Progress in Medical and Industrial
Electron Li near Accelerator Technology

Eiji Tanabe
Varian Associates Inc., Palo Alto, California

94303

A BSTRACT

Recent advancements in the technology of standing wave election 1inear accelerators, which arewidely used in medical(radiation therapy) and i n d u s t r i a l (radiography) applications, are reviewed.

These include comparisons of several different structures and operating modes, in terms of performanceand cost effectiveness.

In addition, a new concept of a niulti-energy, single-section, standing wave

linear accelerator structure, which has been developed in Varian and demonstrated in commercial applications,is described.

INTRODUCTION

In recent years, electron 1 inear accelerators have been widely used in medical and industrialapplications.

The ideal accelerators for commercial applications have to satisfy the following

conditions, i e . , 1 ) compact and light weight, 2)highly efficient and stable,and operate,5) output energy variable over a wide range.

4)

narrow output energy spectrum and3) simple to fabricate

This paper reviews the recent extensive developments of electron 1inear accelerators operatingat room temperature in terms of those conditions.

COMPARISON OF ACCELERATOR STRUCTURES

F i g . 1 shows cross sections of various high beta accelerator cavity structures.

The disc loaded

traveling wave guides have been commonly used in high energy physics as well as in commercial applications,in the last two d e c a d e s J

The structure is very simple yet the shunt impedance is relatively low (about50 MH /m at S - b a n d ) , and at the same time, a considerable amount of RF power is wasted in the RF load at theend of the guide.

The bi-periodic and tri-periodic on-axis standing wave guides are also relatively simple

but have rather higher shunt impedance (about 80 MO/m at S-band) and coupling (about 12%)•

Thisstructure is suitable for application when the transverse space is 1imited, but the interaction between
coupling cavities and the beam may be serious.

The side-coupled standing wave guide developed at LASL

has many advantages, and is consequently widely used for many applications.^

Although the structure israther complicated, it is much less sensitive to mechanical tolerance and beam loading than the otheralternatives.

Moreover, the shunt impedance can be optimized independently from the coupling cavity

structure, and by choosing the accelerating cavity contour and beam hoie diameter, one can obtain ashunt impedance 120 MH/m at S-band frequency.
has several unique features.^

The interlaced side-coupled SW guide developed at Vari an

The ratio of peak E field to accelerating E field can be as 1ow as 1.3

without sacrificing the shunt impedance (90 M/l/m at S - b a n d ).

A1 though the structure is somewhat complicated,

it is suitable for a very compact, high gradient a c c e l e r a t o r •The disc-and-washer structure introduced byAndreev has a very high coupling factor ( 50%) and potentially very high shunt impedance.
stage, further developments will be required for commercial a p p l i c a t i o n s •Yet at thisTTTTTJULLLLTTTTT

C ONSTANT IMPEDANCECO N S T A N T-+

G RADIEN T

BIPERIODICACCELERATO R

STANDING

ACCELERATORD I S C -L O A D E D T R A V E L IN G -W A V E

W AVE

STRU CTURE

T R IP E R IO D IC STANDING WAVEACCELERATO R

STRUCTUREt K . 眺 BSUSSAM,®. T. TRAM AMO O. TRONC•描7Tt

STRUCTURESyK.
^7p

INTERLACED SIDE-COUPLED SW

SIDE-COUPLED STANDING-WAVE

ACCELERATOR STRUCTURE

ACCELERATOR STRUCTURE

Fig.1

DISC-AND-WASHER STANDING-WAVE
ACCELERATOR STRUCTURE

Cross Sections of High Beta Accelerator Structures

VARIABLE ENERGY A CCELERATOR STRUCTURE

Radiation therapy linear accelerators typically produce X-rays in one of three enerav ranqes:
4-6 MeV (low e n e r g y ) ，8-12 MeV (medium energy) and 15-25 MeV (high e n e r g y ) •Depending on the locationof tumors , the optimum treatment can be performed by choosi ng the right X-ray energy.
accelerator would allow optimum treatment over a larger

A m u l t i-energy

range of tumor depths with a sinqle machine.

