SOFT X-RAY SPECTROSCOPY ON THE TFR TOKAMAK

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SOFT X-RAY SPECTROSCOPY ON THE TFR TOKAMAK

C. Breton, C. de Michelis, W. Hecq, M. Mattioli, J. Ramette, B. Saoutic, J.

Schwob

To cite this version:

C. Breton, C. de Michelis, W. Hecq, M. Mattioli, J. Ramette, et al.. SOFT X-RAY SPECTROSCOPY ON THE TFR TOKAMAK. Journal de Physique Colloques, 1988, 49 (C1), pp.C1-119-C1-122.

�10.1051/jphyscol:1988125�. �jpa-00227445�

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SOFT X-RAY SPECTROSCOPY ON THE TFR TOKAMAK

C. BRETON, C. DE MICHELIS, W. HECQ, M. MATTIOLI, J. RAMETTE, B. SAOUTIC and J.L. SCHWOB*

Association Euratom-CEA, D6partement d e Recherches sur l a Fusion Contr61de. CEN Cadarache,

F-13108 Saint-Paul-lez-Durance Cedex 1, France

" ~ a c a h Institute of Physics, The Hebrew University, IL-91904 Jerusalem, Israel

The 2m-grating radius, extreme grazing incidence (1.5") Schwob-Fraenkel spectrograph was developed at the Racah Institute of Physics (under CEA contract) more than 10 years ago. The first results (using photographic plates) on the TFR tokamak permitted the indentification of the spectrum of highly ionised Mo ions in the 5-50

1

spectral region /I/. Subsequently, the system was modified by J.L. Schwob into a duochromator, using two channeltron electron multipliers independently movable along the Rowland circle. It was thus possible to obtain radial profiles of the emissivities of the strongest lines of the H-and He-like isoelectronic sequences of light impurities in the 18-42

a

spectral range /2/.

Recently, the duochromator has been converted into a multichannel spectrometer by equipping it with a microchannelplate (MCP) detector again movable along the Rowland circle. The detector consists of a MgF2 coated, funneled MCP, associated with a phosphor screen image intensifier and coupled by a flexible fiber optic conduit to a 1024 element photodiode array (controlled and read-out by a commercially available PAR-1461 EGG Princeton Applied Research optical multichannel analyser system). The first of this type of detector was developed at Princeton for the PLT and TFTR tokamaks and was described by Schwob et a1 /3/. An identical s stem has been installed on TFR, using a 20 ,um entrance slit and a 600 groove mm-Y Jobin-Yvon holographic grating. This instrument has been routinely used during the last year of TFR operation to monitor spectra of both intrinsic impurities (C, 0, Cr, Fe, and Ni, with traces of Mn, C1, and S ) and purposely injected impurity elements in the 10-330

a

spectral range. The spectrometer has been used in both the spectrographic and the polychromator modes. In the former mode, spectra of highly-ionized, unstudied, heavy elements (injected either by the laser blow-off technique or as gaseous elements) have been obtained /4,5/. In the latter utilization (in which selected individual pixels are read-out as function of time) line radiance evolutions of several different Fe ions have been simultaneously obtained on a single discharge. This has allowed the impurity transport to be modelled /6/ even though the system was not absolutely calibrated, since different ionization degrees have different time evolutions.

In this aper we shall present an overview of the system capabilities in the entire 10-330

1

spectral range accessible with the present grating.

The emitted spectrum of tokamak plasmas depends on the maximum value of the electron temperature Te, since all the ionization degrees exist, from neutral at the plasma periphery up to a maximum reached at the center. Indeed, impurities, produced in the scrape-off region by plasma-wall interactions, enter the peripheral plasma and are successively ionized during their inward movement, the central ionization degree being an increasing function of the ionization rate coefficients (increasing with both Te and the electron density ne) and a decreasing function of the speed of the radial movement. Since the impurity confinement time is much shorter than the discharge duration, central highly- ionized ions are lost, with little recombination, by an outward movement back to the walls.

During the last year of TFR operation, the highest ionization degree was obtained in deuterium plasmas having Te (0) ". keV the electron density continuously increasing up to ne (0) S 1.1 x

loi5

cm-3, during supplementary heating by a 600 kW neutral hydrogen beam between 130 and 250 ms. Figure 1 shows

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988125

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CI-120 JOURNAL DE PHYSIQUE

two spectra, obtained by averaging three successive individual spectra (20 ms read-out time) around the density maximum. Additionally, figure 2 shows that, even in the spectrographic mode, the data display system allows to follow the time evolutions of individual lines. The ne and Te values are sufficient to ionize Fe ions up to the Li-like state, but insufficient to reach the same isoelectronic sequence for Ni ions.

120.84 130.94 140.96 150.65 AIA] 159.85

n~ F N C O C

N E E I R R

nee e n e n n e x + h e

+se e 3 e ^{2 1} ¹ ^{1 1}

I I 6 I 1

100 500 700 pixels 900

Figure 1 : Photoelectric spectra of a high ne, high Te, NBI-heated plasma between 1 16 1 and 160 1 (upper) and between 259 1 and 329 1 (lower) ; the central pixel wavelengths are, respectively, about 141

1

and 290 1. Lower abscissa : pixel number ; upper abscissa : computed wave- length in

1

(see /4/for explanations) ; ordinate scale : count number. The highest observed ionization degree in this series of discharges be- longs to ~ e ~ 3 + . The strongest lines are indicated with their spectroscopic notations (whe- re for computer printing easiness arabic numerals are used instead of the more usual latin numerals) ; the wavelengths are given in hundredths of

1

and t means that the line is a second order.

