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SCIENTIFIC APPLICATIONS OF THE UNIVERSITY
OF CALIFORNIA - SANTA BARBARA FREE
ELECTRON LASER
W. Yen, V. Jaccarino, L. Elias
To cite this version:
JOURNAL
DE PHYSIQUEColloque C7, supplement au nOIO, Tome 46, octobre 1985 page C7-413
SCIENTIFIC APPLICATIONS OF T H E UNIVERSITY OF CALIFORNIA
-
SANTA BARBARA
F R E E ELECTRON LASER
W.M. yen*, V. ~ a c c a r i n o + and L . R . ~ l i a s '
Department of Physics, U n i v e r s i t y of Wisconsin, Madison, WI 53706, U.S.A. and Physikalisches I n s t i t u t der UniuersitBt, 6000 Frankfurt/M, F.R.G. +&uantwn I n s t i t u t e and Department o f Physics, U n i v e r s i t y o f C a l i f o r n i a , Santa Barbara, CA 93206, U.S.A.
Rksumk - Nous p r Q s e n t o n s un rQsum6 des c a r a c t k r i s t i q u e s de performance du Free E l e c t r o n Laser (FEL) de 1 ' U n i v e r s i t k de C a l i f o r n i e - Santa Barbara (uCSB) q u i e s t en o p e r a t i o n depuis aoiit 1984. Ce FEL u t i l i s e des a c c k l g r a - t e u r s Q l e c t r o s t a t i q u e s
2
f a i b l e t e n s i o n de mdme pour l e s z u i d e s d ' k l e c t r o n e t il e s t spkcialement adapt6 pour l a p r o d u c t i o n d'une grande e f f i c a c i t e e t d'une p u i s s a n c e moyenne i m p o r t a n t e dans l a r e g i o n s p e c t r a l e de l ' i n f r a - rouge l o i n t a i n . L 1 a c c o r d a b i l i t 6 de c e l a s e r unique en son genre recouvre une r k g i o n s p e c t r a l e oil aucune a u t r e s o u r c e comparable n ' e x i s t e e t p a r conskquent l e UCSB-FEL peut G t r e u t i l i s e comme s o u r c e de l u m i s r e pour ktu- d i e r l e s p r o p r i Q t 6 s s t a t i q u e s e t dynamiques des e x c i t a t i o n s klkmentaires dans d e s phases condensges. Nous dkcrivons l e s k t a p e s p r k l i m i n a i r e s dans l ' u t i l i s a t i o n de c e t t e source de r a d i a t i o n e t nos p r k v i s i o n s en k t a b l i s s a n t un mode de fonctionnement simple pour l ' u t i l i s a t e u r .A b s t r a c t
-
We p r e s e n t a summary of t h e performance c h a r a c t e r i s t i c s of t h e U n i v e r s i t y of C a l i f o r n i a - S a n t a Barbara (UCSB) F r e e E l e c t r o n Laser (FEL) which has been i n o p e r a t i o n s i n c e August 1984. This FEL u t i l i z e se l e c t r o s t a t i c r e l a t i v e l y low v o l t a g e a c c e l e r a t o r s a s t h e e l e c t r o n d r i v e r s and is s p e c i f i c a l l y designed f o r high e f f i c i e n c y , high average power output i n t h e f a r i n f r a r e d (FIR) s p e c t r a l region. I t is pointed out h e r e t h a t t h e t u n a b l e l a s e r o u t p u t of t h i s unique d e v i c e f a l l s i n a r e g i o n where h i t h e r t o no comparable s o u r c e s have e x i s t e d and t h a t consequently t h e UCSB-FEL can be used a s a FIR s p e c t r o s c o p i c s o u r c e t o i n v e s t i g a t e both s t a t i c and dynamic p r o p e r t i e s of elementary e x c i t a t i o n s of t h e condensed phases. We d e s c r i b e P r e l i m i n a r y s t e p s taken i n t h e u t i l i z a t i o n of t h i s r a d i a t i o n and our e x p e c t a t i o n s i n e s t a b l i s h i n g a modest u s e r ' s f a c i l i t y i n connection with t h i s FEL.
