Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
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Manufacturing Techniques of Superconducting
RF Cavities for Heavy Ions

HIROSHI HARA*1

KATSUYA SENNYU*2

AKIHIRO MIYAMOTO*3

TAKESHI YANAGISAWA*1

The 113th element, nihonium, named for Japan, was listed in the periodic table in November
2016.
In response, The Institute of Physical and Chemical Research (RIKEN) launched a project toincrease the beam intensity and energy of the existing heavy ion linac to allow it to further
investigate new super-heavy elements.
In order to realize this enhancement in a limited space, it is indispensable to introduce
superconducting radio frequency cavities, but those made of pure niobium for heavy ions had notbeen developed in Japan. Mitsubishi Heavy Industries Machinery Systems, Ltd. (MHI-MS) worked
with RIKEN on the development of superconducting radio frequency cavities for heavy ions
utilizing its technology for manufacturing superconducting radio frequency cavities for electrons
and has succeeded in manufacturing 10 actual units. This report describes the manufacturing
technology that realized high-performance superconducting radio frequency cavities for heavyions.

|1. Introduction
A heavy ion accelerator is a particle accelerator that accelerates heavy ions, which usuallyrefer to ions of elements heavier than carbon, to a high energy level. Research activities using
heavy ion accelerators include nuclear physics (such as research on synthesis of elements),
elementary particle physics (such as high-energy heavy ion collision experiments), radiotherapy(such as heavy ion radiotherapy, etc.), research on radiation damage to materials, etc.
RIKEN decided to introduce superconducting radio frequency cavities into a heavy ion linac,
RILAC, in the Radioactive Isotope Beam Factory (RIBF) at the RIKEN Nishina Center to increase
beam intensity and energy.
Figure 1 shows a schematic diagram of the RILAC upgrade plan. One of the features of thesuperconducting radio frequency cavity is that it can perform CW (continuous-wave) acceleration
at a high accelerating gradient (in V/m).
The purpose of this RILAC upgrade was to perform CW acceleration with a highacceleration voltage (in V) within a limited length (in m).
In order to achieve this, it was indispensable to apply superconducting radio frequencycavities(1).
A superconducting radio frequency cavity for heavy ions has a complicated shape. Globally,its development is being carried out at accelerator laboratories in the United States and France. On
the other hand, in Japan, the Japan Atomic Energy Agency has operated a tandem booster using aniobium/copper clad material, but a product made of pure niobium has not been developed.
*1

Machinery Engineering Department, Infrastructure Facilities Business Division, Mitsubishi Heavy Industries Machinery*2

General Manager, Quality Assurance Department, Infrastructure Facilities Business Division, Mitsubishi Heavy Industries*3Manager, Machinery Engineering Department, Infrastructure Facilities Business Division, Mitsubishi Heavy Industries

Systems, Ltd.
Machinery Systems, Ltd.
Machinery Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
2Since the late 1980s, we have accumulated technologies to manufacture superconducting
radio frequency cavities made of pure niobium for electrons that achieved both a high acceleration
gradient and a high Q value, which is a performance index for high frequency loss in cavities (thehigher the Q value, the smaller the ratio of high frequency loss).
In recent years, these performances we offer have satisfied the required specifications of theInternational Linear Collider (ILC) project.
Utilizing these accumulated technologies, we worked on the production of 10
superconducting radio frequency cavities for heavy ions in a short period from 2017 to 2018.

Figure 1

Schematic diagram of RILAC upgrade plan

|2. Outline of superconducting radio frequency cavity for heavy ions
2.1

Configuration

A superconducting radio frequency cavity has a double-vessel structure composed of a vessel
made of a superconducting material (niobium) and a vessel covering the outside thereof.
An electric field is formed inside of the niobium cavity by high-frequency power input fromthe outside and heavy ions get energy from the electric field and accelerate.
Between the vessels, liquid helium is stored and cooled to 4.5 K.
Since the niobium cavity becomes superconducting, the power loss during operation due toelectrical resistance can be minimized.
Superconducting radio frequency cavities are assembled (modularized) with input couplers
for inputting high frequency power, tuners for adjusting the resonance frequency of thesuperconducting radio frequency cavities, magnetic shields for shielding magnetism that affects
superconducting properties, thermal shields for reducing heat load on the superconducting radio
frequency cavities, vacuum chambers for adiabatic retention of superconducting radio frequency
cavities, etc., which are then installed in a beam line. We also designed, manufactured andassembled these module components. Figure 2 presents the configuration of the superconducting
radio frequency module for RILAC.
Figure 3 illustrates the superconducting radio frequency cavities for heavy ions being
modularized.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
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Figure 2 Configuration of superconducting radio frequency module for RILAC

