vacuum uv spectroscopy and impurity behavior in tokamak and stellarator plasmas

• ' Fusion Engineer ing

Fusion Engineering and Design 34 35 (1997)115 t23 and Design E L S E V I E R

Vacuum UV spectroscopy and impurity behavior in tokamak and stellarator plasmas

R.C. Isler

Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Abstract

An overview of applications of vacuum UV (VUV) spectroscopy to studies of fusion plasmas is presented. This spectral region is usually considered to extend from 10 to 2000 A. Most impurity studies in core plasmas and in divertors rely on measuring the intensities of the bright resonance lines that lie in this wavelength range. In the core, these data are supplemented by emissions from charge exchange excitation to determine concentrations of fully stripped low-Z ions. Determinations of individual impurity radiative losses and densities are carried out with the aid of one-dimensional codes in which transport coefficients are adjusted until the modelled intensities are well matched to experimental observation. Analysis of divertor spectra presents special difficulties, because of the inherently two- or three-dimensional nature of the radiation patterns. In addition to impurity studies, VUV lines are used to determine electron and ion temperatures, and plasma rotation. © 1997 Elsevier Science S.A.

1. Introduct ion

Optical spectroscopy is employed for two main purposes in diagnosing fusion plasmas: (1) to assess the production and behavior of impurities which are always present in high temperature discharges; (2) to determine general plasma prop- erties, such as the ion temperature and rotation [1]. The range of spectroscopic applications has been extended considerably from the traditional passive observations by exploiting charge exchange excitation (CXE) from neutral beams of hydrogenic ions [2]. The most reliable characteri- zations of the impurities in core and divertor plasmas are accomplished by analyzing the resonance lines which terminate on the ground or metastable states, and by making use of the strongest CXE lines produced by the beams. Ex-

cept for some of the low ionization stages of light elements, these transitions lie below the transmission cut-off in air at 2000 A. These lines are also utilized for determining the electron and ion temperatures, and for studying plasma rotation, even though most investigations of these quantities take advantage of visible spectroscopy, because of the flexibility of employing standard optics to obtain spatial resolution.

This paper provides a very brief overview of passive and active spectroscopy of fusion plasmas in stellarators and tokamaks. The emphasis is on the VUV region, although some of the examples given utilize transitions that do not strictly meet the criterion of having wavelengths less than 2000 A. In practice, many visible spectroscopic systems are limited to wavelengths above 3500 A, because of the transmission losses of long fibres or of glass

0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00686-2

116 R.C. Isler /Fusion Engineering and Design 34-35 (1997) 115-123

optics, so that emissions between 2000 and 3500 are often monitored through vacuum connec-

tions. Spectroscopy of the crystal X-ray region below 10 A, which is primarily adapted to studies of the He- and H-like stages of intermediate-Z elements, is a subject in its own right and is not addressed here.

SPRED spectrometers [3] are widely used to provide complete low resolution spectra about every 15 ms during a discharge, so that it is possible to develop a qualitative picture of impurity evolution before carrying out detailed modelling. Fig. 1 illustrates spectra at three different times during a PBX-M discharge in which neutral

200E

:::5

i~ lOOC

4000 I

ooc

3 <Z .~200C g

l ooo

6000

5000

-4 , ~ 4000

> ,

.~ 3000

"~ 2ooo

lOOO

o

(a) ' ~ J I 0 V ' ' I

Before H-mode, Before IBW

o VJ

CIV FVI

OIV

OIV OV Cl i l C Iv F Vii

C Ni

(b) Ni XV[I~ Fe xvi During H-mode, During IBW

CL XIV CI XV 500 ms

C1XV C IV

(c) , During H-mode, During IBW Fe XXIII 600 ms

:e XX - e XXI

200 400 600 800 1000

Wavelength (A)

Fig. 1. SPRED spectra at three different times during a PBX-M discharge. The rapid increase in Fe XXIII between 500 and 600 ms compared with the pseudo-continuum of low stage iron lines around 250 ,~ indicates central accumulation.

beam injection (NBI) leads to the development of an H-mode around 420 ms; ion-Bernstein wave heating (IBWH) is initiated at 450 ms [4]. The major impurities are oxygen, carbon and fluorine. The oxygen and carbon are expected, because they are always released from plasma-facing com- ponents in tokamak discharges; the presence of fluorine in such large concentrations indicates that Teflon-coated wires interior to the machine are not shielded adequately from plasma bombard- ment. The very broad feature from about 100 to 350 A comprises a large number of unresolved lines from low ionization stages of metals.