I. INTRODUCTION
A FEL of unique d e s i g n began o p e r a t i o n a t t h e UCSB campus i n l a t e summer of 1984. T h i s FEL i s unusual a s i t is t h e o n l y FEL designed t o r e l y on low v o l t a g e
e l e c t r o s t a t i c a c c e l e r a t o r s o p e r a t i n g a s a e s s t o r a g e r i n g / I / . F u r t h e r , t h e UCSB d e v i c e has been c o n s t r u c t e d with high e f f i c i e n c y , h i g h t u n a b l e average power output s p e c i f i c a l l y i n mind. The t u n a b l e o u t p u t of t h e FEL w i l l span t h e 70pm-1000pm wavelength r e g i o n of t h e e l e c t r o m a g n e t i c spectrum, i . e . t h e m a j o r i t y of t h e FIR. I t is our b e l i e f t h a t t h i s p a r t i c u l a r device can be used advantageously a s a s p e c t r o s c o p i c l a s e r s o u r c e i n t h e FIR where no comparable s o u r c e s of r a d i a t i o n
*Alexander von Humboldt Foundation US Senior S c i e n t i s t Awardee, 1985.
C7-414 JOURNAL
DE
PHYSIQUEe x i s t . The u t i l i z a t i o n of t h i s FEL FIR r a d i a t i o n f o r sundry purposes w i l l h e l p demonstrate t h e tremendous p o t e n t i a l FEL's hold a s a r e s e a r c h t o o l i n general. I n t h i s a r t i c l e we w i l l d e s c r i b e b r i e f l y t h e design and t h e o p e r a t i n g c h a r a c t e r i s t i c s of t h e UCSB-FEL. We w i l l then p r e s e n t s e v e r a l a r e a s i n t h e s t u d i e s of o p t i c a l p r o p e r t i e s of the condensed phases i n which t h e l a s e r used a s a s p e c t r o s c o p i c source can be put t o immediate use and t h e progress we have a t t a i n e d i n t h i s d i r e c t i o n . F i n a l l y we w i l l d e s c r i b e plans f o r t h e formation of an i n t e r n a t i o n a l u s e r ' s f a c i l i t y which w i l l be a v a i l a b l e f o r those i n t e r e s t e d i n u t i l i z i n g t h e r a d i a t i o n f o r r e s e a r c h purposes.
11. DESIGN AND OPERATION OF THE UCSB-FEL
There has been a g r e a t d e a l of a c t i v i t y i n a l l a s p e c t s of EEL development s i n c e Madey and h i s group a t S t a n f o r d f i r s t suggested and then demonstrated t h e s e devices. The design of t h e UCSB-FEL was f i r s t proposed by Madey / 2 / and was then expanded upon by E l i a s / I / . The design c e n t e r s upon t h e u t i l i z a t i o n of
e l e c t r o s t a t i c a c c e l e r a t o r s which a r e capable of a t t a i n i n g p o t e n t i a l s between t h e range of 0-30 MeV. S p e c i f i c a l l y t h e Santa Barbara device uses an a c c e l e r a t o r produced by National E l e c t r o s t a t i c Corporation, t r a d e names P e l l e t r o n , which spans t h e 0-6 MeV. The P e l l e t r o n c o n t a i n s two a c c e l e r a t o r t u b e s , one of which is used t o a c c e l e r a t e t h e e l e c t r o n s , t h e o t h e r i s used t o d e c e l e r a t e t h e e l e c t r o n beam
following t h e i r passage through t h e l a s e r i n t e r a c t i o n region. The recovery of t h e unused energy i n t h e beam through t h e l a t t e r i n conjunction with t h e use of an energy s e l e c t i n g c o l l e c t o r a s s u r e s e f f i c i e n t wall-plug t o l a s e r conversion. Recovery a l s o allows t h e i n c r e a s e of t h e c i r c u l a t i n g c u r r e n t t o a t t a i n threshold values f o r s t i m u l a t i o n or bunching and g e n e r a l l y s t a b i l i z e s t h e beam energy. Readers a r e r e f e r r e d t o t h e l i t e r a t u r e f o r a d d i t i o n a l d e t a i l s on t h e design 131. Physical d e t a i l s of t h e a c c e l e r a t o r , undulator and l a s e r c a v i t y a r e summarized i n Table I . Table I1 summarizes c e r t a i n performance c h a r a c t e r i s t i c s of t h e UCSB FEL under t y p i c a l o p e r a t i n g c o n d i t i o n s . I t is t o be noted t h a t t h e low energy l i m i t of t h e t u n i n g range c i t e d i n t h e t a b l e is determined by t h e c u r r e n t c a r r y i n g
c a p a b i l i t y of t h e beam binding magnets r a t h e r than by any fundamental reason. F u r t h e r , a s i t has been demonstrated t h a t up t o 99.5% of t h e c i r c u l a t i n g charge may be recovered / 3 / . Under t h e s e optimal c o n d i t i o n s , t h e energy of t h e beam w i l l be s t a b l e enough t o produce an extended, quasi continuous output a t a s t a b l e
frequency. Though t h i s type of performance has not been achieved during t h e development phase of t h e FEL, t h e r e is no fundamental o b s t a c l e preventing t h i s attainment. Thus we can expect t h a t a high power, high e f f i c i e n c y t u n a b l e FIR l a s e r source w i l l be a v a i l a b l e f o r experimentation i n t h e near term.
