CONCEPTUAL DESIGN FOR A DOUBLE-SIDED MICROTRON
K.Hayakawa, K.Tsukada, K.Sato and T.Tanaka
Atomic Energy Research Institute, Nihon University
Abstract
A study of a C.W. 1-GeV double-sided microtron for medical
use of pions and a source of free electron laser has been started
at Nihon University. In this paper, we propose an idea for beamtransport system to reduce vertical beam defocusing and makemagnetic field uniform along the orbit of the first turn.
Introduction
A principle of the double-sided microtron was proposed atmiddle of the 1950's. However, because of strong vertical defocusing for the low energy region at the field edge of bending
magnets, no double-sided microtron has been built.
A microtron type electron accelerator, which includes
clasicale, race-track (RTM) and double-sided (DSM), can accelerate in a small room in comparison with a linac for same beamenergy. The clasical microtron employs a uniform magnetic
field. This configuration is not suitable for heigh-energy C.W.
applications because of limitation of the energy gain per turn.
This limitation is eliminated by splitting the guide field magnet
into two equal halves, allowing the insertion of an accelerator
section of arbitrary length, which is the RTM. The RTM appears
to be an excellent choice for energy up to 500-800 MeV, fromconsideration of both cost and performance. In the geometry ofthe RTM, at constant B, the weight of the end-magnets increace
as E3, becoming excessive for energies above about 800 MeV.
The size of the RTM magnets can be reduced by a large factor by
the DSM. The DSM would be significantly less expensive for energies up to at least 2 GeV.
The coherence condition of the DSM is expressed by therelation.
-nL~v cos cf.s = v )\ B(-c-)-~

where 6V is the energy gain of an electron which traverses theaccelerating section at peak RF phase, ~the free-space RF wavelength, V an integer, B the value of the (uniform) magnetic fieldin fuur quadrant magnets, and 1~ the synchronus phase.
Conceptual Design
The basic components of the microtron include an electric gun
a buncher, a SMeV linac injector, a DSM, an extraction system.
A schematic view of the accelerator is shown in Figure 1. Basicdesign parameters of our proposal of the DSM are tabulated inTable 1.
The electron energy at the output of the gun will be 100 keV.
The beam will be bunched and accelerated to 5 MeV and then injected the DSM. The DSM will be built with two 16 MeV linacs andfour 90° sector bending magnets. Each linac will use eight 2meter long disk and washer structurs operated at 2.4 GHz.
-273-

To avoid vertical beam defocusing and make magnetic field
uniform, we adopte rather complicated magnet system for the
first recircuration orbit as seen in Figure 1, in which thesystem is drawn emphasized. Using the system, because the firstorbit can be free from the microtron coherence condition, it canbe used for longitudinal phase matching.
Table 1.

Basic design parameters.

Maximum energy
Current
Injection energy
Energy gain per turnAccelerating field
Number of recircurations
Wave length
Magnetic field
Linac length
Number of linacs
RF power losses
Shunt impedance
Maximum orbit radius
Minimum orbit radius

1300
5321
32.5
12.5
0.97
162. 0. 975
3.59
0.072

LINAC 2

LINAC 1Figure 1.

Schematic view of the DSM.

-274-

GeV
Jv~A

MeV
MeV
MV/memTm
MW
Mrt/mmm