The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DEVELOPMENT OF A MELCO HIGH INTENSITY MICROTRON FOR
INDUSTRIAL APPLICATIONS
T. Nakanishi, H. Tanaka, Y. Pu, T. H. Kim, Y. Makita, A. Maruyama, C. Tsukishima, S. Yamamoto,and S. Nakamura, Mitsubishi Electric Corp., Advanced Technology R & D Center
8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo, 661-8661, Japan
Abstract
A high intensity microtron operated in a cw mode isunder development at MELCO for industrial applications.
The machine is of a racetrack type using a 500 MHz rf
cavity with a total gap voltage of about 1 MV.
Nonsynchronization by change of electron velocity ofeach turn can be compensated by adjusting orbit length ofeach turn by bending magnets with two sections. A 5
MeV-50kW machine, which is suitable for x-ray
sterilization, is under construction to prove this newmethod. A 500 MHz cw gun has been also developed,
and beam tests have showed that the emitted beam
parameters agree well with the calculated results. The
beam acceleration test will be carried out within this year.

INTRODUCTION
More application of a high intensity electron
accelerator is expected since irradiation processing byelectrons and x-ray is safe and environmentally friendly.
High intensity accelerators such as Dynamitron of anelectrostatic type with 5MeV-200kW, and Rhodtron of an
rf type with 10MeV-150kW are used for irradiation [1].
The authors have been developing a compact machineof a self-shielding type which is of a racetrack microtron
type and has a beam power of 50~100kW. Operation in acw mode is effectual to supply a high intensity beam. A
conventional racetrack microtron with an acceleration
cavity of S-band, however, is not suitable for the cw
operation because of heat dissipation in the cavity. Use ofa cavity with a lower acceleration voltage can not satisfy
the condition for synchronous acceleration because
electron velocity changes every turn [2]. The authers
have proposed a new method with adjusting orbit lengthof each turn by bending magnets with two sections andadopting a 500 MHz cavity [3]. An electron gun with
500MHz cw operation has been also developed for alonger lifetime. A 5 MeV-50 kW machine is underconstruction to prove this new method. Beam tests of thegun and a high power test of the cavity have been alreadyfinished.
This paper describes the design, the fabrication, and thetests of the 5 MeV-50kW machine.

therefore the self-shielding can be made easily. The
operation frequency is selected to be around 500 MHz in
consideration of suitability for the cw operation and thetotal size of the machine. The total gap voltage of thecavity is about 1 MV. A higher gap voltage results in areduction in the electric power efficiency of the machine,where the power efficiency is very important for theseindustrial applications.
An 80 keV electron beam from the gun is injected usinga chicane magnet with three bending magnets. The orbit
radius in the BM is not large enough for the first orbit toclear the cavity because of a low energy gain of 0.7 MeV.
The electrons after the first pass through the cavity,
therefore, are displaced by an inverse bending magnet(IBM) and then directly reflected back into the cavity by
the BM. The IBMs are also useful for the verticalfocusing for every turn in the edge regions of the BMs.
The transverse focusing is done by only the edge focusing
and the two quadrupole magnets placed at both sides ofthe cavity.

Figure 1: Schematic drawing of the 5 MeV cw microtron

BEAM DYNAMICS DESIGNA 5MeV microtron is shown in Figs. 1 and 2. Maincomponents are an 80 keV electron gun, two main
bending magnets (BM), and an rf cavity. The height ofthe machine is as low as 1.1 m at the top of the cavity andFigure 2: Photograph of the 5 MeV cw microtron

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

This machine parameters can not satisfy the conditionof synchronous acceleration like a conventional racetrack
microtron because electron velocity changes every turn,where the energy gains per pass are around 0.85 MeV
after the first pass. The nonsynchronization can becompensated by adjusting orbit length of each turn by theBMs with two sections as shown in Fig. 1, and the lower
acceleration frequency such as 500 MHz assists thecompensation. The section of the dark color of the BM inFig. 1 has a larger gap and therefore a weaker field. Thesize and the field strength of the section are optimised
with a beam simulation using the 3D field distribution soas to obtain a large phase acceptance. The optimised fieldstrengths are about 0.025 T at that section and about 0.03
T at the other section, respectively. The optimised beam
orbits are shown in Fig. 3 with calculated and measured
fields. Dots in the figure are 1000 particles’ simulation
results using the measured magnetic fields. The
calculation conditions are as follows: initial acceleration
phases are 0, +10, and –10 degrees, and horizontal andvertical beam emittances are 50 π mm-mrad, respectively.
The study shows that an electron beam can be accelerated
up to 5 MeV with practical beam sizes (x= ±18 mm, y=±12 mm). The phase acceptance considering the beam
aperture is 15 degree with an energy spread of 1.4 % at
FWHM.
An accelerated energy of 10 MeV can be obtained withFigure 3: Beam orbits
calculated using calculated
magnetic fields (lines) andmeasured magnetic fields(dots). Calculation
conditions of the dots:
initial acceleration phases
are 0, +10, and –10 degrees
and a beam emittance is 50π mm-mrad.
the use of two acceleration cavities and a stronger
bending field of about 0.06 T. This energy is the upper
limit for direct electron irradiation. The beam power is
100 kW with the same beam current as the 5 MeVmachine.
The basic parameters of the 5 MeV and 10 MeV
machines are listed in Table. 1.
Table 1: Basic parameters of MELCO microtrons.

