HAL Id: jpa-00227377
https://hal.archives-ouvertes.fr/jpa-00227377
Submitted on 1 Jan 1987
HAL
is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire
HAL, estdestinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
NEW DIRECTIONS OF X-RAY SPECTROSCOPY APPLIED TO HOT, LOW DENSITY PLASMAS
E. Källne
To cite this version:
E. Källne. NEW DIRECTIONS OF X-RAY SPECTROSCOPY APPLIED TO HOT, LOW DENSITY PLASMAS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-335-C9-342.
�10.1051/jphyscol:1987959�. �jpa-00227377�
JOURNAL DE PHYSIQUE
Colloque C9, s u p p l 6 m e n t au n 0 1 2 , Tome 4 8 , dbcembre 1 9 8 7
NEW DIRECTIONS 0F.X-RAY SPECTROSCOPY APPLIED TO HOT, LOW DENSITY PLASMAS
E . K ~ L L N E
JET Joint Undertaking, GB-Abingdon OX14 3 E A . Great-Britain
ABSTRACT
X-ray spectroscopy has frequently been used a s a major t o o l f o r diagnosing t h e center hot p a r t of fusion plasmas. High resolution X-ray l i n e s p e c t r a and broad band X-ray emission s p e c t r a a r e used t o deduce ion temperatures and impurity concentrations i n t h e plasma. X-ray l i n e s mostly studied a r e t h e n=2 t o 1
t r a n s i t i o n s produced by electron impact excitation. However, time resolved s p e c t r a observed during t h e f u l l plasma discharge have shown t h e importance of other e x c i t a t i o n mechanisms. These new r e s u l t s a r e discussed and implications towards next generation of X-ray instrumentation i n plasma diagnostics is outlined.
1
.
INTRODUCTIONThe plasma parameters f o r t h e plasmas produced by t h e presently operating l a r g e tokamaks have entered new regimes compared t o those of t h e plasmas produced by smaller tokamaks previously. Thus, f o r example, i ) t h e ion and electron
temperatures a r e now higher (up t o 30 and 10 keV, respectively, compared t o 8 and 2 keV, previously), i i ) t h e plasma cross s e c t i o n is much l a r g e r (by about a f a c t o r of 5001, i i i ) t h e variations and f l u c t u a t i o n s of parameters a r e l a r g e r both on longer and s h o r t e r time s c a l e s and i v ) the pulselengths a r e longer, up t o 30s (canpared t o
<
500ms before). A l l these changes introduce new requirements on t h e diagnostictechniques applied t o measure t h e plasma parameters. I n this paper t h e present applications of X-ray spectroscopy t o l a r g e tokamaks w i l l be discussed together with sane prospects f o r f u t u r e developments. Several papers have recently ap ared reviewing application of X-ray spectroscopy i n fusion research i n general and
psi
describing i n d e t a i l t h e b a s i s of sane of t h e s p e c i a l applications of X-ray
spectroscopy2. This paper w i l l highlight sane new d i r e c t i o n s of X-ray spectroscopy i n i t i a t e d by r e s u l t s frcm t h e l a r g e tokamaks.
2. THE PLASMA SOURCE
There a r e presently t h r e e l a r g e tokamaks i n operation i n t h e world, J o i n t European Torus (JET) i n UK, Tokamak Fusion Test Reactor (TFTR) i n USA and JT-60 i n Japan.
The goal f o r these projects is t o produce a burning plasma through t h e reaction
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987959
C9-336 JOURNAL DE PHYSIQUE
The r a t e parameters f o r t h e r e a c t i o n ( s e e Fig.1) has a maximum around a n i o n temperature of 20
-
30 keV. Presently, t h e plasmas a r e produced using hydrogen, helium and deuterium as f i l l gas, while t r i t i u n is planned t o be used i n t h e l a s t operation period of JET. Ion temperatures achieved up till now a r e i n t h e range of 10-
30 keV which i s s u f f i c i e n t f o r reaching t h e maximum of t h e c r o s s s e c t i o n of t h e d-t reaction. Electron temperatures a r e around 5-
1 0 keV, e l e c t r o n d e n s i t i e s lo1'-
10'' cm'', pulse lengths 5-
209. Table I s m a r i z e s these parameters and g i v e s t h e t o t a l input power used f o r Ohmic and f o r a d d i t i o n a l h e a t i n g such a s n e u t r a l beams and radiof requency waves.TABLE I
Plasma and operating parameters
FIG.l Reaction r a t e <ov> as a
function of i o n t m p e r a t u r e f o r d+d and d + t reactions.
