The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Development of the Industrial Electron Accelerator Ridgetron
Noriyosu HAYASHIZAKI, Toshiyuki HATTORI, Masatoshi ODERA, Kouichi NAKAYAMA·,Osamu TAKEDA·, Shinichi TAKAMA·, Akiko YAMAGUCHI', Katsushi ISOKAWA*,Kiyokazu SATO*, Keiko IWASAKI*, Takashi FUJISAWA**
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology
2-12-1 0-okayama, Meguro-ku, Tokyo 152-8550, Japan*Toshiba Co.
1-20 Kansei-c~o, Tsurumi-ku, Yokohama-shi 230-0034, Japan··National Institute of Radiological Sciences
4-9-1 Anagawa, lnage-ku, Chiba-shi 263-8555, Japan

Abstract
A compact RF electron accelerator, the Ridgetron, with aridged cavity .resonator has been developed for industrialirradiation. It is of a recirculating acceleration type and isintended to have an energy range of 0.5-10 MeV and a beam
power of 1-25 kW. A prototype system with an energy of
2.5 MeV and a beam power of 6.5 kW was constructed toconfirm the feasibility.

1 Introduction
Many electron accelerators have been widely used forindustrial electron irradiation (curing, crosslinking ofpolymers, sterilization of medical disposables, preservationof food, etc.). Regardless of the acceleration energy, theaccelerators to be installed in industrial facilities have tosatisfy the requires of compact size, low running cost andstable operation. A DC electron accelerator with high beam
power is stable technique, however, its acceleration energy
is up to 5 MeV because of the discharge of an acceleration
column.
We have developed a RF electron accelerator system witha ridged cavity resonator for high-energy industrial
irradiation such as the sterilization of medical diposables. It
is of a recirculating acceleration type and is the firstexample of an electron accelerator which incorporates theridged cavity. Its power efficiency increases in proportion toa number of recirculating acceleration by the same cavity up
to the final energy. Since an acceleration voltage per pass ischosen relatively low in the recirculating type of accelerator,
the heat loss of the cavity can be kept small resulting insignificant power savings and the cw mode operation isfeasible. This accelerator system is named the Ridgetron. It
is intended to accelerate electrons with an energy range of
0.5-10 MeV and a beam power of 1-25 kW. As the first step
to confirm the feasibility, a prototype system with anacceleration energy of 2.5 MeV has been developed.

2 Acceleration scheme
The Ridgetron consists of a cylindrical ridged cavity andsmall bending magnets at its periphery. The cross sectionalview of the prototype machine is shown in Fig. 1. The

ridged cavity which is adopted as the acceleration cavity has
the characteristics as the followings: (1) Its resonant
frequency is lower than that of a same size cylindrical cavity
without a ridged structure, (2) The electric field distribution
can be optimized by varying the ridge shape, (3) The
increase of an acceleration energy can be easily realized byusing an axially longer cavity. The operating frequency ofthe Ridgetron is in a VHF band to allow use of a gridded
power tube for its RF system. Therefore it easily becomesfeasible to realize large beam power by operating in cwmode.
The acceleration scheme of the Ridgetron has been
already described in the previous paper [1], and issummarized briefly here. In the acceleration cavity, twohollow ridge electrodes are equipped in the axial direction.
The electric field excited by the TE110 mode is concentrated
in the gap between the two ridges. The beam modulated by
RF voltage applied to the grid of the electron gun tosynchronize with the acceleration phase of RF field isinjected into the cavity. It is passed through the inside of theridges and the acceleration gap in the radial direction anddeflected with the bending magnet after each pass. By
adjusting the deflecting field of an arbitrary magnet to bezero, it is possible to extract the beam below the final energy.
The acceleration period is every half cycle of the operating
frequency. The drift length at an interval of acceleration isextended in proportion to the electron velocity to keep thesynchronism. The beam focusing is performed with the edge
focusing of the bending magnets equipped with the active
field clamp to compensate the effect of fringing field.

3 Prototype design
3.1 Main cavity

The design performance of the prototype system waschosen at an acceleration energy of 2.5 MeV and a beam
power of 6.5 kW that are quarter of the industrial machine.
The cavity size is defined by its fundamental resonant
frequency. The cavity diameter was optimized by varying
the gap size and the ridge shape in a frequency range of 100200 MHz. Although cavity size becomes compact by use ofhigh frequency, beam cannot pass through the inside ofridges of narrow width required to keep high quality factor-269-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Table 1 The specifications of the Ridgetron prototype

Operating frequency
Input energy
Output energy
Beam power
Maximum gap voltage
Cavity inner diameter
Cavity inner length
Gap length
Ridge width
Quality factor
RF ohmic power loss

