CALIBRATION OF HIGH MAGNETIC FIELDS USING FREE ELECTRON CYCLOTRON RESONANCE IN A PLASMA

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CALIBRATION OF HIGH MAGNETIC FIELDS USING FREE ELECTRON CYCLOTRON

RESONANCE IN A PLASMA

J. Maan, H.-J. Schneider Muntau, A. Dorelon

To cite this version:

J. Maan, H.-J. Schneider Muntau, A. Dorelon. CALIBRATION OF HIGH MAGNETIC FIELDS

USING FREE ELECTRON CYCLOTRON RESONANCE IN A PLASMA. Journal de Physique

Colloques, 1984, 45 (C1), pp.C1-961-C1-963. �10.1051/jphyscol:19841196�. �jpa-00223673�

J O U R N A L DE PHYSIQUE

Colloque C1, suppl6ment au no I, Tome 45, janvier 1984 page C1-961

CALIBRATION OF HIGH MAGNETIC FIELDS USING FREE ELECTRON CYCLOTRON RESONANCE IN A PLASMA

J.C. Maan, H r J . Schneider Muntau and A.

or el on*

Max-PZanck-Institut f i l ~ Festktrrperforschung, 166 X, 38042 GrenobZe Cedex, France

* ~ a b o r a t o i r e de Spectrome'trie, B.P. 68, 38041 GrenobZe Cedex, France

RESUME

D e s mesures precises d e champ magnetique eleve (10-30 T) avec u n e prb- cision d e 2 x l 0 - ~ sont presentQes e n utilisant l a resonance cyclotron d e l'electron libre dans un plasma. Cette methode e s t proposee pour la mesu- r e des champs magnetiques eleves.

ABSTRACT

Acc rate measurents of high magnetic fields (10-30 TI with an accuracy of 2x10-' of the magnetic field are presented using free electron cyclotron resonance in a plasma. This method is proposed for high magnetic field calibration.

Accurate measurements of the absolute value of the high static and pulsed magnetic fields that can be obtained with modern magnets (up to 3 0 T with

static and up to lOOT with pulsed fields) are difficult for both technical and principal reasons. Here w e present accurate magnetic field measurement at high magnetic fields (accuracy 0.005T between 15Tand 25T)based upon the observation of free electron cyclotron resonance i n a plasma. This experiment shows the feasibility of this method for high field calibration.

In a magnetic field free electrons perform a periodic motion with a frequency w =(e/m).B, where w is the cyclotron frequency, m the

free e1ectr:n mass, e the elettron charge and 0 the magnetic field. The factor elm-js a natural constant which is accurately known C1.7588057(49)~10 ^{1 1}

Ckg 1111 and which relates the magnetic field to a frequency. For the range of fields of interest this frequency lies in the region of the Far Infrared Radiation (FIR) , whe e powerful sources with accurately determined emission frequencies (Avlv<lO

F

summarizes some accurately determined emission frequencies of strong laser lines which correspond to resonance magnetic fields in the range from 10 to 3 0 Tesla. Under suitable experimental conditions the linewidth o f the cyclotron resonance absorption of electrons i n a cold noble gas plasma discharge is very narrow I61 allowing an accurate determination of the resonance position.

In the experiment the FIR transmission through the plasma is measured as the magnetic field i s swept. The radiation, generated with an optically pumped FIR molecular laser, is chopped, guided by lightpipes (10mm I0 brass tubes).

through the plasma slab to a detector and measured with lock i n techniques.

