "DEVELOPMENT OF OPTICALFIBER BEAM-LOSS MONITOR" in Japanese, Proc.
of the 8th Annual Meeting of Particle Accelerator
Society of Japan, Tsukuba, August 1-3, 2011.
[2] Y. Yano et al., "OPTICAL FIBER BEAM LOSSMONITOR" in Japanese, Proc. of the 9th Annual
Meeting of Particle Accelerator Society of Japan,Osaka, August 1-3, 2012.

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