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Submitted on 1 Jan 1981
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CESIUM BEAM FREQUENCY STANDARD AT THE RADIO RESEARCH LABORATORIES
K. Nakagiri, M. Shibuki, S. Urabe, R. Hayashi, Y. Saburi
To cite this version:
K. Nakagiri, M. Shibuki, S. Urabe, R. Hayashi, Y. Saburi. CESIUM BEAM FREQUENCY STAN-
DARD AT THE RADIO RESEARCH LABORATORIES. Journal de Physique Colloques, 1981, 42
(C8), pp.C8-253-C8-256. �10.1051/jphyscol:1981831�. �jpa-00221726�
JOURNAL DE PHYSIQUE
CoZZoque C8, suppte'ment au n012, Tome 42, de'cembre 1981 page C8-253
C E S I U M BEAM FREQUENCY STANDARD A T THE R A D I O RESEARCH LABORATORIES
K. Nakagiri, M. Shibuki, S. Urabe, R. Hayashi and Y. Saburi Radio Research Laboratories, Tokyo 184, Japan.
Abstract.- A laboratory type cesium beam frequency standard has been developing at the Radio Research Laboratories, Tokyo. Design consideration, some of the analysis and basic experiments have been made. The construction and perfor- mance of the beam tube and the electronics are mainly reported.
Beam tube.- The design of the apparatus is based on beam optics using hexapole mag- nets because of small cross-section of the beam. (I), (2) Some modifications have been made as the results of preliminary tests. The construction is shown in Fig. 1 and the outline and performance of the main parts are as follows:
(1) Oven and Detector: Both are mounted on a same stand, which is movable perpendicular to the beam axis with a reproducibility of about 5 um using a stepping motor. Cesium oven has a hole of 1 mm in diameter and 3.5 mm length and its opera- ting temperature is about 95 degree centigrade. The hot wire detecter of platinum
(80%) and iridium(20%), 1 mm wide and 0.025 mm thick is used at a current of about 1A with a screen having a hole of 1 mm in diameter for the beam entrance.
(2) Deflecting magnet: The magnet of hexapole, 6 cm in length, 3 mm in dia- meter, 0.8 Tesla of the magnetic field intensity at the pole piece is set symmetri- cally at both sides.
(3) C-field: The main solenoid coil and two correction coils are set inside the cylindrical magnetic shields consist of three layers. The peak-to-peak varia- tion of themagnetic field intensity is about 0.1 percent over the interaction region, and the residual component of the earth's magnetic field perpendicular and parallel to the axis are about 6 nano-Tesla and about 3 nano-Tesla respectively.
500 e/s
Ion pump Fig. 1. Structure of RRL Cs Beam Tube
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981831
C8-254 JOURNAL DE PHYSIQUE
( 4 ) Guiding f i e l d : I n o r d e r t o avoid t h e e f f e c t of t h e Majorana t r a n s i t i o n being i n t r o d u c e d between t h e magnet and t h e C-field, two c o i l s f o r g u i d i n g f i e l d a r e s e t i n such r e g i o n a t both s i d e s . A t t h e g u i d i n g f i e l d i n t e n s i t y more t h a n a b o u t 2 . 5 m i l l i T e s l a , t h e i n c r e a s e of t o t a l beam c u r r e n t and t h e mormal shape of low frequency
zeeman resonance were observed due t o t h e e l l i m i n a t i o n of t h e Majorana t r a n s i t i o n . (5) Pumping system: Two i o n pumps of 500 21s a r e used and o p e r a t i n g p r e s s u r e i s about 2 x 10-6 Pa.
(6) Ramsey c a v i t y : The a x i a l C - f i e l d and t h e s i n g l e U-shape Ramsey c a v i t y having even-order mode l e d t o t h e a d o p t i o n of H-plane bends and a E-plane Tee feed.
Themethod of phase adjustment by o f f s e t t i n g frequency and a l s o by s l i d i n g t h e posi- t i o n of t h e Tee-junction, and t h e use of s p e c i a l c a v i t y , i n which t h e beam h o l e s a r e not a t t h e same d i s t a n c e from each c a v i t y end, were proposed and t e s t e d ( l ) . A t p r e s e n t , however, t h e same method developed by Mungall e t a l . of N R C ( ~ ) ~ O ~ phase adjustment and a l s o a c a v i t y w i t h h o l e s s y m n e t r i c a l l y l o c a t e d from t h e c a v i t y end a r e used f o r t h e t e s t . The c a v i t y i s c o n s t r u c t e d b y e l e c t r o f o r m i n g and t h e d i s t a n c e between h o l e s i s about 50 cm a l o n g t h e beam a x i s .
E l e c t r o n i c s . - The block diagram of t h e e l e c t r o n i c s is shown i n Fig. 2 , i n which two s e r v o systems a r e included. One i s f o r t h e c o n t r o l of 5.00688 MHz c r y s t a l o s c i l a t o r w i t h r e f e r e n c e t o t h e cesium frequency, c a l l e d "cesium c o n t r o l loop". The o t h e r i s f o r t h e c o n t r o l of 5.0 MHz c r y s t a l o s c i l l a t o r on r e f e r r i n g t o t h e frequency of 5.00688 MHz o s c i l l a t o r , c a l l e d " o f f s e t t e r c o n t r o l loop". The system is s i m i l a r t o t h a t proposed by Howe of NBS.
