Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FRP016

DEVELOPMENT OF J-arc SYNCHROTRON RADIATIONMONITOR AT SuperKEKB LINAC
R. Zhang†1, M. Yoshida1, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
N. Toyotomi, T. Kudou, Mitsubishi Electric System & Service Co., Ltd. (MSC), Tokyo, Japan1
also at The Graduate University of Advanced Studies (SOKENDAI), Tsukuba, Japan

Abstract
SupreKEKB Linac injector generates and accelerates
electron beam and positron beam for SuperKEKB HighEnergy Ring (HER) and Low Energy Ring (LER) injection.
At the same time, Linac also undertakes the electron beam
injection tasks for Photon Factory (PF) and Photon Factory
Advanced Ring (PF-AR). Four rings simultaneous injection has been realized from phase II commissioning. During 4 ring simultaneous injection, non-destructive beammeasurements are important for high quality injection, one
synchrotron radiation monitor (SRM) is con-structed after
the last bending magnet of Linac J-arc section. Two dimensional and pulse by pulse beam profiles have been obtained
successfully and applied for non-destructive beam adjustment from 2020c. operation. In this work, the design andoperation results of J-arc SRM are introduced.

INTRODUCTIONKEK Linac injector has achieved the 4 rings simultaneous injection from 2019 for SuperKEKB HER and LER,
PF ring and PF-AR [1]. The layout of Linac injector isshown in Fig. 1. Two kinds of electron source are locatedat the start point of Sector A, one is thermionic gun for generating primary electron beam to generate positron beam at
Sector 1, and electron beams for PF and PF-AR. The otherone is RF gun which is the electron source of SuperKEKB
HER. All the electron beams are accelerated and passed theJ-arc to the later sectors for continuous acceleration. Certainly, screen monitors are installed in the J-arc section, but
the screen cannot be inserted during the simultaneous injection. Therefore, non-destructive beam monitoring system is necessary to ensure highly efficient for simultaneousinjection. SRM is the best candidate to realize the continu-ous non-destructive measurements of beam profiles at Jarc.

LINAC J-arc SRM
About 20 years ago, Dr. Mitsuhashi constructed one Jarc SRM for KEKB project [2]. The synchrotron radiation
light of primary electron beam was observed successfully.
After that, there was no maintenance for the SRM even after the Great East Japan earthquake. We decided to rebuild
the SRM from the end of 2018, but the work was postponedby the fire accident of Linac in April 2019. The rebuilt
work was started from the summer maintenance of 2019
and the synchrotron radiation lights of the electron beam
generated by RF gun and primary electron beam were observed successfully. After that, software and monitor panel
were developed for commissioning. The SRM monitoring
system is being applied all day during the commissioning.
In this section, the experimental setup and data analysis results will be introduced.

SR Light Extraction Setup
As shown in Fig. 2, the 180o arc of J-arc section has 6
bending magnets which can bend the beam of 30o. The
bending radius of these bending magnets is 3.57 m. Electron beam energy at J-arc section is 1.5 GeV. Due to thespace limitation, the light source of synchrotron radiationis selected at the entrance of the 6th bending magnet, which
is marked in the Fig. 2. All the existing vacuum duct andmirror supports are reused in our experiment, but the mirrors, lens, and the other optical components are exchanged
due to the long-term aging damages. According to thespace and imaging condition, single lens imaging layout isdesigned again for the new SRM optics system.

Figure 1: Layout of SuperKEKB Linac injector.

___________________________________________† rui.zhang@kek.jp- 942 -

Proceedings of the 19th Annual Meeting of Particle Accelerator Society of JapanOctober 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FRP016

adjusting the image distance for the best imaging. According to the distance from the light source position to the lensposition, the conjugation ratio of the objective lens is 0.14.
This conjugation ratio is large enough for the beam size
imaging and small enough for CCD resolution, that’s why
additional magnification lens is not necessary and single
lens imaging is just adopted in our system. By the way, adichroic polarizer is not applied here to select the σ-polarization component, the reason is the total radiation poweris very low so there is no need to remove the π polarization
for beam size measurement.
The optical dark box is shieled by lead blocks to avoid
the radiation damages to optical components and CCD camara, all the remote-control units and drivers are placed ina special place where is surrounded by lead blocks.

Observation and Calibration

Figure 2: Location of SR source point at J-arc.

