Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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Manufacturing Technology of Electron Accelerator
for 3GeV Synchrotron Radiation Facility NanoTerasu

HIROSHI HARA*1

KYUSAKU HIGA*1NOBUYUKI SHIGEOKA*2

The 3GeV synchrotron radiation facility NanoTerasu, which has been started operation fromApril 2024, is a facility incorporating the latest accelerator technology, and has an accelerating
cavity and structure manufactured by Mitsubishi Heavy Industries Machinery Systems, Ltd.
(MHI-MS). Accelerating cavities structures are precision devices that require high precision inmachining, assembly, and adjustment. Over the years, MHI-MS has contributed to the constructionof accelerator facilities by providing components and has continued to improve its manufacturingtechnology. This report focuses on the manufacturing technology MHI-MS has developed.

|1. Introduction
1.1

Overview of NanoTerasu (1)The 3GeV synchrotron radiation facility NanoTerasu, which has been started operation from
April 2024, is a facility with the world's highest level of analytical capabilities. Bending the orbitsof electrons accelerated to near light speed produces “synchrotron radiation”. NanoTerasu
generates synchrotron radiation has a brilliance more than one billion times higher than sunlight tovisualize the structure and function of materials at the level of nanometer (one-billionth of a meter).
The range of its applications is diverse, including materials science (development of new energy
materials and elucidation of the properties of materials), life science (analysis of the structure andinteraction of proteins and biomolecules), and medical science (analysis and imaging in thedevelopment of new treatments and new drugs). It is also expected to play a role in supporting
research and development to solve social issues as represented by the SDGs.

1.2

Accelerated cavity for NanoTerasu

NanoTerasu consists of a storage ring, which serves as a synchrotron radiation source, and an
injection linac, which injects electron beams into the storage ring (Figure 1(2)). NanoTerasu wasneeded to be completed with limited resources.
For this purpose, it was designed to be thoroughly compact by actively incorporating theachievements and developed technologies of accelerator laboratories in Japan.
The linac for accelerating electrons generated by an electron source up to 3 GeV employs asystem proven at SACLA (SPring-8 Angstrom Compact Free Electron Laser), an X-ray-freeelectron laser facility at RIKEN, and a 238-MHz accelerating cavity, a 476-MHz Sub Harmonic
Buncher, and a C-band (5,712-MHz) disk-loaded type accelerating structure were installed.
The storage ring has a large number of electromagnets to achieve high brilliance with alimited circumference, and the space for an accelerating cavity to compensate for the energy loss
was limited to about 5m. The conventional accelerating cavity structure could not provide thenecessary acceleration voltage and stable beam, so a beam-accelerating RF cavity with new HOM(higher-order mode) damping structure is installed for the first time in the world.
Figure 2 shows our accelerating cavities and structures installed in NanoTerasu. The above"accelerating cavities" and "accelerating structures" are technically distinguished, but they bothhave a function to accelerate electrons.
*1

Infrastructure Facilities Business Division, Machinery Engineering Department, Mitsubishi Heavy Industries MachinerySystems, Ltd.

*2

Engineering Manager, Infrastructure Facilities Business Division, Machinery Engineering Department, Mitsubishi HeavyIndustries Machinery Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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Figure 1

Layout of the 3 GeV light source NanoTerasu.

Figure 2

Installation of our accelerating cavities and structures|2. Accelerated cavity manufacturing technology
This chapter describes our accelerating cavity manufacturing technology, using theHOM-damped RF cavity as an example. For the manufacturing technology of 238 MHz
accelerating cavity, 476 MHz Sub Harmonic Buncher, and C-band disk-loaded type acceleratingstructure, refer to our previous reports (3), (4).