The i n t e r i o r of t h e vacuum chamber where t h e plasma is contained i s covered with t i l e s f r a n a low Z element ( f o r JET carbon is used). Therefore, t h e main contaminant of t h e plasma emanates from low Z elements ( i e . , carbon, oxygen) but t h e r e a r e a l s o minor t r a c e s of metal contaminants which have been r e l e a s e d frcm t h e walls of t h e vacuum chamber ( i e . , n i c k e l , chromiun).
3. PLASMA PARAMETERS
The s p a t i a l d i s t r i b u t i o n s of t h e plasma parameters a r e important t o diagnose and t h e v a r i a t i o n s (due t o t r a n s p o r t , e t c ) i n t h e s e s p a t i a l parameters a r e t h e c r u c i a l i s s u e s i n fusion research presently. The i m p u r i t i e s e n t e r i n g t h e plasma w i l l be successively ionized, low Z elements w i l l quickly be completely s t r i p p e d while t h e metals w i l l l o o s e most but not all of t h e i r electrons. The p a r t i a l l y s t r i p p e d ions r a d i a t e and it i s t h i s c h a r a c t e r i s t i c r a d i a t i o n t h a t give s i g n a t o r i e s of t h e plasma parameters i n t h e region where t h e i o n s were de-excited. The t r a n s p o r t of ions, both towards t h e c e n t e r and o u t of t h e c e n t e r ( s e e Fig.2) has up till now only been measured i n d i r e c t l y through measurements of r a d i a l d i s t r i b u t i o n s of o t h e r plasma parameters such as Te(R) and ne(R).
Schematic of JET vacuum vessel.
The dimensions a r e b= 4.2m, R=1.5m with a major r a d i u s of 2.96m.
.I
- - - FIG.3 Twporal evoiution of a plasma
discharge with a n FP pulse f ran 10 t o 12. Displayed are f r a n l w e r t o upper t r a c e s : i o n t a n p e r a t u r e (Ti keV)
,
e l e c t r o n tanperature (Te keV), RF power(MW), average e l e c t r o n d e n s i t y - - - * - - -
(ne 1018 ans) and plasma c u r r e n t T
,I
($ MA). (w 2
-
/I n a d d i t i o n t o t h e s p a t i a l d i s t r i b u t i o n s of plasma parameters t h e r e i s a t a p o r a l evolution during t h e plasma discharge ( s e e Fig.3). The plasma pulse can f o r example be characterized by a s t a r t - u p phase, a quiescent phase, an a d d i t i o n a l heating phase and an ending phase. The various plasma parameters as well as t h e impurity
behaviour must be measured during the various phases with a time r e s o l u t i o n s u f f i c i e n t t o d i s t i n g u i s h behaviour during t h e f a s t o s c i l l a t i o n s as well a s record t h e f u l l plasma discharge.
4. X-RAY SPECTROSCOPY
Around t h e tokamak t h e r e is a manifold of d i f f e r e n t diagnostic equipment; h e r e we s h a l l concentrate on t h e X-ray s p e c t r a n e t e r s presently i n use and t h e a n a l y s i s of d a t a f r a n such i n s t r m e n t s .
The X-ray geometry is chosen with consideration t o source s i z e , source s t r e n g t h , accessible aperture. The source s t r e n g t h , i e . , eg, t h e i n t e n s i t y of a character- i s t i c X-ray l i n e can be calculated. For example, we assume a nickel concentration of lo-"
-
lo-' ne, ne being 1013 and an e l e c t r o n temperature of 3.5 keV. With t h e s e parameters t h e source region ( a s h e l l of 100 cm width) emits a l i n e i n t e n s i t y of l o x 2 photons/an2.s.sr f o r Is-2p t r a n s i t i o n s of ~1"'. This i n t e n s i t y isconcentrated i n one l i n e and emitted during s h o r t t i m e s s o t h e s p e c t r a n e t e r must have a high throughput and a high count r a t e c a p a b i l i t y . The l a t t e r is e s p e c i a l l y important considering t h e background f l u x e s of neutrons and Y : s which a r e detected by t h e X-ray d e t e c t o r with an e f f i c i e n c y of 5
-
1 0%.FIG.4 Schematics of t h r e e X-ray c r y s t a l spect r a n e t er g e m e t ri es :
a ) Rowland c i r c l e , b) van Hamos c ) double c r y s t a l .
C9-338 JOURNAX DE PHYSIQUE
Three d i f f e r e n t geanetries have been used f o r X-ray spectraneters a t tokamaks (see Fig.4) m e tht-oughput of these systems a r e canparable (eg., f o r p a r t i c u l a r systems i n use and considered f o r JET: a ) 9.10-' sr.cm2, b) 3.10-' sr.cm2 and c ) 2.10'' sr.cm2 ) while t h e main difference is the accessible bandwidth (0.5%, 7% and oontinuous scanning, respectively) and t h e v e r s a t i l i t y .