Table I
UCSB-EEL Accelerator :
Undulator:
Resonator:
Technical Design Parameters
National E l e c t r o s t a t i c s Corporation, P e l l e t r o n 0-6 MeV
Samarium-Cobalt Permanent Magnets i n Halback Configuration
Period 3.6 cm On-axis F i e l d 460 G
Mirrors
-
Copper, c y l i n d r i c a l with r=500 cm Design-
Hybrid (waveguide-free, Ref. 5 )Mode
-
E,,,Table I 1 Typical Performance c h a r a c t e r i s t i c s * E l e c t r o n Beam Current E l e c t r o n Beam Recovery E l e c t r o n Pulse Lengths S a t u r a t i o n Time Laser T u n a b i l i t y Demonstrated Laser T u n a b i l i t y Proposed Laser Gain Resonator Losses Peak Power
Laser Linewith (Best Case)
1.25
-
1.50 A 90-
99% 50-
100 y s 4v s
120 ym-
1 mm 70 pm-
lmm 30$/cycle 1 l % / c y c l e 10 KW 200 MHz/4OO pm *As of J u l y 1985111. APPLICATIONS OF THE UCSB-FEL FOR FIR SPECTROSCOPY
I t has been long understood t h a t t h e FIR region of t h e spectrum is experimentally a d i f f i c u l t a r e a i n which t o operate. These d i f f i c u l t i e s may be t r a c e d i n l a r g e p a r t t o t h e absence of s u f f i c i e n t l y v e r s a t i l e sources of r a d i a t i o n . Conventional incandescent sources a s well a s gyrotron devices a r e capable of producing r a d i a t i o n i n t h i s submillimeter r e g i o n but do s o a t very low power l e v e l s . S i m i l a r l y C02 l a s e r driven molecular l a s e r s e x i s t throughout t h e FIR but they a r e a l l
monofrequency devices.
I t has been pointed out r e c e n t l y by a Study Panel of t h e US National Academy of Sciences t h a t t h e EEL concept may be expected t o remedy t h e d e a r t h of coherent tunable s o u r c e s i n t h e FIR and t h e VUV r e g i o n s of t h e electromagnetic
spectrum. / 4 / Of t h e l a t t e r two, t h e FIR is most a t t r a c t i v e a s FEL devices possess t h e i r h i g h e s t g a i n i n t h i s frequency r e g i o n ; f u r t h e r a s no matter is involved i n t h e generation of r a d i a t i o n FEL d e v i c e s may be s c a l e d up and hence high peak and average power l a s e r s w i l l be a v a i l a b l e through t h e s e techniques. Indeed, t h e s u c c e s s f u l continuing o p e r a t i o n of t h e USCB-FEL s i g n a l s a new phase i n both l i n e a r and non-linear FIR l a s e r spectroscopy.
JOURNAL DE PHYSIQUE
Wavelength
(microns)
F i g . 1.
SOLID STATE EXCITATIONS
10"
lo3
lo2
10
I
I
I
RUBY R-LINES j U C S 5 - F E L1
'
AFMR METAL- INSULA-TOR GAP
I
PLASMONSI
R e a l i z i n g t h e p o t e n t i a l o f t h e FIR l a s e r r a d i a t i o n , s e v e r a l i n d i v i d u a l s a n d g r o u p s have a c t i v e l y begun t o p l a n e x p e r i m e n t s which would e x p l o i t v a r i o u s o f t h e
p r o p e r t i e s o f t h i s u n i q u e s o u r c e . I n T a b l e 1 1 1 , we p r e s e n t a s a m p l i n g of p r o j e c t s which h a v e been p r o p o s e d . Of c o u r s e , t h e t u n a b l e r a d i a t i o n c a n b e p u t t o immediate u s e f o r c o n v e n t i o n a l s p e c t r o s c o p y i n t h e FIR; t h e s a m p l i n g p r e s e n t e d e m p h a s i z e s c e r t a i n f e a t u r e s o f FIR r a d i a t i o n s u c h as h i g h t u n a b l e powers which h i t h e r t o were n o t a v a i l a b l e f o r e x p l o i t a t i o n i n t h e s p e c t r o s c o p i c s e n s e .
