PRELIMINARY CONSIDERATIONS ON THE ISOTOPE SEPARATION DEDICATEDFREE ELECTRON LASER
Yuuki Kawarasaki and Masayoshi Sugimoto
Department of Physics, Japan Atomic Energy Research InstituteTokai-mura, をaka-gun， Ibaraki 319-11? Japan
Abstract

A free electron laser(FEL) configuration， which is dedicated to the isotope
separation, is conceptually proposed as the preliminary considerations.
The
case of three lines of visible light in simultaneous emission is considered.
Some technological items for realizing the system are discussed in conjunction
with the current R&D program on the superconducting linac driven FEL at JAERI.
Introduction

In the many EEL applications, isotope separation seems to be the firs tdominancein the field of nuclear research and industry application, as
exemplified in uranium enrichment process for nuclear fuel preparation.
As ananother case， the isotope
separation of 90Zr, 91Zr?
61Ni， 57Ee? 53Cr,and 97Mo,
which are used in fuel covering tube(Zircalloy) or as the reactor structural
material, can be considered.
91Zr has a large neutron capture cross section,
while the remainders(90Zr e t c .) have smaller ones.
Optimistically ， theZircalloy without the 91Zr can save the fuel consumption as much as 40 .
There are two ways in laser isotope s e p a r a tio n :( 1 ) atomic vapor laser
isotope separation (AVLIS) and (2) molecular vapor laser isotope separation(MVLIS), however the AVLIS is believed much superior.
It uses either threelines ofvisible light or two ultraviolet lines, because the ionization
potentials of atoms is in the range of 5-7 eV(6.2 eV for U).
If we use theconventionallasers? three visible lines can be got from the combination of
three copper
vapor and dye lasers 9and two ultraviolet lines can be got fromexcimer and dye lasers•
The basic R&D on the superconducting linac driven EEL in minimal
configuration (Phase—I) is now underway at Tokai.
It will go further to thePhase—II with the several options in near
future.
The ’’minimal’1 means
single-pass, mono-light， rather low electron energy(10-20 MeV)5 and laser
wavelength in infrared.
It is aimed at the technological achievement of thecomponents and their combination under the several experimental conditions.
The discussion below is foreseeing nature of the future plans of experiments ordesigns.
FEL configurations

Severalconfigurations can be considered to generate two or three laser
beams with the different wave lengths, simultaneously, using the rf linac.
These are categorized by the scheme of the injection of electron beam and theundulator configuration.
( 1 ) s i n g l e

e n e r g y ,m u l t i

u n d u l a t o r

w i t hd i f f e r e n t

p a r a m e t e r ,d e f l e c t i n gm e t h o d :The consecutive micro bunches of electrons are deflected? one by one, by afast deflector using a cavity, and injected into the separated undulators whichparameters are adjusted to oscillate desirable wave lengths.
After passing
through the undulators, electron bunches are collected by a merging cavity into
an energy recovery lin e .
F i g . 1 shows the sample layout of this configuration and the injector,
accelerator sections described latter.
In this method， the incident electron energy is same for all undulators, sothe undulator parameters define the obtainable laser wave lengths.
Those wave

lengths are tunable by changing the incident energy, but in a dependent manner
given in the formula: \ ニ〜厂1+及2) / 27 2.
( 2 )s i n g l e

e n e r g y ,m u l t i

u n d u l a t o r

w i t hd i f f e r e n t

p a r a m e t e r ,c a s c a d i n g

m e t h o d :The undulators with the d if 士erent parameters same as given in the method(1) ， are configured as a serial or cascading line.
The incident electron beam
passes through all undulators， sequentially.
The exit beam returns into a
recovery line.
To maintain the laser oscillation at all undulators, a mechanism to improvethe beam quality is necessary between the neighboring undulators, such as aphase space compressor.
( 3 )m u l t ie n e r g y ,m u l t i

u n d u l a t o r

w i t hs a m ep a r a m e t e r ,b r a n c h i n g

m e t h o d :The energy dispersion cavity is used to give the energy difference among
the electron micro bunches, and the bunches are analyzed and injected into theseparated undulators for each laser wave lengths.
All electron beams arecollected by the funneling magnet and compensated the energy difference by theanti-dispersion cavity to enter the single recovery line.
In this case? the undulators can have the same parameters when the energy
dispersion cavity can give enough energy difierence for obtaining the desirable
wave lengths.
( 4 )m u l t ie n e r g y ,s i n g l e

u n d u l a t o r

w i t hm u l t i

f r e q u e n c yt y p e ,m e r g i n g

m e t h o d :The energies of the electron micro bunches are modulated as described in
the method (3) ， but these bunches are injected into the single undulator insteadof the multi undulator.
The laser wave lengths are selected by the resonator’ sresonance condition.
The exit beam is compensated its energy dispersion using
the method in (3) and returned into a recovery line.
This method is much d ifficu lt to decouple the multi frequency in the singleresonator (circular type with a correction for chromatic dispersion) and thetuning mechanism is also a tough problem.
Beam Biinch Structure

