A PREPARATORY STUDY OF LINAC .FOR THE FEL PROJECT AT JAERIY* K a w a r a s a k i and L i n a c g r o u pJa p a n A t o m i c E n e r g y R e s e a r c h I n s t i t u t e

AbstractPreliminary consideration on the linac for a free electron laser(FEL) project proposed by Japan Atomic Energy Research Institute ( J A E M )will be briefly described*1• Introduction
The first succ e s s ” of an FEL experiment at HEPL, Stanford Universityin 1976 and 1977 gave a great impetus to R6cD activities on FEL.

The FEL

has characteristics superior to ordinary lasers such as gas and glass
lasers in its abilities to change the wave length and to produce highpower output.
uses•

Wider applications are thus expected in various future

However, only a few experiments have succeeded so far in the FEL

oscillations, because extremely high quality of the electron beam isrequired•
The aim of our project is to produce infrared through visible FEL
l a s e r s , because these lasers can be possibly used for practical purposessuch as uranium enrichment and isotope separation.

Though the laser wave

length depends on the design of the undulator m a g n e t , the electron beamenergy, ranging from a few of tens of MeV to several hundred MeV, isconsidered to be suitable for those p u r p o s e s •This requirement lead us toadopt the linear accelerator as the first stage machine of our project*
The required beam quality, however, cannot be easily achieved fromthe simple modification of the present l i n a c •It is necessary to design acompletely new machine dedicated only to the FEL project.
are not yet ready to fix our s p e c i f i c a t i o n s •At p r e s e n t , weThis study will discuss only

preliminary matters such as how we can choose the suitable type for thededicated machine.

2•

Considerations on linac types
For last several years 9 serious considerations have been given andsome promising results have been already obtained in the development ofsuperconducting linacs operated in a liquid He temperature.

An successful

operation of superconducting cavities was carried out at Tristan

ラAccumulator Ring$) and a new proposal has been mad e ) for the CEBAF(Continuous Electron Beam Accelerator F a c i l i t y ) •At JAERI, a project of aheavy ion boosting linac for the Tandem accelerator is now in progressusing a niobium quarter half line r e s o n a t o r ^ " .

The various techniques ofthe cryogenics and high Q cavities developed there will be adoptable.
When superconducting linacs are chosen, standing wave cavities rather thantraveling wave will have to be newly built due to the requirement of thehigh Q value.

B u t , the most difficult problem will still remain in theelectron current intensity t h a t •superconducting linacs can accelerate.
Several normal conducting linacs operated at room temperature andwith standing wave mode have been built and used for the FEL projects atseveral laboratories such as LANL, U.S.A. and Kelvin Laboratory, U.K.
A new linac with travelling wave mode at Boeing Aerospace C o m p a n y " wasalso designed for only FEL experiments.

This linac will produce a longpulsed beam with more than 100 m s for FEL o s c i l l a t i o n s •Klystrons

designed for these specific purposes will be supplied from Thomson C S F ^ " •They have carried out experiments using the 20 MeV S-band linac in the
p a s t , but they decided to change it to the L-band linac.

Although the L-band linac is expected to be more efficient in electron current andquality, various technical problems will still have to be solved in future.
In addition to all of these c o n d i t i o n s，the availability andfamiliarity with our present S-band linac may also be one of the essential
factors to be considered.

Thus the problems are rather c o m p l i c a t e d , andwe have to compromise between these conflicting choices.

3.

Schedule
The first step will hopefully start next fiscal year 1987, with 4 or

6 year project's p e r i o d •In the first year, the surveys of the existing

FEL programs and technical feasibility studies for the various possible
choices will be carried o u t •In parallel with those w o r k s , the design

studies for the injector, RF power supply and beam diagnostics will bestarted, because these parts, especially an electron gun, can beseparately d e s i g n e d f fabricated and tested.
In the next year, we will start the detailed design works for themain accelerator p a r t s , after the decision about main specifications has
been made、 A bench test of the injector will be performed in this stage.

The design and fabrication of the RF power supply with high stability will
be started at the same time*
In the third year, the fabrication of the main accelerator parts will
be s t a r t e d，and the building and new shielding facility for the newaccelerator will be constructed.

The design and fabrications ofu ndulator and laser optical instrumentations will be also started.
The fourth year will be used for the assembly of the whole system andtest experiments to check the performances for the FEL will be started.

4•Conclusion
Various kinds of electron accelerators are now used for the FEL

experiments in the w o r l d •project•

We are now at the starting point of thisOne of main parts of this project is thus to choose the mostsuited linac in the most successful manner.
The authors wish to thank P r o f • Y* Torizuka, Tohoku University for hissuggestion and valuable discussions*Information from Mr. T® O n o d e r a，

Mitsubishi Electric Cooperation, is also greatly acknowledged* They areindebted Dr® N* Shikazono for his continuous support and encouragement.

References
UD.k.Q* Deacon et a l ” Phys. Rev. Lett, 38 (1977) 892

2* Y« Kojima, Private communication
3• Physics Today, Feb. 1986, p18
4« S* Takeuchi andShikazono, JAERI-M 85-104 (1985) p H

5* J-L. Adamski et a l . , IEEE NS-32 (1985) 3397
6• Thomson-CSF's catalogue