The transmission signal was normalized to the laser power, measured with a second detector to eliminate fluctuations. Due to the multiple reflections in the lightpipe system the light is entirely depolarized. In the present experiment a bolometer was used as a detector for convenience, but the laser power and the l o w transmission losses ot sne l l g n ~ p l p e s and the plasma slab would permit the use of a room temperature Golay cell. The plasma cell is showm in more detail in fig. 1 . It consists of a 10 mm long quarz cilinder with 20 mm diameter which can be filled with high purity gas, between two Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841196

C1-962 JOURNAL

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T a b l e I

A c c u r a t e l y m e a s u r e d f r e q u e n c i e s of s t r o n g e m i s s i o n l i n e s i n o p t i c a l l y pumped F I R l a s e r s , u s a b l e f o r f i e l d c a l i b r a t i o n b e t w e e n 1 5 - 3 0 T.

. . . . . .

A ( u m l CO pump l i n e g a s r e s o n a n t f i e l d LT)

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6 9 9 . 4 2 3 7 9P34 CH3 OH 15.3123

5 7 0 . 5 6 8 6 9P16 CH3 OH 18.7704

469.0234 10R38 CH3 OH 2 2 . 8 3 4 3

4 4 7 . 1 4 2 2 l o p 1 8 CH I 23.9517

4 0 5 . 5 8 5 2 9R18 H C ~ O H 26.4059

3 9 3 . 6 3 0 9R18 HCOOH 2 7 . 2 0 8

3 7 5 . 5 4 5 0 lOP12

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c o n d e n s e r p l a t e s . F o r r o u t i n e c a l i b r a t i o n a s e a l e d o f f c e l l w i t h t h e a b o v e m e n t i o n e d p a r a m e t e r s c o u l d e a s i l y b e u s e d . The F I R i s f o c u s e d w i t h a c o n e a n d p a s s e s t h r o u g h a 5 mm h o l e i n t h e p l a t e s . A p l a s m a i s g e n e r a t e d b y t h e e l e c t r i c f i e l d o f t h e c o n d e n s e r w h i c h i s d r i v e n w i t h a 6OMhz AC v o l t a g e . The v o l t a g e n e c e s s a r y t o b r e a k down and s u s t a i n t h i s d i s c h a r g e was o b t a i n e d b y a q u a r t e r wave c o a x i a l l i n e p o w e r e d b y a 3W f r e q u e n c y g e n e r a t o r . The m a g n e t i c f i e l d i s p r o d u c e d w i t h t h e 2 5 T p o l y h e l i x m a g n e t . C 5 1

Waveguide

Coaxial l ~ n e

Quartz cell Condenser plates Polyhellx coi\

Transmitted FIR

F i g u r e 1.

Plasma s e t u p

F i g . 2 shows t h e m e a s u r e d t r a n s m i s s i o n s i g n a l , as o b t a i n e d w i t h a w a v e l e n g t h o f 447.14vm and w i t h Ar a t 1 p r e s s u r e o f 0 . 5 T o r r . S i m i l a r e x p e r i m e n t s

w e r e a l s o p e r f o r m e d w i t h o t h e r w a v e l e n t h s ( 6 9 9 . 4 2 , 5 7 0 . 5 7 a n d 4 6 9 . 0 2 u m ) . A t a r e s o n a n t m a g n e t i c f i e l d o f 23.95T t h e l i n e w i d t h i s O.1T. T h i s l i n e w i d t h i s c l o s e t o w h a t i s e x p e c t e d o n t h e b a s i s o f e a r l i e r e x p e r i m e n t s . 151 a n d was c a u s e d b y D o p p l e r b r o a d e n i n g . On t h e o t h e r h a n d t h i s v a l u e i s a l s o c o n s i s t e n t w i t h t h e known i n h o m o g e n e i t y o f t h e p o l y h e l i x m a g n e t . I n e i t h e r c a s e h o w e v e r t h e b r o a d e n i n g m e c h a n i s m i s p r o p o r t i o n a l t o t h e r e s o n a n t m a g n e t i c f i e l d v a l u e . T h i s p r o p o r t i o n a l i t y was a l s o o b s e r v e d h e r e y i e l d i n g a r e l a t i v e w i d t h A 8 1 8 o f 0.004. The e r r o r i n t h e p o s i t i o n o f t h e m i n i m u m d u e t o n o i s e c a n b e e s t i m a t e d f r o m t h e r a t i o o f t h e n o i s e i n t h e o f f - r e s o n a n c e s i g n a l t o t h e maximum c h a n g e i n t r a n s m i s s i o n i n r e s o n a n c e , compared t o t h e l i n e w i d t h . I n t h e p r e s e n t c a s e t h i s c o r r e s p o n d s t o a n a c c u r a c y o f 5 % o f t h e l i n e w i d t h . The e l e c t r o n d e n s i t y i n t h e d i s c h a r g e u n d e r s t e a d y s t a t e c o n d i t i o n s c a r O b e d r i v e d f r o m t h e a b s o r p t i o n s i g n a l and i s f o u n d t o b e o f t h e o r d e r o f 1 x 1 0 cm -