CYCLOTRON RESONANCE
-
SEMICONDUCTORSI V . USER'S FACILITIES
ELECTRONIC TRANSITIONS
The UCSB-FEL was c o n s t r u c t e d u n d e r t h e a u s p i c e s o f t h e US O f f i c e o f Naval R e s e a r c h . The f a c i l i t i e s a r e o p e r a t e d w i t h i n t h e Quantum I n s t i t u t e o f t h e U n i v e r s i t y . I n t h e n e a r f u t u r e , t h e C e n t e r f o r F r e e E l e c t r o n L a s e r S t u d i e s (CFELS) w i l l be formed as a m a j o r arm o f t h e Quantum I n s t i t u t e . CFELS i n t u r n w i l l c o n s i s t o f two
s u b d i v i s i o n s . The f i r s t o f t h e s e w i l l be c o n c e r n e d w i t h f u r t h e r development of FEL advanced c o n c e p t s and d e v i c e s . T h e s e t a s k s i n c l u d e t h e p r o p o s e d s e c o n d s t a g e of t h e p r e s e n t FEL which would expand t h e r a n g e o f t h e d e v i c e i n t o t h e n e a r I R . I n
RARE EARTH STARK SPLlTTlNGS SUPERCON. GAP
I
1
T a b l e I11
Sampling of Proposed P r o j e c t s 1 . Magnons
2. Phonons
3. Non-linear Phenomena
4. Homogeneous FIR Lase Spectroscopy
A n t i f e r r o m a g n e t i c Resonance G e n e r a t i o n o f Magnons Through
2-Magnon Absorption
S t u d i e s of P r o p a g a t i n g and L o c a l i z e d Modes Phonon A m p l i f i c a t i o n and Spectroscopy Generation o f Coherent, Narrow Band,
High Frequency Phonons Phonon Induced Phase T r a n s i t i o n s Frequency Mixing and
Harmonic G e n e r a t i o n i n t h e FIR E x c i t a t i o n s i n 2-d E l e c t r o n Systems 2 Photon FIR P r o c e s s e s I n t r a Manifold T r a n s i t i o n s D i s o r d e r e d Modes and F r a c t a l s i n G l a s s e s 5. S u r f a c e s R o t a t i o n a l S p e c t r o s c o p y of Adsorbed Molecules
a d d i t i o n , s t u d i e s w i l l be conducted on t h e p o s s i b i l i t y o f c o n s t r u c t i n g compact and economical FEL's f o r u s e i n t h e FIR.
Of more i n t e r e s t t o t h e p r e s e n t a u d i e n c e s h o u l d be t h e p l a n s t o e s t a b l i s h a U s e r ' s F a c i l i t y a s p a r t o f t h e CFELS. T h i s f a c i l i t i e s would be r o u g h l y modelled a f t e r t h e v a r i o u s S y n c h r o t r o n R a d i a t i o n C e n t e r s c u r r e n t l y i n o p e r a t i o n throughout t h e world. C o n c e p t u a l l y , t h e u s e o f t h e FIR EEL r a d i a t i o n would be a v a i l a b l e f o r a l l t h o s e Who have an i n t e r e s t i n its use f o l l o w i n g t h e customary s c r e e n i n g by a n a p p r o p r i a t e p a n e l . P r o s p e c t s f o r t h e f u l l f u n d i n g o f such a f a c i l i t y l o o k encouraging a t t h i s time.
ACKNOWLEDGMENTS:
The work d e s c r i b e d h e r e h a s enjoyed t h e s u p p o r t of t h e US O f f i c e o f Naval Research and t h e US Department o f Energy. One o f u s (W.M.Yen) wishes, i n a d d i t i o n , t o acknowledge w i t h g r a t i t u d e s u p p o r t from t h e U n i v e r s i t y of Wisconsin and t h e Alexander von Humboldt Foundation.
REFERENCES
/1/ L.R. E l i a s , Phys. Rev. L e t t . s , 977 (1979). /2/ J.M.J. Madey, J. Appl. Phys. 42, 1906 ( 1 9 7 1 ) .
/3/ L.R. E l i a s and G . J . Ramian i n 7 F r e e - ~ l e c t r o n G e n e r a t o r s of Coherent R a d i a t i o n v , C . Brau e t . a l . e d s . , SPIE
%,
137 (1983)./ 4 / Report of t h e EEL Subcommittee o f t h e S o l i d S t a t e S c i e n c e Committee of t h e N a t i o n a l Research C o u n c i l , C.K.N. P a t e l , Chairman ( N a t i o n a l Academy of S c i e n c e P r e s s , Washington. 1982).
