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3 4456 0 2 7 4 3 6 5 4

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OREJIJ TM- 1 0 4 .I 9 D i s t Category UC-20f

Physics Division

DESIGN AND CALIBRATION OF A TWO-CHANNEL LOW-NOISE HETERODYNE RECEIVER FOR USE IN A COz U S E R 'MOMSON SCATTERING

ALPHA PARTICLE DIAGNOSTIC

C. A. Bennett,* R. K. Richards, and D. P. Hutchinson

*Department of P h y s i c s , University of North Carolina, Asheville, NC 28814

Date Published - March 1988

P r e p a r e d by the OAK RIDGE NATIONAL TABORATORY

Oak Ridge, Tennessee 37831 operated by

MARTIN MARIETTA ENERGY SYSTEMS, INC, for the

OFFICE OF FUSION ENERGY U.S. DEYARTbENT OF ENERGY

under Contract No. DE-AC05-84OR2140n

3 4451, 0 2 7 4 3 b 5 4

lid

TABLE OF CONTENTS

Page - . . . . . . . . . . . . . . . . . . . . . . . 1

. . . . . . . . . . . . . . . . . . . . . . 1

. . . . . . . . . . . . . . . . . . 2

Abstract.

Introduction

Heterodyne Detection

System Design and Calibration . . . . . . . . . . . . . 4

Conclusions . . . . . . . . . . . . . . . . . . . . . . 9

References....................... 10

DESIGN AND CALIBRATION OF A TWO-CHANNEL LOW-NOISE HETERODYNE RECEIVER FOR USE IN A C02 LASER THOMSON SCATTERING

ALPHA PARTICLE DIAGNOSTIC

C. A. Bennett,* R. K. Richards, and D, P. Hutchinson

ABSTRACT

A dual channel low noise heterodyne receiver has been constructed

as part of a development effort to build a carbon dioxide laser based

Thomson scattering alpha particle diagnostic for a burnfng plasma

experiment. The receiver employs two wide bandwidth ( > I GHz) HgCdTe

photovoltaic mixers followed by low noise IF amplifiers. A noise

equivalent power of less than 3.0 x W/Hz has been demonstrated.

Design details and calibration methods are described.

INTRODUCTION

It has been established that heterodyne spectroscopy of Doppler

shifted Thomson scattered C02 laser radiation can provide a promising

method for determining the kinetics of fusion produced alpha particles.

A crucial component of such a diagnostic is the heterodyne receiver;

this system must exhibit low internal noise along with 8 detection band-

width large enough to ensure an adequate post detection signal to noise

ratio. In what fo l lows , we describe a receiver system utilizing a

HgCdTe photovoltaic photomixer with noise and bandwidth capabilities

necessary to provide data for alpha particle densities equal to or

greater than 1011 c n ~ " ~ . ~ We also describe a method for producing an

absolute broadband sensitivity calibration by referencing the output

of the receiver to a blackbody standard; this will provide a real time

2

check f o r photomixer s e n s i t i v i t y changes, system alignment d r i f t , and

neutron damage t o t h e o p t i c a l t r a i n .

HETERODYNE DETECTION

The heterodyne s i g n a l r e s u l t s from i n t e n s i t y f l u c t u a t i o n s as two

e l ec t romagne t i c waves of d i f f e r e n t f r equenc ie s i n t e r f e r e a t t h e s u r f a c e

of a l i n e a r power d e t e c t o r . Consider ~ W Q i d e n t i c a l l y po la r i zed plane

e l ec t romagne t i c waves i n c i d e n t normally on a photomixer, g iv ing a t o t a l

e l e c t r i c f i e l d of

Power f l u c t u a t i o n s a t t h e in t e rmed ia t e frequency (IF) w l - w2 produce

t h e signal. c u r r e n t

where PI and P2 a r e t h e powers of t he i n c i d e n t e l ec t romagne t i c waves, hv

i s the i n c i d e n t photon energy, and q i s t h e d.c. quantum e f f i c i e n c y .