After NBI is initiated and the H-mode transition occurs, the metallic lines increase in intensity, while the transitions from low-Z ions are not affected. In addition to the increase in the metal influx, lines from interior ions, such as Fe XVI, Fe XX and Ni XVIII, begin to stand out above the broad low-wavelength feature emitted near the ed g e - - an indication that the metallic profile is becoming more peaked in the interior. (Thomson scattering shows that the central electron temperature changes little during this period.) Finally, the addition of IBWH not only raises the influxes of all impurities, as revealed by the growth of the peripheral emissions, but also causes a very strong central peaking of impurities, as evidenced by the increasing intensity of the Fe X X - F e XXIII signals in Fig. l(c). This interpretation of the spectral data is borne out by bolometer measurements of the radiation profiles, and detailed analysis of the plasma heat and particle transport indeed shows that IBWH can induce an enhanced confinement mode [5].

2. Radiated power and transport

Although the total radiative losses from fusion plasmas are best measured by bolometers, the need to evaluate the fractional contributions of individual impurities often arises. Because it is not feasible to measure all the spectral intensities, the total power emitted from a given ion must be inferred using the few prominent lines that are routinely detected. Most of the power is emitted from the longest wavelength transitions that ter-

R.C. Isler /Fusion Engineering and Design 34 35 (1997) 115-123

Table 1 Measured and modelled column emissivities (photons cm 2 s ~) for an ATF discharge heated by ECH only.

117

Impurity 2 (A.) Measured Modelled Implied Average Radiated (1015 cm -2 (104 cm -2 ntot(0 ) ntot(0 ) power S -1) S -1) (1011 cm -3) (t011 cm -3) (kW)

OV 629 3.5 2.7 1.26 760 1.3 1.2 1.03

O VI 1034 18.8 11.3 1.65 O VIIIo× e 102 2.1 1.1 1.89 Total oxygen

C III 1176 0.8 0.72 1.11 977 2.5 1.75 1.42

C VI 182 0.15 0.05 2.80 C VIc× e 182 1.4 1.72 2.34 Total carbon

Fe XV 284 3.5 17.2 0.20 Fe XVI 343 5,4 30.2 0.18 Fe XVIII 94 0.09 0.53 0.17 Total iron

1.91 33

2.56 48

0.18 16

The transport modelling assumes 1 impurity ion cm-3 on axis. The diffusion coefficient is taken as 1600 cm 2 s 1 and the convection velocity is 0 cm s 1.

minate on the ground state and on the metastable states. However, an accurate evaluation of the radiated power requires that account be taken of as many transitions as possible. For example, most of the power f rom C III, i.e. a Be-like ion, is emitted in the lines near 977 and 1176 ,~, but about 25% is dissipated in shorter wavelengths from levels with n _> 3.

The usual procedure for achieving spectroscopic estimates of radiated power employs a one-dimensional t ransport code to compute the ionization balance for a given impurity, using electron temperature and density profiles as input. The results are then post-processed with the excitation rates for measured spectra lines to calculate their emission rates. The computat ion is iterated by adjusting the transport coefficients until a rea- sonable match between the calculated and measured intensities is achieved. For the low-Z elements, it is usually best to ignore lines f rom isoelectronic sequences below the Be-like stages when making comparisons with code results, because their radiation most likely deviates from toroidal symmetry. It is desirable to use a full collisional-radiative model to calculate the line intensities and total radiative power, particularly for ions such as C III , where a large fraction of

the density resides in the metastable state. An example of such analysis is shown in Table