The first observation of primary electron beam and HER
electron beam was achieved in 2019c operation, as shownin Fig. 4. The two-dimensional distribution can be seenclearly. Especially, the existed beam tail of HER electron
beam at the J-arc source point is known.

Figure 3: Outline of the imaging optics layout.
The optics layout of SRM is shown in Fig. 3. One quartz
extraction mirror is fixed in a vacuum chamber at the 1340
mm from the source point position. Due to the low radiation power of SR at J-arc is very low (~0.01 W) so it is noneed to use water cooling system for the extraction mirror.
High optical surface quality of λ/10 aluminium coating iscoated for the extraction mirror and reflection mirror for
decreasing the wavefront distortion.

Imaging SystemFigure 4: (a) Observed beam profile of HER electron beam;(b) beam profile of the primary electron beam at J-arc.
In order to measure the beam size of electron at J-arc,calibration is essential to carry out soon. The typical calibration method for SRM is building local bump to change
the beam orbit, such as SuperKEKB main ring SRMs andDumping ring SRM. Different from the other SRMs, one
local bump can’t be built in J-arc section. We tried to build
bump in front of J-arc entrance to do calibration, but theaccuracy was very rude.

After reflecting, the SR light is sent to the imaging line
which is set inside a dark box fixed on the floor. As entrance pupil, one aperture is placed before the object lensfor decreasing the diffraction and focusing field to enhance
imaging quality. Meanwhile, a diffraction limited doublet
lens is used as objective lens for imaging, whose diameterand focusing length are 80 mm and 300 mm respectively.
Because the SR light is wide band incoherent light. A bandpass filter with band width of 2 nm is applied to obtain aquasi-monochromatic ray at the wavelength of 500 nm.
One variable ND filter is applied to optimize the intensityof the image. It can be adjusted to the optimized status byremote control for primary electron beam imaging andHER electron beam imaging respectively.
The CCD camera is set on a linear stage at the direct focusing point of the objective lens along the direction of ray
propagation. The CCD camera can be moved ±37.5 mm for- 943 -Figure 5: Calibration result of J-arc SRM.

Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FRP016

In 2020 summer-maintenance, an additional motorized
linear stage was added under the CCD camera. This stage
can move along the horizontal direction which is perpendicular to the ray propagation direction. By measuring thepixel change amount on screen after adjusting the stage by
1 mm step, we can get the ratio of pixel to μm. The resultis shown in Fig. 5, and the calculated ratio is 1 pixel corresponds to 7.443 μm.

Application in Operation
By use of the conjugation ratio of imaging system andthe calibration ratio, we can measure the electron beam sizeat the source point in the last bending magnet of J-arc section. The results are listed in the Table 1.
Table 1: Measured Two-dimensional Beam Size for HER
Electron Beam and Primary Beam Size at the Entrance ofthe Last Bending Magnet of J-arc
Horizontal
Vertical

HER e- Beam

Primary e- Beam

412±9 μm

1208±11 μm

241±34 μm

469±59 μm

ACKNOWLEDGEMENTSThe authors also thank to Dr. T. Mitsuhashi, Dr. H. Ikeda,
Dr. Y. Seimiya, Dr. F. Miyahara and Mr. H. Kumano for
their kind supports and operation.

REFERENCES[1] K. Furukawa et al., “Achievement of 200,000 Hours of Operation at KEK 7-GeV Electron 4-GeV Positron Injector
Linac”, in Proc. IPAC’22, Bangkok, Thailand, June 2022, pp.
2465-2468. doi: 18429/JACoW-IPAC2022-THPOST012.
[2] T. Mitsuhashi and H. Kobayashi, “Optical Profile Monitor for
KEKB Injector Linac”, in Proc. EPAC’2000, Vienna, Austria,June 2000, pp. 1786-1788.

From 2020c operation, the SRM operation panel is usedduring beam operation and commissioning, as shown in
Fig. 6. Especially, beams’ status can be checked non-destructively by the SRM during 4-ring simultaneous injection. In addition, there is no wire scanner setup in J-arc section, so the two-dimensional beam size can be measured by
the SRM. Compare with the wire scanner measurement, instantaneous beam profile could be measured for different
electron beams.

Figure 6: SRM monitoring panel for operation.

CONCLUSIONThe SRM at linac J-arc section is rebuilt. One lens imaging layout is adopted for different electron beams’ profiles measurement. Accordingly, the calibration for our imaging system has been achieved for beam size measurement. During 4-ring simultaneous injection, it plays important role in continuous non-destructive monitoring forstable operation.

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