Figure 3 Superconducting radio frequency cavities for heavy ions being
modularized

2.2

Design of superconducting radio frequency cavity for heavy ions

The typical shapes of superconducting radio frequency cavities are classified according to theresonance frequency and the velocity of the accelerating particles. Figure 4 gives the classificationof typical shapes of superconducting radio frequency cavities(2).

Figure 4 Classification of typical shapes of superconducting radiofrequency cavities(2)The superconducting radio frequency cavity for RILAC is called a quarter-wave resonator(QWR) and the radio frequency design was carried out by RIKEN.
Table 1 shows the design parameters of the superconducting radio frequency cavity forMitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
4(3)RILAC .
Table 1

Design parameters of superconducting radio frequency cavity for RILAC(3)Item
Resonance frequency (4.5 K)
Accelerating gradient Eacc
Particle velocity βopt
Performance index for high frequency loss Q0

Design value73.0
6.75
0.078
1.0×109[MHz][MV/m]

Since superconducting radio frequency cavities store liquid helium, they are subject to theHigh Pressure Gas Safety Act. There was a problem in that the evaluation formulas of applicablelaws and regulations could not be used due to the special materials and shape, but criteria for thesafety evaluation of the material strength and structural strength were defined through the detailed
standard pre-evaluation system of the High Pressure Gas Safety Institute of Japan, and thestructural design was carried out after obtaining the approval of the relevant ministries andagencies.
Figure 5 presents an exploded view of the superconducting radio frequency cavity for
RILAC.
The major parts were shaped from pure Nb sheets (purity 99.85% or more) of 3.5 mm or 4.0
mm in thickness by press-forming.

Figure 5 An exploded view of thesuperconducting radio frequency cavity for
RILAC|3. Superconducting radio frequency cavity manufacturing
technology that achieves high acceleration gradient and high Q
value
3.1

Electron beam welding (EBW)

Impurities on the inner surface of a superconducting radio frequency cavity become heatsources and lead to deterioration of the superconducting characteristics.
Such impurities include not only particulate matter such as grit and dust, but also gas
components such as oxygen and nitrogen.
Therefore, it is necessary to assemble parts in a cleanroom and to perform welding invacuum.
In order to meet this requirement, we have a shop dedicated to accelerator manufacturing,which has an ISO Class 7 (formerly Class 10000) cleanroom with a 10-kw low-voltage electron
beam welder.
Since defects in the welding process cause suppression of the performance of thesuperconducting radio frequency cavity to be restrained, a wide variety of items are managed, suchas the thickness, gaps and steps of welding grooves, the cleanliness of welding groove surfaces, thecleanliness of assembly areas and the stability of welding parameters.
This time, all of the welding processes were performed from one side and a smooth weld

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
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bead is formed on both the front and back sides without mechanical polishing. Figure 6 shows anexample of a weld bead.

3.2

Frequency tuning

The resonance frequency of a superconducting radio frequency cavity is determined
depending on the shape of its inner surface but may deviate from the design value depending on themanufacturing accuracy during the manufacturing process.
The frequency was measured during the manufacturing process and the part length wasadjusted.
As a result, the error of the resonance frequency at the completion of welding assembly waskept within a few kHz.
This is equivalent to processing and welding assembly with an accuracy of 1000 mm for thetotal length of the superconducting radio frequency cavity of about 0.1 mm. Figure 7 shows theresonance frequency measurement performed at room temperature.

Figure 6

3.3

Example of weld bead

Figure 7 Resonance frequency measurement(at room temperature)

Buffered chemical polishing (BCP)

The surface treatment of superconducting radio frequency cavities is a very important
process that affects the performance thereof. In order to finish the inner surface smoothly,
electro-polishing or buffered chemical polishing is usually performed.
This time, we introduced in-house surface treatment equipment for buffered chemicalpolishing.
As the buffered chemical polishing solution, we adopted a mixed solution of nitric acid(HNO3) that oxidizes the niobium surface, hydrofluoric acid (HF) that dissolves the niobium oxide
film (Nb2O5) and phosphoric acid (H3PO4) that suppresses the polishing rate. It should be notedthat in the buffered chemical polishing of niobium, the polishing speed rises sharply when theliquid temperature increases due to the heat of the polishing reaction and polishing with the residual
chemical solution progresses if it takes time to drain the liquid after polishing. Therefore, weconstructed a circulation system to control the liquid temperature and flow rate. As shown in thetitle photograph, the inner surface of a superconducting radio frequency cavity after surface
treatment resulted in a semi-mirror finish unique to buffered chemical polishing.