(4)The o u t l i n e and c h a r a c t e r i s t i c s of two c o n t r o l l o o p s a r e a s follows:
(1) Cesium c o n t r o l loop: Squarewave frequency modulation i s performed d i r e - c t l y by changing t h e c o n t r o l v o l t a g e of 5.00688 MHz c r y s t a l o s c i l l a t o r . I n o r d e r t o a t t a i n low n o i s e d e t e c t i o n of cesium t r a n s i t i o n and t o p r e v e n t t h e u n f a v o r a b l e e f f e c t
X 1836
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CES I UM BEAM TUBE
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Fig. 2. Block Diagram of E l e c t r o n i c s
of l i n e a r d r i f t of analog d e v i c e s , i t i s designed t h a t one c o n t r o l p e r i o d of a b o u t 1 0 s e c c o n s i s t s of 4 s t e p s of frequency switching, t h a t i s , low, h i g h , h i g h and low, f o r s e n s i n g t h e cesium c e n t e r frequency. The a n a l y s i s of t h e s t a b i l i t y of t h i s loop was made by t h e u s e of t r a n s f e r f u n c t i o n model, and a l s o t h e experiment a p p l i e d t o acorn- m e r c i a l beam tube was c a r r i e d o u t . ( 5 ) The measured v a l u e s a r e shown i n Fig. 3 , i n which t h e s o l i d l i n e s r e p r e s e n t t h e c a l c u l a t e d v a l u e s t a k i n g c o n s i d e r a t i o n of t h e beam s h o t n o i s e . The r e s u l t approves t h a t t h e most a p p r o p r i a t e loop g a i n i s around 1.5 from t h e viewpoints of dynamic r e s p o n s e and t h e e f f e c t of d r i f t of c r y s t a l o s c i l - l a t o r .
Loop g a i n dependence
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=0.972 r g a i n = 0 . 4 8 6
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I I I I 1 1 1 1 I I I I i I I I ~ I I I , 1 1 1 110 100 1000 10000
Measurement t i m e ( s )
F i g . 3. Frequency s t a b i l i t y o f t h e commercial cesium beam t u b e f o r d i f f e r e n t open l o o p g a i n .
( 2 ) O f f s e t t e r c o n t r o l loop: A s t h e 5.00688 MHz c r y s t a l o s c i l l a t o r i s modula- t e d i n frequency,
iti s needed t o use t h i s o f f s e t t e r f o r s t a n d a r d frequency o u t p u t . I n p r i n c i p l e , a 5.0 MHz c r y s t a l o s c i l l a t o r i s c o n t r o l l e d s o a s t o c o i n c i d e i t s frequ- ency-divided s i g n a l w i t h t h e mean frequency of f i n a l b e a t s i g n a l a g a i n s t t h e 5.00688 MHz o s c i l l a t o r . The p e r i o d of about 2.1 s e c f o r t h i s b e a t s i g n a l i s used a s a timing u n i t f o r sequence c o n t r o l l e r i n cesium c o n t r o l loop. As a r e s u l t of t h e s t a b i l i t y measurement of 7 Hz b e a t frequencr;, i t i s e s t i m a t e d t h a t t h e system n o i s e of t h i s loop i s c h a r a c t e r i z e d by 2 x 10-I
/T.(3) Microwave sideband s p u r i o u s e f f e c t : Many c o n s i d e r a t i o n s on t h e frequency s h i f t due t o s p u r i o u s s p e c t r a l component i n t h e i n t e r r o g a t i n g microwave s i g n a l have been made, f o r example, by C. Audoin e t a l . (6) I n o r d e r t o e s t i m a t e t h e e f f e c t of t h e sideband s p u r i o u s i n t h e microwave o u t p u t of t h e e l e c t r o n i c s mentioned above, t h e measurements of phase n o i s e a t 9 GHz and of power s h i f t u s i n g a commercial beam tube were made. The measured phase n o i s e was l e s s t h a n -40 dB w i t h r e s p e c t t o t h e c a r r i e r
i n t h e r e g i o n of 1 H z t o 300 H z , and t h e measured frequency s h i f t due t o t h e change of power i s shown i n F i g . 4. From t h e c o n s i d e r a t i o n s of t h e s e r e s u l t s and t h e ana- l y s i s i n Reference ( 6 ) , i t would seem t h a t t h e s p u r i o u s e f f e c t of our e l e c t r o n i c s i s comparatively s m a l l .
Methods of t h e d e t e r m i n a t i o n of V e l o c i t y D i s t r i b u t i o n . - Both an automatic measuring
system and a method of a n a l y t i c a l d e t e r m i n a t i o n have been developed, and t h e i r app-
l i c a t i o n s t o a commercial cesium beam tube were examined. The r e s u l t s o b t a i n e d
JOURNAL DE PHYSIQUE
by these two methods show fairl~~good agreement, so it would be possible to get a precission of a few parts in 10 in the correction of the second-order Doppler shift .
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Overall test and problems.- The beam intensity measured by the flop-out typedetector at an oven temperature of about 95 degree centigrade is around 300 pA and the obser- ved line width and the signal amplitude (peak-to-valley) of the Ramsey pattern are about 230 Hz and 30 PA, respectively. With the observed signal-to-noise ratio,short term frequency stability of about 3 x 10-12fi was expected, but the measured stabil- ty is three times worse. In preliminary measurement, the frequency with corrections for magnetic field, velocity distribution and beam reversal agreed with TAI within around 1 x 10-12. The measurement using a cavity recently adjusted in phase has just been started, but it shows large and unreasonable characteristics in power shift, the cause of which is not clear enough at present.
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