Mitsubishi Heavy Industries Technical Review Vol. 51 No. 3 (September 2014)
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Particle Accelerator Products for Advanced Researchand High-precision Radiation Therapy Equipmentusing Accelerator Technology

Advanced Mechanical Systems
Sales & Marketing GroupB u s i n e s s D e v e l o p m e n t D e pa r t m e n t
Business Strategy Division
M a c h i n e r y, E q u i p m e n t & I n f r a s t r u c t u r eParticle accelerators, which are used for advanced research in fields such as high energy
physics and radiation physics, accelerate charged particles (e.g., electrons and protons) to nearly thespeed of light at which particles possess high energy. Mitsubishi Heavy Industries, Ltd. (MHI) has
provided domestic and overseas research institutions with several types of accelerating cavities orstructures, which are the heart of an accelerator. We are also internally developing state-of-the-art
accelerator manufacturing technologies. The ultra-compact accelerating structure we have thus
developed is mounted on our Vero4DRT high-precision radiation therapy equipment (marketed
under the MHI-TM2000 Linear Accelerator System brand name). This report introduces oursuperconducting accelerating cavities, normal-conducting accelerating structures and radiation
therapy equipment as applied products thereof.

|1. Superconducting accelerating cavities
1.1Features

Superconducting accelerating cavities can lower electrical resistance to nearly zero bycooling the niobium constituting the cavities down to an extremely low temperature by using ofrefrigerants such as liquid helium and thereby achieving superconductivity. These cavities aremainly used in accelerators that are operated in high acceleration electric fields and in acontinuous-wave mode. This is because radio-frequency power loss in superconducting
accelerating cavities can substantially be reduced relative to normal-conducting accelerating
structures in which oxygen-free copper is used.

1.2Applications

As superconducting process requires a large-scale refrigerant supply/recovery system, thesetypes of accelerating cavities are mainly used in large particle accelerators for the research ofelementary particle physics. MHI provided the High Energy Accelerator Research Organization(also known as KEK) with a “superconducting crab cavity” intended for the electron-positron
collider (called KEKB). KEKB contributed to the validation experiments that led to the 2008 Nobel
Prize in Physics. As an overseas example, MHI supplied the “superconducting acceleratingmodule” to be installed at the Taiwan Photon Source (TPS) of the National Synchrotron Radiation
Research Center (NSRRC), which is currently under construction.
MHI also has developed and supplied superconducting accelerating 9-cell prototype cavity
for the International Linear Collider (ILC) Project, which attempts to unravel the mysteries of theorigin of matter and the creation of the universe (and there is an expectation that Japan will host thefacility), as well as for the Energy Recovery Linac (ERL) Project, which is involved in research forthe next generation of synchrotron light sources (Figure 1).

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1.3

Manufacturing technologies

In the superconducting accelerating cavity, contamination by even the microscopic impurities
in the niobium or minute defects/projections on the inner surface of the cavity can cause the failureof generating a high electric field. For this reason, the following manufacturing technologies areused.
(1) High-precision forming: A press forming technology to manufacture the hemispherical cells
that constitute the cavity with high accuracy.
(2) High-precision electron-beam welding: A technology that is used to assemble thehemispherical cells, by which the surfaces of a cavity to be assembled are smoothed out
inside of a highly clean vacuum environment to prevent impurity contamination while
precisely welding the cells using an electron beam (Figure 2).

Figure 1
Superconducting accelerating 9-cell
prototype cavity for the ILC Project

Figure 2 Electron-beam welding
machine for superconducting
cavities|2. Normal-conducting accelerating structures
2.1Features

Normal-conducting accelerating structures are mainly made of oxygen-free copper and areoperated at room temperature. Therefore, compared with superconducting accelerating cavities,
these structures require fewer auxiliary devices and the cleanliness control of accelerators can beless strict. On the other hand, as their radio-frequency power loss is greater than superconducting
accelerating cavities, these structures are not suitable for continuous-wave operation, but can createa very high acceleration electric field with pulsed mode operation. The electron acceleratingstructure designed to accelerate electrons (which are of little mass), and the proton acceleratingstructure intended for the acceleration of protons (which are of large mass) are different in theirbasic structures/dimensions. The proton accelerating structure is generally larger in size.

2.2Applications

The electron accelerating structure is used as beam injection into large-scale ring accelerators
at KEK or SPring-8. The main accelerator of the X-ray free-electron laser of RIKEN, known as theSPring-8 angstrom compact free electron laser (“SACLA”), has adopted our original “C-bandchoke-mode accelerating structure.” As an overseas example, we have provided a“quasi-symmetrical S-band accelerating structure” intended for the X-ray free-electron laser that isunder construction at the Pohang Accelerator Laboratory (PAL) of South Korea (Figure 3).
The proton accelerating structure is used in the main accelerators of the Japan Proton
Accelerator Research Complex (J-PARC), which was jointly constructed by KEK and the Japan
Atomic Energy Agency (JAEA). Accelerated protons are beamed to the special targets to generate
neutrons. Efforts to utilize these neutrons for the public benefit have begun, such as used for cancer
treatment or non-destructive inspection (for the industrial sector).