Ampl i f ica t ion of t h i s signal. by IF a m p l i f i e r s and subsequent s i g n a l pro-

c e s s i n g g ive an i n t e g r a t e d r e c e i v e r ou tput p r o p o r t i o n a l t o <1s2> and

hence p r o p o r t i o n a l t o t h e product of the i n c i d e n t b e a m powers. I n prac-

t i ce , one o f the beams i s produced by a l o c a l o s c i l l a t o r (LO) w i th beam

power PLO, and the o t h e r beam con ta ins the s i g n a l power Ps. For a given

Ps, t h e signal. is enhanced by i n c r e a s i n g PLo u n t i l t h e po in t where the r -

mal inpu t i n t o the photomixer begins t o produce excess ive noise . Under

i d e a l c i rcumstances , PLo >> P,, and t h e system n o i s e c o n s i s t s mainly of

quantum mechanical or shot no i se wi th a c o n t r i h u t t o n due to thermal

3

noise generated within the photomixer and within the IF amplifiers. For

a reversed bias photovoltaic photomixer, the noise equivalent power per

unit frequency interval (NEP) is :4

hv = - rl'

( 3 )

where k = Boltzman's constant, Tm = photomixer temperature, TVIF =

effective input noise temperature of the IF amplifiers, % = reverse

shunt conductance of the photodiode, Io = LO induced photocurrent, e =

elementary charge, q = d.c. quantum efficiency, n' = heterodyne quantum

efficiency, and E, = 3 dB cutoff frequency of the photodiode. The first

term in Eq. (3) represents the shot noise contribution while the second

term accounts f o r thermal noise.

Contributions from the signal power and from the noise power appear

at the output of the amplifier; this signal is subsequently rectified,

averaged, and amplified t o give a d.c. output linearly proportional t o

the signal power. The post detection signal to noise ratio accounts for

this signal averaging, and is given by5

where I3 = IF bandwidth, 'G = slgnal integration time, Ps = signal power,

and pN = noise power with

4

Note t h a t t h e SNRpd becomes independent of t h e s i g n a l power when

Ips >> PN.

SYSTEM DESIGN AND CALIBRATION

Figure 1 shows she o p t i c a l l ayout of t h e two-receiver system. The

output of a pulsed C02 laser produces t h e s c a t t e r e d s i g n a l . Since i t

w i l l be necessary t o u t i l i z e s m a l l forward s c a t t e r i n g a n g l e s , 9 a hot

cel l f i l l e d with C02 w i l l be used t o p r o t e c t t h e d e t e c t o r s from s t r a y

r a d i a t i o n a t t h e s c a t t e r i n g laser frequency.6 The l o c a l o s c i l l a t o r

beams are prepared from t h e a t t e n u a t e d fundamental mode o u t p u t s of s m a l l

waveguide lasers o p e r a t i n g a t f requencies a p p r o p r i a t e l y s h i f t e d from t h e

s c a t t e r t n g laser frequency.l Two percent beam s p l i t t e r s are used t o

c o l i n e a r l y combine t h e L O and s i g n a l beams, while t h e f o c a l l e n g t h s of

t h e ZnSe o b j e c t i v e leases are c a r e f u l l y chosen t o g i v e an optimum spo t

s i z e t o d e t e c t o r dimension r e l a t i ~ n s h i p . ~ Three d i f f e r e n t WgCdTe photo-

d iodes were u t i l i z e d i n t h i s s tudy: a 125-pm-diam d e t e c t o r from Santa

Barbara Research Center (SBBC) and two 100-p-diam d e t e c t o r s from

Societe-Anonyme de TelecomunicatFons (SATC). Each d e t e c t o r w a s

equipped wi th a Dewar capable of maintaining an o p e r a t i n g temperature

of around 7 7 K. I n t h e o p t i c a l t r a i n i l l u s t r a t e d i n Fig. 1, a 50-50

b e a m s p l i t t e r d i v i d e s t h e incoming l i g h t so t h a t t h e two d e t e c t o r s s h a r e

t h e same s l g n a l beam; t h i s would be a p p r o p r i a t e f o r t h e case. where t h e

s i g n a l power w a s l a r g e enough t o s a t u r a t e t h e SNRpd. For smaller s i g n a l

powers, an a d d i t i o n a l hot ce l l could be added.