2.1Overview of HOM-damped RF cavity

The HOM-damped RF cavity was basically designed by RIKEN(5), (6). This accelerating
cavity enables both beam acceleration with a short cavity length and countermeasures against beam
instability by utilizing resonant modes (which characterize the configuration and oscillation of theelectromagnetic field) different from those of conventional acceleration cavities for beam
acceleration. On the other hand, the inner diameter of the accelerating cavity becomes larger, which
requires precision surface machining, assembly and bonding technologies, and frequency tuningtechnology for manufacturing.
Figure 3 shows the configuration of this accelerating cavity, which is a re-entrant type with acenter inner diameter of approximately 1,000 mm, an overall length (distance between beam portends) of approximately 450 mm, and a beam port inner diameter of approximately 70 mm. For thecavity body, oxygen-free copper for electron tubes (grain size requirement conforms to ASTM F68CLASS1) is used, and for the flange, JIS-standard SUS316L is used.
This accelerating cavity has a space on the wall at a certain distance from the center axis andincorporates a HOM absorber for damping HOMs that cause harmful vibrations to the accelerated

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
3electrons, thereby eliminating the instability of the electron beam. The HOM is converted to heat
when absorbed by the ferrite block for UHF band RF absorption on the HOM absorber and the heatis eliminated by cooling water.

Figure 3

2.2

Configuration of HOM-damped RF cavity

Machining

Since the machining of the cavity body requires high resonance frequency accuracy and ahigh Q value (a performance index for RF power loss of the cavity; the higher the Q value, thelower the RF power loss), we used turning (a machining method that applies a cutting tool to amaterial rotating at high speed) to achieve precision machining with a dimensional error of ±0.05
mm or less and a surface roughness of Ra 0.8 µm or less.
When machining copper, it is important to control the temperature (the linear expansion
coefficient is large, so temperature conversion of dimensions is necessary), to maintain the cutting
tool edge (cutting resistance is large and tends to cause plucking), and to prevent rust (rust stains orcorrosion caused by cutting fluid may occur).
After the machining was completed, we cleaned the surface of the machined parts with dilute
sulfuric acid and immersed them in a chromate solution to protect the surfaces against rust.

2.3

Assembly

Oxygen-free copper parts were joined by brazing in a large vacuum heat treatment furnace
owned by us, as shown in Figure 4. The uniform spreading of molten braze over the entire joining
surface results in a strong joint with no gaps, and good electrical and thermal conductivity can beobtained.
During brazing assembly, brazing material selection (considering brazing metal flow andstep brazing temperature), brazing material placement (considering brazing metal flow and amountof brazing material), gap control (considering brazing metal flow and fillet formation), andtemperature control are important.
In the brazing of the cavity body, the temperature was gradually increased over a period of
about 24 hours so as not to cause a temperature difference in the parts. Cooling by natural heatrelease in a vacuum took about 48 hours. The airtightness of the brazed surfaces was confirmed bya helium leakage test with a detection sensitivity of a leakage rate of 1 x 10-10 Pa-m3/sec or less that
there was no leakage.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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The HOM absorber was assembled by a single brazing step. As shown in Figure 5, 12 ferrite
blocks, each with a length of 25.5 mm x width of 10.26 mm x height of 4 mm, were joined to each
HOM absorber.
To confirm that the ferrite blocks were joined with sufficient strength, a load of 4 to 5 kgf
was applied from the side of the ferrite blocks, and no cracking or delamination was observed.
Also, after vacuum baking at 150°C for degassing prior to installation in the accelerating cavity, nocracking or delamination was observed.

Figure 4 Large vacuum heat treatment
furnace

2.4

Figure 5

HOM absorber

Frequency adjustment

The resonant frequency of an accelerating cavity is determined by its internal dimensions,and the frequency is generally adjusted by deforming the whole or a part of the inner wall surface
or by inserting another part to change the volume of the space. For the frequency adjustment of thiscavity, we used a method to precisely adjust the frequency by machining a specific part of the innerwall surface. We confirmed the correlation between the amount of machining and the amount offrequency change by calculation using SUPERFISH, the electromagnetic field simulation code in
two-dimensional Cartesian in advance.
Figure 6 shows the history of frequency adjustment by inner wall adjustment machining.
The slopes of the plots of the calculated values and the frequency adjustment results were almost
identical. Then we predicted processing volume and adjusted to the target frequency.