1 for an electron-cyclotron-heated plasma in the ATF torsatron. Modelling was performed with the S T R A H L code [6], used in conjunction with beam attenuation codes for calculating the density of fully stripped ions f rom CXE. A diffusion coefficient of 1600 cm 2 s -1 without a convection term gives the best agreement with the measurements. The total impurity densities are obtained by dividing the measured emission rates by the rates modelled for 1 particle cm -3 on axis. The consistency of the modelling is assessed by the spread of the results derived from the intensities emitted by the different ion species of a given impurity. Note in Table 1 that the individual densities of the ground and metastable states of the important Be-like ions are determined sepa- rately and must be summed to obtain the total density of oxygen and carbon. The deviation from the average value for a given impurity is less than _+ 20% for all three elements shown in Table 1. Spectroscopically derived evaluations of radiated power in the advanced toroidal facility (ATF) torsatron differed by no more than 20% from bolometric results obtained by a well-calibrated array, and were ordinarily within 10% [7].

118 R. C. Isler / Fusion Engineering and Design 34-35 (1997) 115-123

ORNL-DWG 85C-2833 FED A

1.2 I I I I I / I F I (C) / , (")/ >. 0 .9 Sc X lX Sc XVl

,1 DISRUPTIO 0 . 3

1.2 e. ,

C

> 0 . 9 eo

"F-, 0 . 6

v

" 0 . 3 < z L9

o lOO

I I I

(b)

Sc XVII -

i,,., '"""'. •

150 2 0 0 2 5 0 3 0 0

T I M E ( m s )

I I I

(d)

100 150 2 0 0 250 3 0 0

TIME (ms)

Fig. 2. Time behavior of scandium following laser ablation into the TEXT tokamak. Dashed lines represent operation well within the density limit; solid lines are from discharges operated very near the density limit.

Impurity transport is usually defined in terms of a diffusion coefficient and a convection coefficient. The larger the ratio of inward convection to diffusion is, the more centrally peaked is the impurity distribution. Modelling of the steady state impurity profiles allows the rat io--but not the individual values--of these coefficients to be obtained. The method that has been employed very successfully to provide additional informa- tion on transport coefficients is the ablation tech- nique, in which a laser pulse directed onto a thin film injects trace amounts of metals into a plasma. Fig. 2 shows the time dependence of scandium lines for two different types of discharges in the TEXT tokamak [8]. The dashed lines represent discharges that were operated well away from the density limit and which reflect very typical results. Injection takes place at around 110 ms and the peak intensity is quickly reached for all lines. (The very rapid initial spike observed in the Sc XIX trace comes from an interfering low ionization stage.) After this, the intensities decay monotoni- cally and the central ions can be characterized by

a single confinement time that reflects the rate at which the injected particles are lost from the plasma. The lower stages disappear faster, because they are lost by diffusion back to the limiter (metals are non-recycling) and also because they are ionized to higher stages.

The solid lines in Fig. 2 illustrate the time behavior for operation that is near the density limit, i.e. very near the disruption threshold. Here, it is seen that the temporal evolution of the scandium is not a simple decay. In fact, the test particles remain in the machine up to the time of a soft disruption near 210 ms. The Sc XIX and Sc XVII emissions are constant for several tens of milliseconds after injection, then begin to decay between 170 and 180 ms as the plasma cools. However, the intensities of the other two ions at larger radii increase slightly after this time. The total scandium content is probably retained, but the ionization balance is evolving. Analysis of similar experiments in the ISX-B tokamak using titanium as the test impurity showed that essen- tially none of the probe particles was lost [9].

R.C. Isler /Fusion Engineering and Design 34-35 (1997) Ii5-123 119

Inward convection that leads to long confinement times appear to be very strong for these discharges near the density limit. This result is sub- stantiated by the behavior of the soft X-ray signals which, although sawtoothing strongly, exhibit a steady increase that results from intrinsic impurity accumulation up to almost the end of the discharge.

3. Charge exchange spectroscopy

expected for electron excitation. However, by 250 ms, the central radiation from CXE has become quite prominent, even though the electron-excited peak near the edge is still distinct. As the ionization balance changes as a result of the charge exchange depleting the C VII fraction, the CXE emission overwhelms the peripheral radiation. At later times, the C V signal becomes broadly dis- tributed over the plasma cross-section as the temperature falls and the discharge collapses.