F igure 2 shows t h e c a l i b r a t i o n setup. The chopper wheel i s covered

w i t h an absorbing l a y e r of h igh e m i s s i v i t y material and is maintained at

5

ORN L - DWG 88- 7355

MIRROR Le CELL 4

LA IRR n R

\-- B € A M SP Ll TTER

Fig. 1. Optical layout of the two-receiver system.

ORML-DWG 88-7356

. ... . -

LUCK-IN AMP

CHOPPER B L A C K B O D Y T = 293°K

T = 527°K

r- WAVEGUIDE CCI, LASER

.RF DETECTOR

G= 33 db NF= 1.3 BW= 1 Gt iZ

Fig. 2. Cal ibra t ion setup.

6

room temperature A v a r i a b l e temperature blackbody source produces t h e

s i g n a l power. An a d d i t i o n a l ZnSe l e n s forms a r e v e r s e p r o j e c t i o n of t h e

LO beam which f i t s e n t i r e l y w i t h i n t h e a p e r t u r e of t h e blackbody

source.8 Ampl i f ica t ion i s achieved wi th dual Miteq AM-3A-001 10 a m p l i -

f i e r s equipped wi th an a t t e n u a t o r between t h e ampLiSier s t a g e s t o pre-

ven t s a t u r a t i o n of t he second a m p l i f i c a t i o n s t age . Each a m p l i f i e r has

3 3 dB of ga in , a no i se f i g u r e of 1.3 and a 1-GHz bandwidth. An RF diode

r e c t i f i e s t h e ampl i f ied s l g n a l and t h i s vo l t age i s measured wi th a d i g i -

t a l vo l tmeter . The a.c. component of the r e c t i f i e d s i g n a l is analyzed

by a lock-in a m p l i f i e r which is re ferenced by the chopper.

F igure 3 i l l u s t r a t e s t h e s i g n a l a t t he inpu t of t h e lock-in ampli-

f i e r . The r e c t i f i e d I F s i g n a l i s modulated beneath an approximately

square wave envelope o s c i l l a t i n g betweeii V1 and V 2 a t the chopping f r e -

quency. These vo l t ages are given by

V1 = C[PNBA f PR ( 2 B ) A]

V2 = C[PNBA -i- P3 ( 2 B ) A]

where A = a m p l i f i c a t i o n , PR = blackbody power s i g n a l per unit: frequency

i n t e r v a l from t h e chopper wheel, PB = blackbody power s i g n a l per u n i t

frequency i n t e r v a l from the e l eva ted temperature source , C = c o n s t a n t ,

and B = I F a m p l i f i e r bandwidth. Note t h a t the heterodyne s i g n a l i s

genera ted over a double-side band whereas the noise is genera ted w i t h i n

a s i n g l e bandwidth only [see Eq. ( S ) ] . Solving f o r PN i n t e r m s of V 1

and v2 and then r e c a s t i n g i n terms of t he measured vo l t ages Vac and Vdc,

w e o b t a i n

7

-- V G C

...... F’B

O R N L - G W G 88-7357

! -- A B S n L U I- E

Fig. 3. lock-in ampl i f i e r .

Representa t ion of t h e s i g n a l vo l t ages at t h e inpu t of t he

where OL accounts f o r t h e f a c t t h a t t he lock-in a m p l i f i e r meaisures only

t h e fundamental component of t he modulating envelope; our chopper wheel

a p e r t u r e and LO b e a m shape gave 01 = 2.26. The blackbody s i g n a l s are

c a l c u l a t e d according to8

hv = hv/kT - 1 e

Under broadband cond i t ions , t h e observed vo l t age l e v e l s cons i s t ed

of an a.c. component of around 0.5 mv and a d.c. component of about 10

t o 20 mv; these vo l t ages were convenient ly monitored on an o s c i l l o s c o p e

p laced a c r o s s t h e RF diode as t he system alignment and LO power were

8

a d j u s t e d f o r optimum r e c e i v e r performance. The system noise l e v e l s and

quantum e f f i c i e n c i e s were e a s i l y optimized t o t h e va lues i n d i c a t e d i n

Table I..