Figure 62.5

Frequency adjustment history

Low-power RF test

We measured the resonant frequency, unloaded Quality factor, coupling coefficient β, andshunt impedance using a network analyzer. Figure 7 shows the low-power RF test system.
The unloaded Quality factor and coupling coefficient β were calculated using a Smith(7)chart .

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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We measured the shunt impedance using the bead perturbation method. While measuring theresonant frequency of the cavity, the metal bead is pulled at a constant speed through a string on thecentral axis of the cavity. The shunt impedance can be calculated from the change in resonancefrequency.
Table 1 shows the results of the low-power RF test.

Figure 7

Low-power RF test system
Table 1 Results of low-power RF test
Specification value

Measured value (S/N 10014-5)

Resonant frequency(25°C, vacuum)

508.759±0.1 MHz

508.827 MHz

unloaded Quality factor

57000 or higher

59934coupling coefficient β

Around 1.1

1.09

Shunt impedance

6.6 MΩ or higher

6.709 MΩ

|3. Conclusion
In this report, the acceleration cavity manufacturing technology of MHI‐MS was described,
and the acceleration cavity for NanoTerasu was taken up as the latest production activity.
NanoTerasu has begun commissioning in April 2023(8), and a first beam (the first observation ofsynchrotron radiation X-rays from an insertion source installed in a circular accelerator into theexperimental hall) was achieved on December 7, 2023(9). And the operation started as scheduled onApril 1, 2024 (10). This has renewed MHI-MS’s achievements through practical use of the 238
MHz accelerating cavity, 476 MHz Sub Harmonic Buncher and 5,712 MHz (C-band) accelerating
structure, and also through the first practical use of the HOM-damped RF cavity, which we haveworked on since its early development. We will continue to improve our technology to contributeto the planning of new accelerator facilities in the future.

References
(1)
(2)
(3)
(4)

(5)

https://nanoterasu.jp/nanoterasu_online_poster4/index-eng.html
N. Nishimori et al., A 3 GeV Light Source “NanoTerasu”, Vacuum and Surface Science Vol.65 No.12(2022) p.560～565 (in Japanese)
K. Okihira et al., A C-band Accelerator for an X-ray Free-electron Laser (XFEL) Facility, Mitsubishi
Heavy Industries Technical Review Vol.49 No.2 (2012) p.27~31
K. Higa et al., Manufacture of RF Cavities and C-Band Accelerating Structure for the Injector Linac,
Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan, Kyoto, Jul. 31-Aug.3,
2019, WEPI005. (in Japanese)
H. Ego et al., Design of a Hom-Damped RF Cavity for the SPring-8-II Storage Ring, Proceedings of the
11th Annual Meeting of Particle Accelerator Society of Japan, Aomori, Aug. 9-11, 2014, MOOL14. (inMitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
6Japanese)
H. Ego et al., High Power Tests of the Prototype Hom-Damped RF Cavity for the SPring-8-II Storage
Ring, Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan, Kyoto, Jul.
31-Aug.3, 2019, WEOH03. (in Japanese)(7) M.Yoshida, Fundamentals of RF Devices in Electron Linear Accelerators, High Energy Accelerator
Seminar OHO (2008) Chapter 2
http://accwww2.kek.jp/oho/oho08/txt08/02%20yoshida3.080825p.pdf(8) N. Nishimori et al., Commissioning of NanoTerasu accelerator system, Proceedings of the 20th Annual
Meeting of Particle Accelerator Society of Japan, Funabashi, Aug.29-Sep.1, 2023, TUOP02(9) https://www.qst.go.jp/soshiki/152/20231212.html (in Japanese)
The English literature on the same subject can be found at the following address
https://www.tohoku.ac.jp/en/news/university_news/nanoterasu_enters_a_new_phase_historic_first_beam_achieved.html(10) https://www.qst.go.jp/site/press/20240401.html (in Japanese)(6)