Similarly, a multi-energy i n d u s t r i a l 1 inear accelerator would significantly increase the useful rangeof subject t h i c k n e s s 。

Most of the single-section accelerators for medical and industrial applications are optimi zed for anarrow energy spectrum at a single energy, since the optimum electron "bunching" occurs at a certain levelof accelerating electric field.

As acontrol output energy by changing theOne approach to varying the beam
which are independently excited, withresult, the energy spectrum deteriorates rapidly if one tries to
input RF power l e v e l .

energy over a wide range is to cascade sections of

1 inear accelerators

independent control of accelerating field a m p 1 it tide and p h a s e ノ

Another approach is the double-path single-section linear accelerator, which uses a m o v eable reflecting
magnet to obtain phase variation between p asses.8

H o w e v e r , these techniques are rather complicated andthe results are often costly and less reliable.

A novel concept for varying the energy of a single-section standing-wave linear accelerator over.

a wide range without degrading the energy spectrum was recently developed at Varian.

o’]o 11’The

technique is to utilize the unexcited coupling side-cavity to vary the relative amplitude and/or sign of
the accelerating fields in adjacent c e n t e r 1 ine cavities.

Technical d e t a i 1s of this approach are givenin the references.
Table I shows the basic design parameters of two cl asses of vari able-energy standing-wave
accelerators, namely Clinac 2500 for radiation therapy and Linatron 200A for radiography.

Table I.

Variable Energy Accelerator Guide
Design Parameters

Clinac 2500 Guide
Guide RF Length (m)Linatron 200A Guide

1.9

No of c e l 1s

J+0.1
36

J + 1 + J

Frequency (MHz)

2856

2997

Effective Shunt Impedance (Mn/m)

9565

Qo

16,00013,000

Coupling Factor K t0.040.01

Energy ModeHigh Energy

Low Energy

High Energy

Low Energy

Energy (MeV)

2462

1.2

Beam Current (mA)

50

180200200

Available RF Peak Power (MW)

4.5

2.0

1.6

L6

28.5™85i

17.0-601

2.8-3.01

1.4-0v9i

Load Line(MeV vs A)

ACKNOWLEDGEMENT

The author wishes to express many thanks to A. McEuen who helped to prepare this manuscript.
REFERENCES

1•

P.M. Lapostolle and A •し Septier, "Linear Accelerators,"

2.

J. McKeown and S.O. Schriber, "Nondimensional Tuning of Accelerating Structures,"
5 1 (I), Jan (1980).

3.

A Bensussan, D.T. Tran and D. Tronc, "Standing - Wave Triperiodic Structure for a 10 MeV Medical
A ccelerator," N u c l • Instr. & M e t h 。 140, 2 3 1 , ( 1 9 7 7 ) .

4.

E.A. Knapp, B.C. Knapp and J.M. Potter, ’Standing Wave High Energy
Rev. S c i • Instr. 39 l 7)s July, (1968)•

5.

V.A. Vaguine, "Standing Wave High Gradient Accelerator S t r u c t u r e , " Rev. S c i • Instr. 48 (11),Nov., (V977).

6.

S.O. Schriber, "High-Beta Linac Structures," proc. 1979 Linear Accelerator C o n f . , 8 N L - 5 1 134, 392(1979).

7.

E.L. Ginzton, "Variable Output Linear Accelerator," U.S. Patent No. 2920228 ( D e c . 1 3 , 1 954).

8.

S.O. Schriber, et a l , "Experimental Measurement on a 25 MeV Reflexotron," IEEE Trans. NS, Vol
NS-24, No. 3»June ( 1 9 7 7 )。

9.

E. Tanabe, and V. Vaguine, "Variable
No. 4286192, Aug. 2 5 , ( 1 9 8 1 ) .

10.

G. Meddaugh, E. Tanabe and V. Vaguine, "Variable Field Coupled Cavity Resonator Circuit," PatentPending.

North Holland Publishing C o . , ( 1 9 7 0 ) .

Energy Standing Wave Linear

Rev. S c i • Instr.

Linear Accelerator Structure/'Accelerator Structure," U.S. Patent

1 1 . E. Tanabe and G. Meddaugh "Variable Energy Standing Wave Linear Accelerator S t r u cture," Proc. 1981
Linear Acc. C o n f ., LA-9234-C (1981).