Table 1. Optimum performance f o r t h e t h r e e photomixers i n v e s t i g a t e d

__s _-__I - D e t ec t o r r n P Heterodyne Quantum E f f i c i e n c y

S BRC 5.6 x 10-20

SATC2 4.7 x 10-20 SATC 1 2.7 x

38 % 70% 40%

It should be noted t h a t t h i s method measures t h e i n t e g r a t e d s e n s i -

t i v i t y of t h e e n t i r e r e c e i v e r system. An opti.ca1 layout which would

a l l o w t h e f i e l d of view of t h e rece.-lver t o be f i l l e d with a blackbody

source upon demand w i l l r e s u l t i n a system wi th t h e c a p a b i l i t y t o be

a c c u r a t e l y c a l i b r a t e d a t r e g u l a r i n t e r v a l s . This c a p a b i l i t y w i l l be

important f o r t h e above mentioned alpha p a r t i c l e experiment s i n c e the

neutron f l u x may degrade t h e t ransmiss ion of t h e o p t i c a l e lements and

s i n c e vacuum v e s s e l exctirsions may p e r t u r b t h e system al ignment .

By using a spectrum analyzer as a tunable f i l t e r , t he frequency response

of t h e system could be determined. Figure 4 shows t h e NEP vs TF f r e -

quency f o r t h e SBRC and t h e SATCl photomixers when t h e bandpass of t h e

spectrum analyzer w a s set t o 3 MHz. The lowest frequency d a t a were

f a i r l y close t o t h e c e n t e r b u r s t of t h e spectrum analyzer while t h e

s t r u c t u r e i n t h e SATC d a t a w a s probably due to a VS

9

ORML-DWG 87C-13564

0.30

0.25

0.20

0.15

0.10

0 .05

0 0 3 6 9 12 15

COLLECTIVE INCREMENTAL POWER (1 .QE-22 Watts /Hz )

Fig. 4. System NEP vs IF frequency.

The l i n e a r i t y of t he r e c e i v e r t o changes i n s i g n a l power i s i l l u s -

t r a t e d i n Fig. 5. These d a t a r ep resen t t he change i n system response as

t h e temperature of t h e blackbody was changed between 45°C and 254'C.

The s i g n a l power l e v e l s w e r e c a l c u l a t e d according t o Eq. (8).

CONCLUSIONS

A l o w no i se , l a r g e bandwidth two channel heterodyne r e c e i v e r has

been developed wi th s e n s i t i v i t y more than adequate €or a proposed C02

laser based Thomson s c a t t e r i n g a lpha p a r t i c l e d i agnos t i c . An abso lu te

broadband c a l i b r a t i o n procedure has been designed which r e fe rences t h e

10

ORNL-DWG 87C-13563

1.00E-19

1.00E-20

I I

S B R C DEGTECYOR

S A T C DEYECTOR

t l v = l . 886-20 W / H z L

S B R C DEGTECYOR

ATC DEYECTOR

0 3 0 0 6 0 0 9 8 0 1200 1500

IF FREQUEN

0 3 0 0 6 0 0 9 8 0 1200 1500

IF FREQUEN

P i g . 5. System l i n e a r i t y as a f u n c t i o n of changes i n s i g n a l power.

system noise t o t h e output of a blackbody s tandard. This c a l i b r a t i o n

procedure can be e a s i l y i n t e g r a t e d i n t o the. a lpha p a r t i c l e experiment so

t h a t system s e n s i t i v i t y checks can r o u t i n e l y be made.