The most canmon system i n use a t all t h e t h r e e l a r g e tokamaks is t h e Rowland c i r c l e geometry, and here we w i l l highlight sane of t h e r e s u l t s obtained with such systens with examples f r a n t h r e e l a r g e tokamaks JET, TFIB and JT-60.
4.1 Line p r o f i l e measurements
The Doppler broadened l i n e width o f , f o r example N i 2
'+,
can e a s i l y be measured. The instrumental contribution t o t h e l i n e width is t y p i c a l l y a t e n t h of t h e t o t a l broaderling (Fig.5). The simplicity of t h e method, a l i n e p r o f i l e of a well-resolved l i n e has made t h i s measurenent i n t o one of t h e most important diagnostic measurements f o r ion tenperatures f r a n t h e center of the plasma. However, these measurenents have a l s o revealed t h a t a s i n g l e line-of-sight observation, a s i s usually performed, is not s u f f i c i e n t t o disentangle t h e pure thermal broadening mechanism f r a n e f f e c t s o f , f o r example, r o t a t i o n and turbulence. A t conditions presently prevailing i n t h e l a r g e tokamak plasmas, t h e s h i f t of .the l i n e is comparable t o t h e l i n e broadening during neutral beam i n j e c t i o n (see Fig.5).Effects of r a d i a l d i s t r i b u t i o n s of eg., t o r o i d a l velocity (Fig.61, ion temperature and electron temperature, a l l influence t h e Doppler broadened 1 i n e p r o f i l e and more detailed analysis is necessary. For example, the absolute l i n e position during the quiescent phase of t h e discharge i s not e a r n , s o it i s not hown whether t h e plasma is r o t a t i n g o r not i n t h i s phase. I n order t o answer t h i s question it would be necessary t o perform absolute wavelength measur ents t o an accuracy of ppn l e v e l , i e . , as is required f o r Lamb s h i f t measurments? Lacking such measurenents (these a r e r e a l l y not f e a s i b l e a t t h e l a r g e tokamaks) data is interpreted as being r e l a t i v e measurements with the assmption of a non-rotating plasma i n t h e quiescent, ohmic phase (Fig.7).
/
FIG.6 Schematic f o r l i n e of s i g h t observations indicating t h e plasma t o r u s with plasma current I
,
neutral beam i n j e c t i o n (NBI)&d angle of line-of-sight $.
FIG.5 A typical X-ray l i n e p r o f i l e fran N i 2 6 + f o r JET before and during (lower) NBI i n j e c t i o n . A Voigt p r o f i l e f i t t e d t o t h e experimental data is shown.
There a r e l a r g e p e r i o d i c f l u c t u a t i ons i n plasma p a r m e t e r s , e s p e c i a l l y i n e l e c t r o n t m p e r a t u r e (sawteeth o s c i l l a t i o n s ) , which can be observed a l s o i n t h e i o n
tenperature ( s e e Fig.7). Hmever,. it i s d i f f i c u l t t o s e p a r a t e effects of
f l u c t u a t i o n s i n l i n e widths and l i n e positions and thereby i n t e r p r e t t h e r e S U l t s as being due t o v a r i a t i o n s i n thermal broadening (ie., i o n tenperature) o r plasma r o t a t i o n . A f u r t h e r i n d i c a t i o n t h a t t h e r e a r e phenanena not y e t understood a r e t h e d i f f e r e n t r e s u l t s on thermal broadening obtained by X-ray s p e c t r a and by v i s i b l e s p e c t r f excited by charge exchange with t h e e n e r g e t i c n e u t r a l beams e n t e r i n g t h e plasma
.
Thus, w e have come t o a point i n X-ray spectroscopy when t h e r e q u i r m e n t s f o r l i n e p r o f i l e a n a l y s i s ( i n o r d e r t o g e t s u f f i c i e n t diagnostic r e s u l t s ) are both t o be a b l e t o absolutely c a l i b r a t e t h e l i n e positions t o ppn accuracy and t o be a b l e t o record l i n e p r o f i l e s with higher s e n s i t i v i t y than presently can be achieved.RF Pulse
5
4
3
FIG.7 Time evolution of i o n
P
i-- 2tenperature (upper) and t o r o i d a l v e l o c i t y (lower) f o r a JET
plasma discharge ( # 9687) w i t h 1
both n e u t r a l beam i n j e c t i o n (NBI) and r a d i o frequency heating (RF)
.