Except the method (2 )， the multi micro bunch structure is necessary togenerate the lasers interlaced to each other.
The time structure of the beam
bunch is usually separated by the round trip time of the light in a resonator ，and its repetition rate is rather low compared with the accelerator frequency.
So the electron bunches for the other wave lengths are just fille d between thebasic bunch structure ， which should be matched to the accelerator and thedeflector (method ( 1 ) ) or energy dispersion cavity (methods (3 ) ，（4)) phases.
The time delay of the laser pulses in the order of several tens nsec can be
adjusted by the length of the light beam transport line.
Electron Accelerator Section

The electron beam energy is recovered using a decelerator cavity to savethe operation cost.
This would be done by a separated decelerator tank or byusing the decelerating phase of the accelerator tank to save the construction
cost and raise the eificiency of energy recovery.
The latter can be achievedettectively when the superconducting accelerator(SCA) is used in cw mode
operation.
The R&D study carried out at JAERI now employs the SCA, so it canbe a good basis for such a purpose.
To decelerate the electron beam after passing through the undulators, its
deteriorated phase space distribution must be modified or compacted by using
some devices before return to the accelerator tank.
One of these devices issuch as energy compressor， which is a combination of the bunch elongation (usingthe a丄pha-type magnet) and the energy compacting cavity.
The accelerator system can be configured as to recirculate the beam to gain
the energy doubled or tripled.
This would be also done by using the SCA as amain accelerator.
The example layout is shown in f i g . 1 . The electrons are
ected at 3MeV and accelerated with energy gain of 39 MeV by a single pass,；Eichi is achieved by six sets of the five-cells tank of 500 MHz cavity. The
beam is recirculated triple times and the final energy becomes 120 MeV.
In the case of multi turn acceleration, the higher order mode (HOM) waves
can be excited by the beams strongly.
Particularly, the beam deflecting modes
should be avoided to get the low emittance and stable beam.
These can besuppressed if the phase space distributions of the neighboring beam bunches arevery different.
So the phase space adjustment before the injection of each
recirculation line is effective to suppress the growing up the HOM power level.
In this context, the beam from the energy recovery line contributes almost
independently about the HOM excitation because its phase space distribution isvery different to that of the accelerated beam.
Another issue in the main accelerator is the control of the rf feeding.
The SCA can be operated in cw mode and its power dissipation at cavity structure
is very low.
If the beam power can be recovered properly, the main accelerator
section consumes a l i t t l e rf power.
The small rf power fed at an instancewould be stored in the energy of the beam bunches in the recirculation lines andthe beam intensity is gradually raised, as in the storage ring.
The control ofthis type operation is a key issue to realize this operation.
Injector Section

The injector provides multi bunch structured electron beam with the energy
around 3 MeV.
Since the requirements for the multi bunch structure may not be
unique? it is desirable to be arranged to produce an arbitrary time structure.
The conventional thermionic cathode with grid pulser and sub—harmonic bunchingsystem cannot be used to produce these bunch structures.
The possible
candidate is a photo cathode gun driven by the mode locked laser system which
can provide any sequence of timing pulses.
However5 the intensive R&D isnecessary to realize such injector system.
The pre—accelerator section is important that it must provide a goodquality beam with a controlled intensity discussed in the rf feeding problem.
The most attractive method is the superconductive rf gun operated in cw mode,
but the separated tank scheme (gun + couple of superconductive cavities) is morefeasible.
Conclusion

O6t

f

The multi color EEL gives a unique and valuable laser source for many
applications in research and V； 一 3MeV
FEL Beam
industry
field ,e.g.
laser 2/6 - 42MeV
Undulator
driven isotope enrichment.
The 3/5 -8 leV-120MeV
severalmethods
canbeconsidered to achieve this.
Anyway, the best choice for the\\ Energy
Branching
main accelerator is the SCA to
H Compensator Recirculation B.LCavity
reduce the operation and theconstruction cost.
Some parts
are required to intensive R&D torealize, e.g. multi beam energy
recovery line or photo cathodeinjector system.
Superconduct.Cavity
DuI
InjectorrSp
F i g . 1 . Schematic Diagram
sepration dedicated FEL