CXE takes advantage of the fact that the cross- sections for an energetic neutral hydrogen atom to charge exchange into excited states of highly stripped impurity ions are large. The subsequent radiative cascades are readily detected in the VUV region. Neutral heating beams and dedicated diagnostic beams have both been used for this purpose. With respect to characterizing impurity densities, this process provides the only direct measurement of the fully ionized stages of low-Z ions, which are the dominant species in the interi- ors of hot fusion plasmas, and it has been widely used for this purpose.

Although most CXE spectroscopy studies con- centrate on transitions from hydrogenic ions, it was found quite useful in the ATF torsatron to measure lines from the He-like ions excited by charge transfer on the H-like species; at the low electron densities and temperatures in the ATF torsatron the concentrations of C VI and C VII were comparable, even in the center of the machine. Moreover, NBI itself alters the species mix, because charge exchange acts as an effective re- combination mechanism for impurity ions. (This aspect of charge exchange is of minor importance in typical fusion plasmas operating at higher densities and temperatures.)

It was also found extremely useful to exploit the 23p-23S transitions excited by charge transfer for ion temperature measurements. The evolution of the radial profile of this transition in C V from a discharge with NBI beginning at 200 ms is shown in Fig. 3 [10]. The solid lines are the raw chordal data and the dashed lines represent the Abel inversions. Before injection, at 183 ms, the emission is narrowly peaked in the periphery, as

4. Ion temperatures and plasma rotation

Charge exchange spectroscopy in the visible region has become the standard method of measuring impurity ion temperatures and plasma rotation in most tokamaks and stellarators. Nevertheless, the utilization of the VUV region has proven to be quite useful for some experiments and promises to be of great interest for divertor spectroscopy (see Section 5). In the ATF torsatron, the resonance lines of 'Li-like carbon and nitrogen (1549 A and 1240 A), and both the electron-excited and the CXE emissions from the 3p-38 transitions of He-like carbon, nitrogen and oxygen (2271 A, 1902 ,~ and 1623 A) were used in third or fourth orders for determining impurity ion temperatures.

An example from a typical discharge is shown in Fig. 4. Prior to 200 ms, the plasma is heated only by electron cyclotron heating (ECH) and all the emissions are electron excited. The O VII signal comes mainly from the center but the C V radiation is peaked more toward the edge. Even so, the temperatures of the two ions are nearly the same, i.e. around 100 eV. The two edge ions, i.e. N V and C IV, exhibit very low temperatures, however, indicating a strong gradient in the periphery. When the neutral beams are initiated, the CXE contributions from O VII and C V transitions are greater than the electron-excited compo- nents. The temperatures rise for about 50 ms, but a collapse begins, because the electron density is too low for a significant fraction of the beam power to be absorbed. All these transitions are very strong and free from interfering lines. They offer good possibilities for ion temperature mea-

120 R.C. Isler / Fusion Engineering and Design 34-35 (1997) 115-123

1.2

z 1.0 O

~ 0.8

0.6

- J

< 0.4 n- O Z 0.2

0

1.2

1.0 (o .~ 0.8 uJ

0.6

- J

< 0.4 rc O z 0.2

0

I I I I

(a)

183

t i

1

- - m* •

"- L "°°''" I I

I I I I (b)

; 250 ms _ ~,

_ :'~ s

- , , , ; ", ,•° \

0 5 10 15 20 Z (cm)

25 0

ORNL-DWG 89M-2319 FED

I , , ,

, ( c )

267 ms

$

I I I I _ ~ (d)

• *", 283 ms

I i

i

i

5 10 15 20 Z (cm)

25

Fig. 3. Signals from C V as a function of the distance Z above the mid-plane in the ATF torsatron: solid lines are for chord-integrated emission, and dashed lines denote Abel inversions.

surements, particularly in the edge and divertor regions.

It is also feasible to use the VUV lines for rotation studies. The initial observations using CXE for determining NBI-induced plasma rotation were made in the VUV region by observing the 633 .& CXE line of O VIII. As confirming evidence, the triplet transitions from O VII (1623 it) and Ne IX (1248 A) were also monitored [11].