15
5' E 10
s
3 05 05 10 15
WBI
Through a more d e t a i l e d l i n e p r o f i l e a n a l y s i s including t h e f a r wings of t h e l i n e it might be possible t o o b t a i n information on t h e e l e c t r o n and i o n v e l o c i t y
d i s t r i b u t i o n s ; departures frcm Maxwellian d i s t r i b u t i o n s could discern themselves i n t h e t a i l s of l i n e p r o f i l e s , f o r example.
4.2 Line I n t e n s i t i e s
Spectra of H- and H e - l i k e i o n s p e c i e s have been recorded up t o N i presently1 **
(Fig.8). I n t e r p r e t a t i o n of t h e s p e c t r a and t h e associated s a t e l l i t e s t r u c t u r e has advanced t h e atomic theory s o t h a t c a l c u l a t i o n s now e x i s t which can well predict t h e experimental spectra. To simulate f o r example t h e s p e c t r a of Nil6* (Fig.8) it i s n e c g s a r y t o include not only e l e c t r o n impact e x c i t a t i o n f r a n t h e ground s t a t e of N i 2
'+
but a l s o inner-shell e x c i t a t i o n of Li-, Be-, B-like i o n s , d i e l e c t r o n i c recanbination of H-, He-, i-, Be-, and B-like, r a d i a t i v e recanbination and cascade e f f e c t s f r a n high n l e v e l s'5 .
JOURNAL DE PHYSIQUE
X-ray s p e c t r a from N i 2 ' + (b) and Ni2'+ ( a ) f r a n JET with a t h e o r e t i c a l f i t t o t h e expande
t
d a t a included as a s o l i d l i n e s
.
Canparisons of experimental and t h e o r e t i c a l X-ray s p e c t r a f o r He-like systems have usually been made using s p e c t r a f r a n the quiescent phase of t h e plasma discharge
"'
observing through a c e n t r e midplane. Single line-of-sight observations of X-ray s p e c t r a a r e influenced by r a d i a l p r o f i l e e f f e c t s f r a n e l e c t r o n tanperatufje, e l e c t r o n density and ion density. Only very few r a d i a l measurements e x i s t s o f a r and we must r e s o r t t o modelling our results with several uniaown parameters9. I n t h e near f u t u r e , however, we w i l l be a b l e t o observe r a d i a l p r o f i l e s of X-ray spectra. This w i l l be a considerable s t e p forward i n t h e information content f r a n X-ray
diagnostic. Fig.9 shows schematic of a double c r y s t a l monochranater t o come i n t o operation a t JET.
FIG.9 Schematics of X-ray double c r y s t a l monochranator on t o p of JET t o observe r a d i a l p r o f i l e s of impurity r a d i a t i o n i n t h e region (1-24) A''.
Time resolved observations of X-ray s p e c t r a a r e presently made during t h e complete plasma discharge which i n d i c a t e s t h a t during t h e various phases of t h e plasma discharge d i f f e r e n t e x c i t a t i o n m e c h a n i s s a r e i n e f f e c t . So, f o r example, it has been observed (Fig.10) t h a t t h e i n t e n s i t y of t h e resonance l i n e of t h e He-like t i t a n i u n occurs e a r l y i n t h e discharge, much t o o e a r l y t o be explained by t h e ionisation-recanbination balance p i c t u r e which is norm
at t
y w e d t o i n t e r p r e t t h e X-ray s p e c t r a form t h e quiescent phase of t h e discharge.
I n s t e a d , it i presently proposed t h a t t h e s e e a r l y s p e c t r a a r e excited by e n e r g e t i c e l e c t r o n beans 72.
Detailed a n a l y s i s of t h e f u l l s p e c t r a with t h e s a t e l l i t e l i n e s is needed f o r i n t e r p r e t a t i o n as well as recording of spectra during t h e s t a r t - u p phase w i n g d i f f e r e n t controlled s t a r t - u p scenarios.
INTENSITY \ - I S M A 8, -45T
I 1
4lvM
FIG.10 Resonance l i n e i n t e n s i t y of r.
Ti2'+ during a plasma discharge ( f r a n ref.11).