Bending
MagnetDesigned

Improved

100
2092.8 [MHz]20 [keY]
25 [MeV]
7.0 [kW]
0.5 [MY]966 [mm]
990 [mm]
140 [mm]
80 [mm]21000
50 [kW]

2565
0.5
966
990
140
80
27000
42

3.2 Electron gun

Fig. 1 The cross sectional view of the Ridgetron prototype

at the high resonant frequency. By choosing ridge width togive reasonable aperture for beam, the resonant frequency of
100 MHz was found to satisfy conditions of a compact sizeand low power consumption after optimization. Since theridges function in the cavity as a capacity, the resonant
frequency is 45 % lower than that of the same size
cylindrical cavity without ridges. The electric field
distribution was measured with a quarter scale cold modelf~bricated by using the calculated result. The overhangs ofndge ends have effects to improve uniformity ofacceleration field along the ridge, so that the field drops at
the first and final acceleration gaps are less than 5 %,compared to those at the central region of ridge. The cavity
wall and the ridges are cooled by deionized water. RF poweris fed into the cavity by inductive coupling with a loop
coupler. The maximum output power of the amplifier is 60kW. The specification of the prototype system is shown inTable 1.
The number of passes in the Ridgetron are limited by thecavity length and the deflecting radius of the bending
magnet. Therefore small bending magnet with a deflecting
radius of 65 mm was designed. The energy gain of theprototype is 05 MeV per pass to keep down the power
consumption, so that the number of pass is five. Although
the magnets are placed with space of only 20 mm between
neighbors, each field is not disturbed as to affect beam
trajectories.

The prototype system must accelerate an averaged currentof 2.5 rnA to satisfy the designed beam power. Therefore theelectron gun has to produce a peak current of 20 rnA or
more when the longitudinal beam acceptance is taking intoaccount. A thermionic electron gun with the cathode madeof a single crystal of LaB 6 was manufactured. The electrodeconfiguration was optimized with the computer code
EGUN2e and the flat type of Whenelt electrode was adopted.
It can operate in both of cw mode and pulse mode by changeof modulation mode of the grid electrode voltage. The beam
current measured at the bench test was 30 rnA.

3.3 Beam focusing
The beam focusing of the Ridgetron is performed with anedge focusing and an active field clamp of the bending
magnets. A magnet with a gap in its pole necessarily has
fringing field, which reduces the focusing in the verticalplane. This effect is enhanced in proportion to the ratio ofgap length and deflecting radius of the magnet [2].
Therefore the active field clamp which consists of a clamp
uon and a sub-coil to excite an inverse field was developed[1]. It appropriately compensates a fringing field byadjusting the exciting current of the sub-coil. The results
were verified with the computer code Power Trace which
simulates a beam envelope including a space charge effect.
In the calculation, the magnetic field region of the bending
magnet with fringing field was separated into 17 segments
and the field inside each segment was assumed uniform. The
beam trajectory at the bending magnet is deflected morethan 180 degrees because of use of the inverse field partly,
where beam is deflected in reversed direction. By thescheme, a good focusing effect was obtained with themodest edge angle. The calculated beam envelope is lessthan ±10 mm throughout acceleration trajectory.

-270-

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

4 Prototype experiment

5 Conclusion

The prototype system was constructed at Toshiba
Corporation. Fig. 2 shows the view of the experimental
setup. The first resonant frequency was 101 MHz and themeasured quality factor was 23300. The electric field
distribution measured by perturbation method was in good
agreement with that of the calculated field.
The acceleration test with a low current beam has been
carried out by using the first acceleration gap in the pulsemode operation. The beam profile was measured byfluorescence of ZnS(Ag) targets placed at the straight duct
attached to the bending magnets. As a result, it wasobserved that the accelerated beam was deviated. by theeffect of RF magnetic field. Therefore the prototype wasimproved by equipping the ridges with metal blocks tominimize RF magnetic field in the acceleration gap. The
improved resonant frequency is 92.8 MHz and the quality
factor is 21000.
The cavity and the beam chamber of the bending magnets
are evacuated with a turbo molecular pump to keep thevacuum pressure of 10-5 Pa in normal operation. The
electron gun and the acceleration cavity are separable by agate valve and an ion pump is provided for the electron gun
to secure high vacuum.

A cw electron accelerator with a ridged cavity has beendeveloped for industrial irradiation. As the first step, theprototype system was constructed. The electrical property isin good agreement with the design value. The beam test up
to the final energy has been prepared at present.

References[1] N. Hayashizaki, T. Hattori, M. Odera, Nucl. Instr. andMeth A427 (1999) 28-32[2] K. G. Steffen, High-Energy Beam Optics, Interscience
Monographs and Texts in Phisics and Astronomy,
Vol.17 (John Wiley and Sons, Newyork, 1965) 97-101

Fig. 2 The experimental setup of the Ridgetron prototype-271-