5. D i v e r t o r s p e c t r o s c o p y

Efforts are currently under way with several machines to understand divertor operation and to make detailed measurements of the divertor plasma parameters. Spectroscopy is employed to study impurities and particle temperatures, just as in the core plasma. However, because of the

complicated geometry, it is often not feasible to obtain the ideal experimental arrangement to un- fold spatial distributions of the emitting regions, particularly in the VUV region. Because the electron temperatures tend to be low--in DIII-D, they are less than 30 eV everywhere in the divertor-- the emission from low-Z impurities is expected to come mostly from the isoelectronic sequences below the He-like stages.

To evaluate the absolute radiation losses from carbon along the line of sight of an SPRED spectrometer that views the divertor in DIII-D, an effective electron temperature is computed from ratios of the spectral lines. Fig. 5 shows the ratio 460 A/538 A of C III calculated from a collisional-radiative model as a function of the electron temperature. The measured ratio is 1.3, which implies an effective electron temperature of 7.5 eV over the volume in which C III radiates.

R.C. Isler /Fusion Engineering and Design 34-35 (1997) 115-123 121

3>

LU r r :D F.- < f t . w 13..

w I..-

Z o

>

LU r r

r r LU 13_

IJJ I-- Z o

3 0 0

200

100

0

200

150

100 -

50 -

0 0

I 1 I

(a)

O VII

- ~ ! 1 6 2 3 A -

l r

k'x~'~ NB'

/ ~ / / / , ~ J / / ~ / ' / / / / / / / / / ~ ' / ' / / . ~ / / / / ~ J - E C H

I I

(b)

3 [ C V

2 2 7 1 A

I

25

2o ul n-

f r Ill

uJ F-

Z

2

2o - - W

r r

15 - - r r '

w 10 -

W -- F-

ORNL-DWG 89M-2316 FED

I I I

_ ~ - (c) _

_ 1239 A _

g

I

I

z 5 - 0

I I :IT 0 I I

100 200 300 400 0 100 200.

TIME (ms) TIME (ms)

I

I

(d)

C IV

1548 A

I

3 0 0 400

Fig. 4. Ion t empera tu re s m e a s u r e d f rom several spectral lines in the A T F torsa t ron .

The electron temperature determined from the 977 A/460 A ratio indicates a somewhat lower temperature of 6.6 eV, but this result is considered to be less reliable, because of greater uncertainties in the spectrometer calibration at longer wavelengths. A similar analysis for C II from the 904 A/687 A ratio yields an electron temperature of 4.5 eV at the location of this ion. The spectroscopically determined temperatures are generally consistent with those deduced from Thomson scattering.

As a confirmation of the inferred electron temperatures, ion temperatures were obtained from the profile of the C III transition array around 4650 ]~. The electron ion collisionality in the divertor is great enough that the two temperatures are expected to be comparable. Extracting such low ion temperatures from the profiles requires exact calculation of the Zeeman or Paschen-Bach effect on the intensities and the splittings of the

individual lines between magnetic sublevels, as well as an accurate determination of the instru- mental profiles. The results are presented in Fig. 6, where it is seen that the ion temperature at 4 s is indeed close to 7.5 eV, as expected.

The power loss analysis in the DIII-D divertor is shown in Table 2 for assumed temperatures of 5, 7.5 and 10 eV. It is concluded that C IV is the strongest radiator, but the power losses must be determined from the An ~ 0 transitions, because the resonance lines at 1550 A are above the range of the SPRED instrument. Because the intensities of these lines are so temperature sensitive, the power emitted in C IV is very uncertain, but the correlation with the bolometer which has the same geometrical view is quite good for an assumed electron temperature of 7.5 eV. It is esti- mated from the spectroscopy that impurities other than carbon account for less than 10% of the power lost, so the agreement is considered to be most satisfactory.

122 R.C. Isler /Fusion Engineering and Design 34-35 (1997) 115-123

3

2.5

°< 2 CO O 3

o 1.5 ¢ O

_= 1 o.

0.5

~ T I O = 1.3

/ 0 . . . . i , , ] , i . . . . i . . . . i i i i i i i i , i i , i , ,

0 5 10 15 20 25 30 35

ELECTRON TEMPERATURE

Fig. 5. Calculated ratio of the 460 ,~/538 ,~ emission rates for C Ill as a function of the electron temperature. The electron density is assumed to be 10 TM c m - 3 Measured ratios indicate that the electron temperature is 7.5 eV.