3.4

Vacuum heat treatment

Vacuum heat treatment is the process of removing hydrogen occluded in niobium material
during buffered chemical polishing. Exposing the superconducting radio frequency cavity withresidual hydrogen to a temperature of about 100 K for a long time increases the surface resistanceand causes its performance to be restrained. We carried out vacuum heat treatment for 3 hours inthe large vacuum heat treatment furnace owned by our company depicted in Figure 8.

3.5

High-pressure rinse (HPR)

High-pressure rinse is the process of injecting high-pressure ultrapure water on to the innersurface of a superconducting radio frequency cavity after buffered chemical polishing to obtain aclean surface. Foreign matter (particulate matter and chemical residue) remaining on the innersurface of the superconducting radio frequency cavity becomes the basis of field emission.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
6This time, we introduced in-house high-pressure rinse equipment.
Based on the results of superconducting radio frequency cavity for electrons, thespecifications of high-pressure rinse were set to a water pressure of 8 MPa and an electricalconductivity of 18 MOhm-cm or more
The flange assembly after high-pressure rinsing was carried out in a cleanroom adjacent to
the high-pressure rinsing equipment.
In consideration of installation cost, maintainability and assemblability, an openable ceilingsystem and a horizontal clean air flow system were adopted for the specifications of the cleanroom.
As a result, the cleanliness performance of ISO Class 4 (formerly Class 10) was obtained,
which is comparable to the performance of general methods that use HEPA filters in the outlets,
and walls, ceilings, or floors as suction ports.

Figure 8

Large vacuum heat treatment furnace|4. Performance of superconducting radio frequency cavity forheavy ions
RIKEN has confirmed the performance of 10 superconducting radio frequency cavities that
had completed the welding process.
As shown in Figure 9, each of the superconducting radio frequency cavities was placed in a
vertical test stand constructed at the RIKEN campus. They were then cooled to 4.2 K with liquid
helium and a low power radio frequency was input to measure the accelerating gradient and the Qvalue.
Figure 10 shows the results of the vertical performance test (3).
Stable performance exceeding the specification value was obtained in all 10 superconducting
radio frequency cavities for heavy ions, and RIKEN evaluated this result as follows (1).
- The integrity and validity of the series of manufacturing processes (machining, welding andsurface treatment) of the superconducting radio frequency cavities were confirmed.
- The surface conditions of superconducting radio frequency cavities that were very close toideal were realized.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
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Figure 9
Installation of bulk
superconducting cavity
into the test cryostat forvertical testing (VT).

Figure 10

The results of the vertical performance test (3)|5. Conclusion
By utilizing and developing our technology for manufacturing superconducting radio
frequency cavities, we succeeded in manufacturing Japan’s first superconducting radio frequency
cavity made of pure niobium for heavy ions. All 10 of the superconducting radio frequency cavities
were installed in the subsequent stage of RIKEN heavy ion Linac (RILAC) and are currently being
operated day and night in order to synthesize 119th element and other new elements. Heavy ion
accelerators are being built or planned in various places around the world, and are also being
considered as next-generation accelerators in Japan. We will contribute to the technologicaldevelopment thereof.

References
(1)
(2)
(3)

Naruhiko SAKAMOTO, Development of Superconducting Linac-Booster for Low-beta Heavy Ions at
RIKEN RIBF, Journal of the Particle Accelerator Science of Japan Vol.17 No.2 (2020），p.70~80
M. Kelly, TEM-class Cavity Design, SRF2013, Paris, France, Sep. 2013, Tutorial Program
K. Yamada et al.，CONSTRUCTION OF SUPERCONDUCTING LINAC BOOSTER FOR HEAVY-ION
LINAC AT RIKEN NISHINA CENTER, in Proc. SRF2019, Dresden, Germany, Jul. 2019, TUP037