ROTATING X-RAY CRYSTAL SPECTROMETER

A soft x-ray spectrometer, based on six flat rotating crystals that demonstrate Bragg diffraction (n=2d sin) has been developed, constructed, and put into use on FTU.

The detector consists of three stacked multiwire proportional chambers (MWPCs). The different absorption efficiencies of the three MWPCs allows resolution of up to three different orders of diffraction [1]. The spectrometer scans a wide energy range (2 to 20 keV) with a high counting rate (up to 2 MHz), time resolution for a single spectrum of ~ 33 ms, and resolving power for spectral features ~ 300. The spectrometer is absolutely calibrated and can operate with different crystals. It is compact and, because of its straightforward setup, has low maintenance requirements and great flexibility, and operates reliably in difficult environmental conditions.

The data obtained with the spectrometer allow the study of line transitions and the corresponding background (consisting of both bremsstrahlung and a recombination continuum). At the high end of the energy range (1.6 to 2.6 Å) scanned by the spectrometer, emissions of Ni, Fe, Cr and Ti are present. Because of the low spectral resolution of the crystal, each feature is composed of transitions from HeI-, LiI, and BeI-like ions of the respective impurity atoms. At lower energies (4.3 to 5.3 Å), the spectrum is dominated by strong L-shell emission features of highly ionized molybdenum (Mo33+, Mo32+, ...). Since these characteristics ( and the L-shell) are only emitted from a region of the plasma where the electron temperature >1 keV, their observation gives information on the central impurity content in FTU. By computing the charge state distributions as a function of temperature for the elements in question and using detailed collisional-radiative models for the emissivity of the above transitions, it is possible to assess the core-averaged impurity densities of the Ni, Fe, Cr, Ti, and Mo ions. It has been found that in the FTU plasma discharges, these impurities have central densities ranging from approximately 1015 to 1017 m-3.

Due to the complexity of the L-shell emission spectra of the highly ionized molybdenum ions, a fruitful collaboration has been undertaken between ENEA-Frascati, the Lawrence Livermore National Laboratory, and the John Hopkins University (Baltimore MD, USA). A quasi-steady-state collisional-radiative model for Mo33+ to Mo30+ has been developed [2]. The model employs ab initio relativistic atomic structure calculations and takes into account resonance contributions to collisional excitation rates and processes involving continuum states such as excitation-auto-ionization and dielectronic recombination. The model has been used to interpret emission features from ions near the NeI-like charge state; the low-resolution soft x-ray spectra taken on FTU are well reproduced.

Detailed analysis of the spectra of Mo33+ to Mo30+ not only provides a value for the total concentration of molybdenum in the plasma, but also valuable information on the transport processes affecting the ion distribution in the plasma and the excitation mechanisms that give rise to the observed spectral features.

Reference

[1] R. Bartiromo, et al. : Nucl Instrum. & Meth. Phys. Res. B95,537 (1995)

[2] K.B. Fournier et al: Phys. Rev E5,1,1084 (1996)


FTU Diagnostics