Beam power
Beam current
RF frequency
RF power
Accelerator
dimension
Weight

5 MeV
machine
50 kW
10 mA
500 MHz90 kW
1600 × 3900
× 1100 mm
10 ton10 MeVmachine
100 kW
10 mA
500 MHz
180 kW1600 × 4700
× 1100 mm
11 tonELECTRON GUN AND INJECTION LINEThe gun is a conventional triode type with a 0.5 cm2
dispenser cathode, and is operated in a cw mode. Toobtain a pulse beam for a longer lifetime of the gun, thevoltage between the grid and the cathode is oscillated by a
500 MHz signal synchronized with the acceleration
frequency [4]. The peak current is changed by varying
the grid bias and the rf voltages.
The 80 keV electron beam from the gun is transported
through two solenoid magnets, a 45 degree bending
magnet, and a quadrupole magnet, and is injected using achicane magnet with three bending magnets.

BENDING MAGNET
The BM has a pole face as large as 1650 mm × 830 mm.
The gap lengths are 46.0 mm and 54.25 mm, respectively.
Several blocks of SUS316 are put in the gap to obtain thegap uniformity with an accuracy of ±0.05 mm. The large
gap section of the pole was machined with a millingmachine, and an additional pole was attached to the edgeof the BM.
The magnetic field distribution on the beam orbit plane
calculated with TOSCA is shown in Fig. 4. Ununiform
region of the magnetic boundary between the two sections
is 70 mm and small enough compared with an interval of
adjacent beam orbits, about 120 mm, where theununiformity is worse than ± 1×10-3.
Figure 4: Magnetic
field distribution of
the bending magnet
calculated withTOSCA.

500 MHZ RF SYSTEM
A schematic drawing of the rf cavity and a photographof the rf cavity installed in the microtron are shown inFigs. 5 and 6. The rf cavity is a conventional 2-cell cavity
with nose cones and inductive coupling slots, which isfrequently used for an electron storage ring. The outer
diameter is 0.51 m and the length is 0.63 m. It is made ofoxygen-free copper. Two inductive tuners are mounted toadjust the balance of the electric fields in the two cells
and resonant frequency.
They are also used tocompensate a shift of resonant frequency due to variationsof temperature. The required total gap voltage is about 1MV. The cavity is evacuated within 1×10-4 Pa using aturbomolecular pump with a capacity of 500 L/s.
The rf source with two IOTs (Inductive Output Tube)
generates 100 kW cw power, where the IOT is frequently
used for broadcasting systems. The power is fed into theThe 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Figure 5: Schematic
drawing of the rfcavity.

Figure 6: RF cavity
placed in the microtron
and a coaxial line for rf
power feed.

A precise beam current from the gun was measured byreplacing the connector type with a large Faraday cup. A
peak current was evaluated from the average current
measured and the above-mentioned bunch structure.
Figure 8 shows the peak current measured as a function of
the grid peak voltage together with the calculated values.
It is found that they have a good agreement. A peakcurrent of 250 mA is required for an average current of 10mA giving the 50 kW beam power.
Figure 9 shows the emittance measurement results
together with the calculated ones. They were measured
by the use of the quadrupole magnet and fluorescentscreen. The measured emittances are around 60 π mmmrad for both the x and the y emittances, and agree wellwith the calculated results.
A beam transport test of the injection line was
performed placing the beam monitors at a position of thecavity before the cavity was installed. The measured
beam sizes of about ±5 mm horizontally and ±4 mm
vertically correspond to the calculated ones, and a beam
can be transported within an accuracy of ± 0.2 mm.
Figure 8. Measured
and calculated peakcurrents as afunction of the gridvoltage.

cavity through a loop coupler and a coaxial line. A wave
guide is not used for a compactness of the system.
A balance of the gap voltages in the two cells has beenadjusted within 1 % using the tuners together withresonant frequency tuning of 500 MHz. The unloaded Q
value measured 31000, which corresponds to 82 % of thecalculated value. An rf power of 40 kW required for thedesigned total gap voltage has been supplied into thecavity[5].

Figure 9: Beamemittances as afunction of thegrid voltage.

BEAM TESTS OF THE GUN AND
INJECTION LINE
Beam tests were performed with a straight line
consisting of two solenoid magnets, a quadrupole magnet,
and beam monitors which are a fluorescent screen and a
Faraday cup. To observe the bunch structure, an N-typeconnector was used as the Faraday cup because theelectric capacitance of the detecting part is very small,though it is smaller than the beam size. The detectedsignal is observed with a 50 GHz sampling oscilloscopeas shown in Fig. 7. The bunch width is about 300 ps andabout 60 degree in a phase length, which corresponds tothe calculated value well. The phase stability of the beammeasured within 1 degree.
Figure 7: Bunchstructures with a50 GHz sampling
oscilloscope: (1ns/div. and 5mV/div.).

SUMMARY
The high intensity microtron operated in a cw mode isunder development. The new method has been proposedfor the cw operation. A 5 MeV machine is underconstruction and the beam tests of the electron gun andthe injection line were done successfully. The beamacceleration test will be carried out within this year.

REFERENCES[1] J. Pottier, “A new type of RF electron accelerator: therhodotron”, NIM B 40/41, (1989) 943.
[2] P. Lidbjork, “Microtrons”, CERN 94-01, Vol. 2,(1994), 1994.
[3] H. Tanaka, “Beam Dynamics in a CW Microtron for
Industrial Applications”, EPAC 2000, Vienna, (2000),1005.
[4][5] Y. Makita, et al., .,"HIGH POWER TEST OF A 500
MHZ RF CAVITY OF A CW MICROTRON FOR
INDUSTRIAL APPLICATIONS", in these proceedings.