REFERENCES

*Department of Physics , Univers i ty of North Carol ina a t A s h e v i l l e , Ashevi l le , NC 288 1 4 .

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- II____.

11

2. L. Vahala, 6. VahaPa, and D. J. Sigmar, "Ef fec t s of Alpha P a r t i c l e s on the S c a t t e r i n g Funct ion i n CO2 Laser S c a t t e r i n g , " - 26, 51 (1986).

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13

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ORNL/ 'I'M- 104 19

INTEIPNAL DISTRIBUTION

17, J. S h e f f i e l d 18, K. L. Vandep: S l u i s 19. Fusion Energy Div is ion L ib ra ry 20. Cent ra l Research Library 21. Document Reference Sec t ion

24. Laboratory Records, RC 25. ORNL Patent Sec t ion

22-23. Laboratory Records

EXTfXNAL DISTRIBUTION

Of f i ce of the A s s i s t a n t Manager f o r Energy Research and Development, Department of Energy, Oak Ridge Operat ions, Box E, Oak Ridge, 7% 37831 C. A. Bennet t , Dept. of Physics , Univ. of Worth Caro l ina , Ashev i l l e , NC 28814 B ib l io thek , I n s t i t u t f u r Plasmaphysik, KFA, Post fach 1913, D-5170 J u l i c h , Federa l Republic of Germany Bib l io thek , Max-Planck I n s t i t v t f u r Plasmaphysik, D-8046 Garching bei Munchen, Federa l Republic of Germany Bib l io theque , Centre de Recherches en Physique des P lasmas , 21 Avenue des &ins:, 1007 Lausanne, Switzer land Bib l io theque , Serv ice due Confinement des Plasma, CEA, B.P. 6 , 92 Fontenay-aux-Roses (Se ine ) , France N. L. Bretz , Pr ince ton Plasma Physics Laboratory, P.O. Box 451, P r ince ton , N J 08544 J. D. Ca l len , Department of Nuclear Engineer tng, Univers i ty of Wisconsin, Madison, W I 53706 R. W, Conn, Department of Chemical, Nuclear, and Thermal Engineer ing, Un ive r s i ty of C a l i f o r n i a , Los Angeles, CA 90024 D. H. Crandal l , ER-542 GTN, 0ffic.e of Fusion Energy, U.S. Department of Energy, Washington, DC 20545 S. 0. Dean, Fusion Energy Development, Science Appl ica t ions , Inc . , 2 P ro fes s iona l Drive, Gai thersburg , MD 20760 Documentation S.I.G.N., Department de l a Physique du P l a s m a et d e la Fusion Cont ro lee , Centre d'Etudes Nuclea i res , B.P. No. 8 5 , Centre du T r i , 38041 Cedex, Grenoble, France 6. A. E l i s eev , I. V. Kurchatov I n s t i t u t e of Atomic Energy, P.O.

D. Evans, Culham Laboratory, Abingdon, Oxon. OX14 3DB, United Kingdom M. F o r r e s t , JET, Culham Laboratory, Abingdon, Oxon. OX14 3DB, United Kingdom

BOX 3402, 123182 MOSCOW, UmSmSeR,

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45 .