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Figure 3

2.3

Quasi-symmetrical S-band accelerating structure

Production technologies

Because normal-conducting accelerating structures require precise resonance frequencies
control and therefore their required dimensional accuracy is very strict, the following production
technologies are used (Figure 4).
(1) Ultra-precise machining to achieve a surface roughness of 0.1 μm or less and a dimensional
error of 2-3 μm or less.
(2) Low-strain multilayered and dissimilar materials brazing, using one of the largest vacuum
brazing furnaces in Japan (maximum acceptable workpiece: 1.2 m in diameter and 3 m inheight).
(3) Accurate frequency measurement and adjustment, which can cover a wide range of
frequencies (300 MHz to 12 GHz).

Figure 4 Part of an accelerating structure being processed by ultra-precise
machining (left) and the vacuum brazing furnace (right)|3. Vero4DRT – high-precision radiation therapy radiation therapy
equipment
3.1Features

The Vero4DRT (marketed under the MHI-TM2000 Linear Accelerator System brand name)is high-precision image-guided radiotherapy equipment that was developed with a concept ofsophisticatedly integrating beam delivery technologies and imaging technologies. The mostMitsubishi Heavy Industries Technical Review Vol. 51 No. 3 (September 2014)57

distinguishingly unique features are the design of the entire structure, the configuration and thesub-devices mounted on the Vero4DRT, which are intended to realize dynamic tracking
radiotherapy to deliver beams to tumors moving inside the body because of factors such asrespiration.
The Vero4DRT is equipped with our original ultra-compact C-band standing-wave
accelerator, which delivers therapeutic beams (one-third the size of traditional accelerators and isbeing used in medical equipment for the first time anywhere in the world). The Vero4DRT also has
an “irradiation head” containing accelerator mentioned above and a multi-leaf collimator (MLC) by
which the X-ray beam is adjusted to fit the shape of the target tumor, and the irradiation head ismounted on a gimbal mechanism that can be swung. Thus, it has become possible to correct
extremely small displacements of the irradiating direction and perform tracking radiation therapy
onto a tumor moving by breathing.

3.2

Applications

The Vero4DRT has been installed at more than 18 domestic and overseas hospitals,
producing many clinically satisfied outcomes for cancer treatment.

3.3

Production technologies, configuration and capabilities

In radiation therapy equipment, the accuracy of delivering radiation concentrated to thelocation of the cancer while avoiding irradiation to the healthy cells determines the accuracy oftreatment. It is necessary to precisely align the diseased region with the precisely controlled
irradiation area. To realize a mechanical accuracy of ±0.1 mm, which is the top level in industry,
we combined the basic technologies we have accumulated through developments of various
products including (1) micro-positioning technologies (as used in machine tools), (2)image-processing technologies (printing machines), (3) system control technologies (iron-makingmachines) and (4) ultra-compact accelerating structure technologies to generate X-rays.
The equipment is comprised of a highly-rigid O-ring structure (a gantry) approximately 3.3
m in diameter, an irradiation head mounted on the swing mechanism (a gimbal), two perpendicularpairs of X-ray imaging systems (each consists of a kV X-ray generator and a flat-panel detector),and the patient’s bed (a couch) movable in five axial directions (Figure 5).

Figure 5

Mechanical configuration of Vero4DRT

In image-guided radiation therapy, the imaging systems installed on the rotatable gantry
along horizontal axis enable the three-dimensional capturing of a tumor from any direction,
whereby exact alignment can be performed quickly. The rotation of the gantry along vertical axis
allows mechanical movements that had not been possible with conventional equipment, enabling
non-planar irradiation without moving/rotating the couch. Not only does this reduce the patient’s
burden caused by couch rotation in existing equipment, but it also eliminates the patient’s

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positional deviation and maintains the accuracy of patient positioning, which is vital forhigh-precision irradiation. Research for new treatment methods using this innovative gantry
rotation technology is under way.
Using its dynamic tracking irradiation function, the Vero4DRT can capture the movement ofa tumor in real time through the imaging systems and track the tumor using the swing mechanismof the interlocking irradiation head with the system. Irradiation can also be performed incombination with intensity-modulated radiation therapy (Figure 6). This functionality has beendeveloped with the financial support of the Japan Society for the Promotion of Science, based onthe Funding Program for World-Leading Innovative R&D on Science and Technology systemizedby the Council for Science and Technology Policy. Clinical support has been provided by Kyoto
University and the Institute of Biomedical Research and Innovation.

Figure 6

Conceptual diagram of dynamic tracking system