-
2 4 6 8
TIME (a) wm
During a d d i t n a l heating t h e r e l a t i v e i n t e n s i t i e s i n t h e X-ray s p e c t r a a l s o change dramatically@ i n d i c a t i n g t h a t ion-ion c o l l i s i o f i beccine i m p r t a n t . Such changes i n t h e X-ray s p e c t r a have been observed previously without a d e t a i l e d explanation f o r t h e observed r e l a t i v e l i n e i n t e n s i t i e s i n t h e He-like spectra. X-ray s p e c t r a f r a n t h e l a r g e tokamaks under t h e d i f f e r e n t operating scenarios, however, show dramatic i n t e n s i t y f l u c t u a t i o n s . It is c l e a r t h a t d e t a i l e d a n a l y s i s of t h e t i m e - r e s q l v e d s p e c t r a is needed i n o r d e r t o understand t h e various atomic processes caning i n t o operation during t h e d i f f e r e n t phases of t h e plasma discharge.
5. FUTURE X-RAY DIAGNQSTIC REQUIREMENTS
I n a d d i t i o n t o n e u t r a l beam i n j e c t t o n frozen p e l l e t s a r e i n j e c t e d i n t o t h e plasma t o increase t h e c e n t r a l e l e c t r o n density. During t h e s h o r t periods of the p e l l e t i n j e c t i o n , t h e s e n s i t i v i t y of t h e presently a v a i l a b l e X-ray s p e c t r a n e t e r s is not s u f f i c i e n t ; thus we need new approaches i n X-ray diagnostics. Given t h e f a c t t h a t t h e X-~gy s p e c t r a can give a multiparameter information f r a n t h e c e n t r a l core of t h e plasma it is a challenge t o think about how t o improve t h e s e n s i t i v i t y by maybe two orders of magnitude, i f possible keeping s u f f i c i e n t r e s o l u t i o n (E/AE = 2
-
5000) i n t h e wavelength region of i n t e r e s t and a l s o being a b l e t o observe t h e s p a t i a l d i s t r i b u t i o n of X-ray s p e c t r a . I t is q u i t e c l e a r t h a t t h e t r a d i t i o n a l approaches presently i n use w i l l not be a b l e t o achieve t h e s e goals and we must use imagination t o s e e t h e next generation of X-ray diagnostic applied t o h o t , low density plasmas.One approach is t o optimize t h e geanetry t o s u i t t h e source function better16.
Hwever, t h e requirenent of both a l a r g e increase i n t m p r a l r e s o l u t i o n and a p o s s i b i l i t y of s p a t i a l mapping of l a r g e extended sources mjlfht call f o r X-ray instrumentation of s a n e new type, e g , an X-ray calorimeter
,
which can give s u f f i c i e n t energy r e s o l u t i o n and work i n a s p a t i a l a r r a y mode s i m i l a r t o what is presently used f o r X-ray tanopaphy.C9-342 JOURNAL DE PHYSIQUE
REFERENCES
1. E KUlne and J KUlne, Physica S c r i p t a Tl7, 152, 1987
2. R Bartirano, i n Varema Workshop on Plasma Diagnostics, Varenna 1986 3. E KXlne and J KUlne, i n Atomic Physics 1 0 , eds. H Narumi and I Shimmura,
1987. ~ 3 9 5
4. M von ~ e l l e r m a n n e t dl., Varema Workshop on Plasma Diagnostics, Varema 1986 5. F Banbarda, e t dl., t o appear Phys. Rev. A (1987)
6. M B i t t e r , et al., Phys. Rev. A 32, 3011(1985)
7. S Sakasai
,
et dl., US-Japan Workshop on Assessment of Atanic Data f o r Fusion, Nagoya, 19878. J E Rice, E S Mamar, E K a l n e and 3 K a l n e , Phys. Rev. A 35, 3033(1987) 9. K H Behringer, e t dl., Nucl. Fusion
26,
751 (1986)10. J Fink, H W Morsi
,
H R5hr and U Schunacher, Max Planck I n s t i t u t f fir Plasmaphysk, Garching, IPP 1 /240, Feb 198711. T Kubo, e t dl., i n Proc. Atanic C o l l i s i o m f o r Fusion, Oxford 1987
12. T Kato and K M a s a i , i n W and X-ray Spectroscopy of Astrophysical and Laboratory Plasmas, IAU Coll. No. 102, Beaulieu s u r Mer, France 1987
and T Kato, S Morita, K Masai and S Hayakawa, Phys. Rev. A36,1987
13. H Hsuan, e t al., Phys. Rev. A35, 4280(1987) and i n Proc. Atanic C o l l i s i o n s f o r Fusion, Oxford 1987
14. E KQlne, et dl., Phys. Rev. Lett.
52,
2245(1984)15. E KXLlne, J K x l n e , E S Matmar and J E Rice, Physica S c r i p t a
2,
551 (1985) 16. H W Schnopper and'B P Byrnak, Danish Space Research I n s t i t u t e , Lyngby, Dennark,2-87
17.