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H. K. Forsen, Bechtel Group, Inc., Research Engineering, P.O. Box 3965 , San F ranc i sco , CA 94105 J, R. G i l l e l a n d , GA Technologies, Ins., Fusion and Advanced Technology, P.O. Box 85608, San Diego, CA 92138 V. A. Glukhikh, Sc ien t i f ic -Research I n s t i t u t e of Elec t ro-Phys ica l Apparatus, 188631 Leningrad, U.S.S.R. R. W. Gould, Department of Applied Phys ics , C a l i f o r n i a I n s t i t u t e of Technology, Pasadena, CA 91125 C. Gowers, JET, Culham Laboratory, Abingdon, Oxon. OX14 3DB, United Kingdom R. A. Gross, Plasma Research Laboratory, Columbia Un ive r s i ty , New York, NY 10027 F. C. Jahoda, Los Alamos S c i e n t i f i c Laboratory, Un ive r s i ty of C a l i f o r n i a , P.O. Box 1663, Los Alamos, NM 87545 Libra ry , Culham Laboratory, UKAEA, Abingdon, Oxfordshire, OX14 3DB, England L ib ra ry , FOM I n s t i t u t voor P lasma-Fys ica , Ri jnhuizen , Ju tpbaas , The Netherlands L ib ra ry , I n s t i t u t e of Physics, Academia S i n i c a , Be i j ing , People ' s Republic of China L ib ra ry , I n s t i t u t e of Plasma Physics, Nagoya Un ive r s i ty , Nagoya 4 6 4 , Japan L ib ra ry , I n t e r n a t i o n a l Centre f o r Theoretical . Phys ics , T r i e s t e , I t a l y Library , Labora tor io Gas I o n i z z a t i , F r a s c a t i , I t a l y L ib ra ry , Plasma Physics Laboratory, Kyoto Un ive r s i ty , Gokasho U j i , Kyo t 0, Japan N. C. Luhinann, 7702 Boel te r H a l l , Electrical Engineering Department, School of Engineering and Applied Science , 405 Hilgard Ave., Un ive r s i ty of C a l i f o r n i a , Los Angeles, CA 90024 K. McCormick, Max-Planck I n s t i t u t f u e r Plasmaphysik, 8046 Garching b i e Munchen, West Germany D. M. Meade, Plasma Physics Laboratory, Pr ince ton Un ive r s i ty , P.O. Box 4 5 1 , Pr ince ton , N J 08544 R. R. P a t t y , Dept. of Phys ics , North Caro l ina State Univ., Raleigh, N C 27650 Plasma Research Laboratory, Aus t r a l i an Nat iona l Un ive r s i ty , P.O. Box 4 , Canberra, A.C.T. 2000, A u s t r a l i a D. H. P r i e s t e r , ER-542 GTN, Of f i ce of Fusion Energy, U.S. Department of Energy, Washington, DC 20545 P. J. k a r d o n , Brookhaven Nat iona l Laboratory, Upton, NY 11973 D. D. Ryutov, I n s t i t u t e of Nuclear Physics, S i b e r i a n Branch of t he Academy of Sciences of t he U.S.S.R., Sovetskaya S t . 5, 630090 Novosibirsk, U.S.S.R. I. Spighel , Lebedev Phys ica l I n s t i t u t e , Leninsky Prospect 5 3 , 117924 Moscow, U.S.S.R. W. M. Stacey, Jr., School of Nuclear Engineering, Georgia I n s t i t u t e of Technology, A t l a n t a , GA 30332 D. S t e i n e r , Rensselaer Poly technic I n s t i t u t e , Troy, NY 12181

15

70. J. D. Strachan, P r ince ton Plasma Physics Laboratory, P.O. BOK 451,

71. Thermonuclear L ib ra ry , Japan Atomic Energy Research I n s t i t u t e ,

7 2 . V. T. Tolok, Kharkov Physical-Technical I n s t i t u t e , Academical St.

73. R. V a r m a , Phys ica l Research Laboratory, Navrangpura, Ahmedabad,

74. M. A. Walker, 199 Warren St , NE, At lan ta , GA 30317 75. M. Yamanaka, Tkpt. of Applied Physics , Osaka Un ive r s i ty , Yamada-

76. K. M, Young, Pr ince ton Plasma Physics Laboratory, P.O. Box 451,

77. S. Zweben, Pr ince ton Plasma Physics Laboratory, P.O. Box 451,

78- Given d i s t r i b u t i o n as shown i n TIC-4500 under UC-20f 203. WE-Experimental Plasma Physics

P r ince ton , N J 08544

Tokai , Naka, I b a r a k i , Japan

1, 310108 Kharkov, U.S.S.R.

I n d i a

kami, S u i t a , Osaka 565, Japan

P r ince ton , NJ 08544

P r ince ton , N J 08544