Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)
26Particle Accelerator Products for
Advanced Research and Technology Development

M A C H I N E RY & S T E E L
HE ADQU ARTERS

STRUCTURESA D VA N C E D T E C H N O L O G Y D E PA R T M E N T

In the fields of nuclear physics and elementary particle physics, accelerator cavities andstructures are used to accelerate particles (primarily electrons, positrons, and protons) to highenergies. Mitsubishi Heavy Industries, Ltd. (MHI) has been developing a variety of production
technologies in order to provide accelerator cavities and structures needed for new research andexperiments. Described here are a superconducting accelerator cavity, conventional
accelerating structures, and their derivatives intended for medical use.

|1. Superconducting accelerating cavity
1.1

Characteristics

The superconducting accelerating cavity requires that liquid helium be used as a refrigerant
to cool the niobium cavity to achieve superconductivity. These cavities are used mainly inaccelerators that are operated at high electric fields and in continuous-wave mode. This is because
high-frequency losses in superconducting accelerating cavities are as small as one hundred
thousandth to one millionth of the loss in normal-conducting accelerating structures that usecopper.

1.2Applications

The superconducting accelerating cavity is used mainly in accelerators intended forlarge-scale research, because such accelerators need a large refrigeration system to maintainsuperconductivity. MHI developed and supplied a superconducting crab cavity intended for anelectron-positron collider (KEKB) for a high-energy accelerator research organization (KEK). KEK
contributed toward a validation test that was performed for the 2008 Nobel Prize in Physics(Figure 1).
MHI has also been developing a 1.3-GHz superconducting accelerating cavity for theInternational Linear Collider (ILC) project, which is a linear electron-positron collider project thataims to further understanding of particle physics, and the Energy Recovery Linac (ERL) project,which is involved in research for the next generation of synchrotron light sources.

Figure 1 Superconducting accelerating
cavity intended for ILC R&D(Courtesy of KEK)

1.3

Production technologies

Because superconducting accelerating cavities cannot achieve a high enough electric field ifeven small amounts of impurities are present in the niobium or if there are any defects or bumps on

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3(Sep. 2009)27

the inner surface of the cavity, the following production methods are used:(1) High-precision forming technology: A technology to precisely form the hemispheric cells that
constitute the cavity (Figure 2).
(2) High-precision electron-beam welding: A technology to precisely weld cells directly using anelectron beam in an ultra-high vacuum so as to reduce the intrusion of impurities andsmoothen the inner surface of the cavity when cells connect to assemble the cavity.

Figure 2 Outline drawing of thehydroforming method, utilizing
counter pressure fluid, of thelarge-diameter thin-wall cell usedinthesuperconducting
accelerating cavity|2. Normal-conducting accelerating structure
2.1

Characteristics

Normal-conducting accelerating structures operate at room temperature and therefore requireless auxiliary equipment and less maintenance than superconducting cavities. These structures havegreater high-frequency loss than superconducting accelerator cavities and are therefore notappropriate for continuous-wave operation, but they can achieve a high electric field in
pulsed-mode operation.

2.2Applications

Normal-conducting accelerating structures are supplied to KEK and to Spring-8, both ofwhom use them for beam injection into large-scale ring accelerators. They are currently being
mass-produced and are the main accelerators used by the X-Ray Free Electron Laser (XFEL)
project, which is currently under construction by the Institute of Physical and Chemical Research(RIKEN). They are also used as intensifiers of the main accelerators by the Japan Proton
Accelerator Research Complex (J-PARC). J-PARC is a high-intensity proton accelerator
constructed jointly by KEK and the Japan Atomic Energy Agency (JAEA), which conducts
neutrino experiments and promotes the industrial use of neutrons. They are also used in medical
equipment and in electronic sterilization systems.

2.3

Production technologies

Because normal-conducting accelerating structures require precise resonant frequencies, thefollowing production technologies are used.
(1) An ultra-precise lathe to achieve a surface roughness of <0.1 μm and an error of <3 μm(Figure 3).
(2) Low-strain multilayer and intermaterial brazing technologies (Figure 3), realized by using oneof the largest vacuum brazing furnaces in Japan (accepting work pieces 1.2 m in diameter and
3 m in length).

Figure 3 Cell for the C-band
accelerator tube under highprecision processing (left) andthe brazing furnace (right)

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3(Sep. 2009)28|3. Application to medical equipment
3.1

Characteristics

The MHI-TM2000, which is a highly advanced radiotherapy machine with its own
accelerator technologies that has been on the market since last year (see Figure 4), isacknowledged as patient-friendly equipment. Downsizing the accelerator is a key aspect in creatinga high-precision compact irradiation head that can be driven to rotate in pan and tilt directions andcompensate for the mechanical distortion caused mainly by its weight. The mechanical distortion
problem was solved by using an ultra-light and compact C-band (5.7 GHz) accelerator for which
the length was reduced to one third of the conventional structure. This C-band accelerator canrealize the application of a high-power klystron, which offers stability and a higher frequency.

Figure 4 TM2000
radiotherapy machine
and the C-band
accelerator tube

3.2

Applications
TM2000 machines are distributed to hospitals both at home and abroad for clinical treatment.

3.3

Production technologies

This machine contains various MHI elemental technologies such as image processing,technologies for normal-conducting accelerating structures explained in article 2 and technologies
for precise-positioning machine tools.