0 I

- I I I

0 200 400 600 800 pixels 1000

Figure 2 : Time evolutions of 6 selected Fe and Cr lines for the same - 1 0 series of discharges as in figure 1. The time resolution corresponds to

O the 20 ms read-out time ;

-

on the spectra three pixels are read for each line, one at the line 0 peaks and the other two at two selected near-by

-

4 backgrounds ("fondl1 in french) on each side of the line. The wavelengths corresponding to the all

0 200 tlms] 400 0 200 tlmsl 400 these pixels are given in

hundredths of 8.

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or small values of the safety factor q, the hot plasma column disrupts with sudden cooling. The cooled plasma lasts, in the case of TFR, for about 5 ms while the plasma current Ip decreases to zero. Since this cold plasma is highly emitting, the spectrum obtained during the last read-out time is practically totally due to cold plasma emission. An example of such spectra is shown in figure 3. A numerical simulation of the time evolution of metal ion line radiances allows the electron temperature of the cold plasma to be evaluated, but the impurity diffusion coeffi- cient must be increased by approximately one order of magnitude with respect to the ohmic phase values /7/. For the disruption of figure 3, the estimated value is Te = 150

-

200 eV, since lines up to Fe XI11 are strong. The relative. temperature in a series of disruptions can be quite accurately estimated by comparing the intensity ratios of near-by lines belonging to different isoelectronic sequences of the same impurity. In practically all cases, Te = 100

-

200 eV, in agreement with evaluations from other diagnostics /7/.

The utilisation of the spectrometer in the spectro raphic mode (see figu-

re 1 ) has the advantage that a large spectral region (20 to 60 A, depending on

the carriage position on the Rowland circle) is recorded, but the time constant of the radiance time evolutions is too long (the minimum read-out time in this mode being 20 ms). This is insufficient to follow the effects of sudden variations of either Te, ne, impurity density or plasma impurity transport. For these cases, the polychromator mode of operation is more useful /6/ ; an example is shown in figure 4, referring to the injection of a frozen hydrogen pellet into an ohmic plasma.

168.62 180.12 197.36 A[A] 202.24

FF F D O FF F FF F O F F F F F F F F OFF F F F F C F

€€ E EL E EE L t l E E E E E E E E E E E R E

6 6 1 1 ¹ 11 1 6 1 1 1 1 1 1 S l i ^{1 1}¹ ⁱ 1 1; a s ee e $ 1 1 e e s a i l e r l e e 3 r 3 7 s

0 1 _I _I _I I ^I

200 400 600 pixels 800

Figure 3 : "Post-disruption" spectrum between 167

8

and 204

8.

The strongest lines are indicated in the same way as in figure 1. Besides a few oxygen lines, only low ionization Fe lines (between Fe VIII and Fe XIII) are detected. Ni and Cr ions of the same isoelectronic sequences have also been observed in cold plasma spectra at shorter and longer wavelengths, respectively.

Figure 4 : Time evolu- FFXIX(1083) fP' tions in the polychroma-

FEXWI(103.9) 1'' I tor mode of the line

FEXXII 1172

100

i 50 '

0 50 100 150 200 250 t(ms)

FEXXIII 132 9

200

o 50 100 150 200 250 Nmr)

radiances of six successive Fe ionisation degrees for a plasma in which frozen hydrogen pellets are injected at 180 ms (PI). The lines are indicated with their spectroscopic notations and their wavelengths are giveh in

8 .

Individual, preselected, line (plus background) pixels are followed with a read-out time of 1.5ms. The effective time constant is longer as a consequen- ce of the large noise level.

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CI-122 JOURNAL DE PHYSIQUE

Six successive Fe ionization degrees are simultaneously detected with a read-out time of 1.5 ms. At pellet injection (t = 180 ms) the radiances of Fe XVIII and XIX increase, whereas those of Fe XXII and XXIII decrease (the radiances of Fe XX and XXI remaining roughly constant within the large noise level). Numerical simula- tions /6/have allowed all these time evolutions to be simulated as a combined effect of sudden plasma cooling and ne increase, together with a reduction of the incoming peripheral neutral Fe flux.

Finally, the spectrometer can be useful also at the shortest available wavelengths, in spite of the insufficient spectral resolution obtained with the 600 groove mm-1 grating. Figure 5 shows a Ne spectrum in the 10-23

8

spectral range, obtained after injection of a Ne-doped frozen hydrogen pellet. The limited spectral resolution is sufficient for recording strong Ne lines, but insufficient for intrinsic metal ion lines, since the 10-20 fi range is very crowded due to n = 2 to n = 3 transitions of Ne-like and higher ionization ions.

Figure 5 : Neon spectrum in the 10-23

1

range. The Ne X Lyman a line is well isolated, whereas the resonance and intercombi- nation lines of Ne IX are not resolved, appearing as a single strong line.

The strongest Ne and 0 lines are indicated in the same way as in figure 1

.

R E F E R E N C E S

/ I / Schwob J.L., Klapisch M. et al., Phys. Lett.

62

A, 85 (1977) /2/ TFR Group, Doyle J.G. and Schwob J.L, J . Phys B

15,

813 (1982)

/3/ Schwob J.L., Wouters A.W. et al., PPPL Report 2419 (1987), submitted to Rev.

Scient. Instrutn.

/4/ TFR Group and Wyart J.F, EUR-CEA-FC Report 1323 (1987), to be published in Physica Scripta

/5/ Breton C., De Michelis C. et al., EUR-CEA-FC Report 1328 (1987), to be published in Physica Scripta

/6/ Equipe TFR and FOM ECRH Team (presented by B. Saoutic), Proc. XIVth Europ.

Conf. Contr. Fusion Plasma Phys., Madrid June 1987, part 11, 682 /7/ TFR Group, Nuc.L. Fusion 25, 919 (1985)