9

t h i s v a l u e i m p l i e s t h a t t h e c y c l o t r o n r e s o n a n c e f r e q u e n c y i s o r d e r s o f m a g n i t u d e h i g h e r t h a n t h e p l a s m a f r e q u e n c y s o t h a t d i s p e r s i o n e f f e c t s o f a wave p r o p a g a t i n g i n a p l a s m a c a n b e n e g l e c t e d . A t t h e same t i m e t h i s l o w e l e c t r o n d e n s i t y , and c o n s e q u e n t l y t h e same l o w i o n i z e d a t o m d e n s i t y . l e a d s t o

negligible ion-electron interaction which in principle could influence the resonance position. Therefore the accuracy of the absolute value of the magnetic field with this method i s estimated to be 2x10-' of the field value.

We believe that this method has important advantages over other techniques used or proposed for high magnetic field calibration.

-The measurement is absolute since the magnetic field is related to a natural constant.

Figure 2

Measured plasma absorption as a function of the magnetic field with h =447.1422 pm in Ar at 0.5 Torr.

-The linearity of the method is unquestionnable, which is not the case for Hall probes 171 and the ESR in rubyC81.

- I t does not depend on materials properties. therefore physical conditions such as temperature, pressure, sample handling etc. d o not influence the results. Consequently results from different laboratories can directly be compared.

-No geometrical factors are important therefore no initial calibration is needed as is the case for integrated flux methods,Hall probes etc.

-The method is convenient considering that almost every high magnetic field laboratory posesses a FIR set-up. The simplicity (strong signals and n o high radiofrequencies) and the absence of the need of high homogeneity are an advantage compared to NMR C91.

REFERENCES

1) Recommended value. ICSU. CODATA Bull no. 1 1

2) S.F.Dyubko, L.D.Fesenko. Conf. Digest 3rd Int. Conf. on "Submillimetre waves and their applications",Univ. of Surrey,UK, ed. Institute of Physics, Bristol,UK.(1978)

3) S.F.DyubRo,V.Svich,L.Fesenko. JETP Lett.16,418,(1972)

4 ) F . R . P e t e r s e n , K . M . E v e r s o n , D . H . J e n n i n g s , A . S c a l a b r i n , IEEE J.Quantum Electron.

QE-16,319,11980)

5) H.-J.Schneider Muntau.IEEE T r a n s . M a g n . . M A G - 1 7 , 1 7 7 5 , ( 1 9 8 1 ) 6 ) A.Dorelon,M.Lombardi,).C.Maan, J.Phys.D:Appl.Phys. 15,605.(1982) 7 ) L . G . R u b i n , D . R . N e l s o n , H . H . S a m p l e , R e v Sci. Instrum.46.1624.(1975) 8 ) G . K i d o , N . M i u r a , A p p l . P h y s . L e t t . 4 1 , 5 6 9 , 1 1 9 8 2 )

9 ) L . R a h f . J . D . S i e v e r t , V . Z e h l e r , J . M a g n e t i s m and Magnetic Materials,9,261.(1978)