BROADBAND (1-25Å) X-RAY CRYSTAL SPECTROSCOPY OF IMPURITY IONS IN THE DITE TOKAMAK

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BROADBAND (1-25Å) X-RAY CRYSTAL SPECTROSCOPY OF IMPURITY IONS IN THE

DITE TOKAMAK

R. Barnsley, K. Evans, N. Hawkes, N. Peacock

To cite this version:

R. Barnsley, K. Evans, N. Hawkes, N. Peacock. BROADBAND (1-25Å) X-RAY CRYSTAL SPEC-

TROSCOPY OF IMPURITY IONS IN THE DITE TOKAMAK. Journal de Physique Colloques,

1988, 49 (C1), pp.C1-207-C1-210. �10.1051/jphyscol:1988140�. �jpa-00227460�

JOURNAL D E PHYSIQUE

Colloque C1, Supplement au n03, Tome 49, Mars 1988

BROADBAND (1-25A) X-RAY CRYSTAL SPECTROSCOPY OF IMPURITY IONS IN THE DITE TOKAMAK

R. BARNSLEY, K.D. EVANS, N.C. HAWKES* and N.J. PEACOCK*

Dept. of Physics. University of Leicester, GB-Leicester LEI 7 R X . Great-Britain

" ~ u l h a m Laboratory, GB-Abingdon OX14 308, Oxon, Great-Britain

Abstract.- We present results from a Bragg rotor spectrometer and refer to wide-ranging studies of intrinsic and injected impurity ions in the DITE Tokamak.The instrument has absolute calib~ation for flux and wavelength and can detect impurities at the part per million level in the band from 1 to 24A.

Monochromator and fast spectral survey modes, combined with a spatial scan facility, result in a versatile and powerful diagnostic tool.

To date, most crystal spectroscopy of laboratory plasmas has been carried out using Rowland circle [1,21 or Von Hamos [ 3 1 spectrometers with relatively high resolving power and narrow spectral range. In contrast, the results presented here were obtained with a Bragg spectrometer [4,5] which is a development of the flat crystal, gridded collimator, spectrometers which have been more comon in x-ray astronomy [6,71.

The Bragg rotor spectrometer has absolute calibration for flux and wavelength, and uses slotted or gridded collimators to achieve a resolving power of between about 200 and 2000. A hexagonal rotor supports six interchangeable crystals and may be locked in a monochromator mode, or rotated at up to lOHz for spectral surveys. The stationary detector is a multiwire gas proportional counter (MWPC) which can operate at count rates up to ~OMHZ. All spectrometer functions and data acquisition are computer-controlled, and radial profiles can be obtained by tilting the spectrometer vacuum system on a shot-to-shot basis.

The spectrometer has provided valuable diagnostic information in

experiments including: studies of transport of injected impurities for a wide range of DITE operating conditions[8,9]; detatched plasmas [lo]; the effect of limiter design on impurities [to be published]; and the recently installed electron cyclotron resonance heating ( E m ) . The measured time evolution and radial profiles of injected impurities have also been modelled with a transport code [ll].

A major function of the spectrometer has been to detect and monitor all impurities of Z ) 7 (N). Figure 1 shows two survey spectra recorded with a rotor speed of 4Odeg/s. The feature at 3.87A results from an increase in detector efficiency below the Argon K-edge (detector fill gas: 9 0 W 1 1 0 % ~ 4 ) and confirms that the continuum signal between the lines is from the plasma bremsstrahlung. The intrinsic impurities 0, Si, S, C1, Cr and Fe can all be surveyed in a single discharge, and on different occasions unplanned incidences of F, Ng, Al, Si and Ni have been detected immediately, allowing diagnosis of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988140

CI-208 JOURNAL DE PHYSIQUE

faults within the tokamak. The wide wavelength coverage and good sensitivity (typically, impurities can be detected at the level of one part per million of the electron density ) have also been utilised for the study of injected Ne, I d , Si, Ar,and Ti.

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Figl Broadband spectra of intrinsic impurities in the DITE Tokamak.

(a) Recorded with an RAP crystal (2d

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Figure(2a) shows silicon spectra recorded during a single DITE discharge.

The time resolution of looms was achieved with three PET crystals on the rotor, and a rotor speed of 3.33 Hz. Additional heating (ECRH) was applied between 330ms and 530ms and the effect on the Si XIV / Si XI11 ratio can be seen clearly. Radial profiles of such a spectrum allow the ionisation balance to be determined, and hence the ionisation temperature profile to be derived for the inner region of the the plasma where there is approximate coronal equilibrium.

The effect of non-equilibrium conditions is shown by the radial dependence of the He-like Silicon spectrum in figure (2b) where, in the low temperature (Te = 100 eV at 16cm) outer region of the plasma, the Si XIII is recombining.

The Al XI11 profile in figure (2c) shows that although the radiation from the outer plasma is relatively small, it is still many orders of magnitude greater than would be predicted by the coronal model. This radiation cannot be explained by electron impact excitation of ions which are out of thermal equilibrium, and is due to radiative recombination of bare A . nuclei which have been transported from the core. This radiation, which is extremely sensitive to the ion transport, has been observed and modelled for Ar in Alcator-C [12].

In surrnnary the Bragg rotor spectrometer is a versatile diagnostic which has been used as a routine impurity monitor,and for several plasma physics experiments.

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Wovelength IAI Wavelength (il ^Minor Rodius ( c m l I L i m i t e r : Z l r m l

~ i g 2 (a) Silicon spectra from a single DITE discharge, recorded with three PET crystals on the rotor, and a rotor speed of 3.33 Hz. Additional heating ( E m ) was applied from 330ms to 530ms. (b) Radial dependence of the spectrw of He-like Si. ( c ) Abel-inverted emission profile of A. XI11 1s-2p (7.17A). The excess radiation at outer radii (compared with a coronal model) is due to the

radiative recombination of bare A1 nuclei.

Cl-210 JOURNAL DE PHYSIQUE

References

[I] Hill,K.W., et all Rev-Sci .Instrum. 56(6)(1985)1165

[2] Platz,P., Ramette,J., et all J.Phys.E:Sci.Instrum. 14(1981)448 [3] Kallne,E., et all Physica Scripta 31(1985)551

[4] Barnsley,R., et all Plasma Diag. Tech., Conf. proc., Varenna Sept. 1986 (publ. by Int. School of Plasma Physics "Pier0 Caldirola" Milano) [5] Barnsley,R., et al, Rev.Sci.Instrum. 57(8)(1986)2159

[6] Evans,K.D.,et all Space.Sci.1nstrum. 2(1976)339 [7] Phillips,K.J.H., et a1,Ap.J. 256(1982)774

[8] Allen,J., et al, Paper A-1B-4, Proc .llth Conf. on Plasma Physics and Contr. Nucl. R~sion. Kyoto Nov. 1986 (Publ. by IAEA Vienna) [9] Barnsley,R., et a1 Culham Report Cm-P792

[lo] McCracken,G.N., et al, Nucl.Inst.Meth. 145-147(1987)181

[ll] Hawkes,N.C., et a1 Proc -14th Eur. Conf. on Contr. Fusion and Plasma Phys., Madrid 1.987 (publ. in ECA 11D Ptl)

I121 Rice,J.E., et a]., Phys.Rev.A 35 (1987)3303