All the analysis of the divertor impurities dis- cussed so far is based on the assumption that the electron distribution functions are Maxwellian. However, recent theoretical work has shown that energetic electrons that have low collisionality may drift into the divertor from upstream [12]. Studies are needed to understand how this popu- lation might affect the spectral intensities and interpretations.

Table 2 Carbon radiated power inferred from the divertor SPRED spectrometer in DII I -D for various assumed temperatures

Carbon radiated power (W cm -2)

Ion T e = 5 e V T e=7 .5 eV T e = 1 0 e V

C II 0.7 0.6 0.5 C III 12 13 13 C IV 1328 93 37 Total 1341 107 51 Bolometer = 105 W cm -2

The ratio of the 460 and 538 A lines of C III implies Te = 7.5 eV.

6. Summarizing remarks

VUV spectroscopy is vital to the analysis of impurities in fusion plasmas. Most studies rely on emissions in the region from 100 to 1100 A, but a significant broadening of capabilities can be obtained by exploiting longer and shorter wavelength regions. Several VUV transitions are also quite useful for determining electron and ion temperatures, and for measuring plasma rotation. All these applications will be important in the field of divertor studies.

10 . . . . , . . . . , . . . . , . . . . , . . . . ~ . . . . , . . . .

>=9

8

F- 7

= 6 o

5 i i 1 , 1 1 1 , , i , 1 1 , 1 1 1 1 1 1 1 1 , , ~ r l l , l l ' 1 1

1.5 2 2.5 3 3.5 4 4.5

Time (s)

Fig. 6. C III ion temperature in the DIII-D divertor, determined from line profiles of the 33p-33S transition array at 4650 A,.

Acknowledgements

This work was sponsored under DOE Contract DE-AC05-84OR21400 with Oak Ridge National Laboratory.

References

[1] R.C. Isler, Impurities in tokamaks, Nucl. Fusion 24 (1984) 1599.

[2] R.C. Isler, An overview of charge-exchange spectroscopy as a plasma diagnostic, Plasma Phys. Controlled Fusion 36 (1994) 171.

[3] R.J. Fonck, A.T. Ramsey and R.V. Yelle, Multichannel grazing-incidence spectrometer for plasma impurity diag- nostics: SPRED, Appl. Opt. 21 (1982)2115.

[4] A.P. Post-Zwicker et al., Impurity behavior during ion Bernstein wave heating in PBX-M, Nucl. Fusion 35 (1995) 215.

R. C. Islet/Fusion Engineering and Design 34-35 (1997) 115-123 123

[5] B.P. LeBlanc et al., Active core profile and transport modification by application of ion-Bernstein wave power in the Princeton beta experiment modified, Phys. Plasmas 2 (1995) 741.

[6] K. Behringer, Description of the impurity transport code STRAHL, Rep. JET-R(87)08, JET Joint Undertaking, Abingdon, 1987.

[7] R.C. Isler et al., Radiative losses and improvement of plasma performance following gettering in the advanced toroidal facility, Nucl. Fusion 31 (1991) 245.

[8] R.C. Isler, W.L. Rowan and W.L. Hodge, Long time impurity confinement as a precursor to disruptive instabil- ities in ohmically-heated plasmas, Phys. Rev. Lett, 55

(1985) 2413. [9] R.C. Islet et al., Impurity transport and plasma rotation

in the ISX-B tokamak, Nucl. Fusion 23 (1983) 1017. [10] R.C. Isler et al., Temperature behavior during neutral-

beam injection in a torsatron (ATF) with field errors, Nucl. Fusion 29 (1989) 1391.

[11] R.C. Islet and L.E. Murray, Plasma rotation measurements using spectral lines from charge-transfer reactions, Appl. Phys. Lett. 42 (1983) 355.

[12] R.W. Harvey, O. Sauter and K. Kupfer, Parallelization of the FEPT Fokker-Planck edge transport code; coupled electron ion distribution, Bull. Am. Phys. Soc. 40 (1995) 1835.

vacuum uv spectroscopy and impurity behavior in tokamak and stellarator plasmas

Documents