Investigation of C IV line broadening mechanisms for plasma diagnostics in magnetic fusion devices
Introduction
Spectroscopic techniques play an important role in the characterization of different plasmas including those produced in magnetic fusion devices. It is known that plasma diagnostics based on spectroscopic measurements of line emission use line intensities, line shapes or both to obtain one or several plasma parameters like the densities and temperatures of the different particles (ions, electrons or/and neutrals). For instance in the JT-60U Tokamak, the electron density and temperature characterizing the plasma in the divertor region around the X-point have been deduced from the measured intensities of several C IV lines emitted by C+3 ions. In that case, a collisional-radiative model has been used to calculate the population densities of the C+3 excited levels involved in the measured C IV emission lines. The comparison of these theoretical population densities with the experimental ones (obtained from the line intensities) allows to evaluate the plasma parameters of the emissive region [1], [2]. Typically, electron densities in the range 1020–1021 m−3 and electron temperatures 1 eV ⩽ Te ⩽ 10 eV have been deduced for detached plasmas with a MARFE in JT-60U [1], [2]. Moreover, electron densities up to 1022 m−3 have been estimated near the X-point from the measured data on this machine [1]. Such high electron densities have still to be confirmed by other independent methods, and among them Stark broadening is the most convenient. For a given species (carbon in our case), the lines which are most sensitive to Stark effect are those resulting from highly excited upper levels of the emitter which has the highest ionization stage, i.e. hydrogen-like carbon ions C+5 (fully stripped ions are excluded since they do not emit any line). However, H-like carbon ions are mainly present in central regions of the plasma where Te is of the order of few hundred eVs (e.g. Ti = 100–300 eV in TEXTOR [3]). Like in other Tokamaks, the region around the X-point in JT-60U is characterized by an electron temperature of only few eVs which favours the predominance of Li-like carbon ions C+3 over all other ionization stages of carbon. Therefore only lines emitted by C+3 ions are considered in this paper. In addition to that, the higher the upper principal quantum number of the transition, the more important is the Stark effect. Our choice to study the profiles of the n = 5–6 and n = 6–7 lines is imposed by the previous considerations and the quality and availability of the spectra which have been measured in JT-60U. The ultimate aim of such a study is to compare the calculated profiles to spectra from JT-60U and other fusion devices. This work is ongoing and the results of such comparisons will be published elsewhere in the near future.
This paper is organized as follows. In Section 2, we will show how the atomic physics data basis is built. In Section 3, the broadening mechanisms affecting the considered C+3 lines and their profiles will be presented. The results will be discussed in Section 4 and a conclusion is drawn in Section 5.
Section snippets
Construction of the atomic physics data basis
The line shape code that we have used for this work is the PPP code [4] which is based on the standard model of Stark broadening [5]. Similarly to all other line shape codes, PPP requires an input file which contains in addition to the plasma parameters some important atomic physics data like the energies and the dipole matrix elements of the atomic transitions. To build the atomic data basis we have proceeded as follows: first we have applied the Cowan’s code [6] to the C+3 levels for
Broadening mechanisms and line shape calculations
We are interested here by the profiles of the n = 5–6 and n = 6–7 lines emitted by C+3 ions in a deuterium plasma characterized by a relatively high electron density 1019–1021 m−3 and a low electron temperature 1–10 eV with the presence of a typical magnetic field B = 1–3.5 T. From the examination of the energies of all levels, it clearly appears that level splitting due to fine structure effect is negligible in comparison with that caused by Stark and Zeeman effects.
For clarity and simplicity, the
Results and discussion
Let us start with profiles calculated without Zeeman effect. As Doppler broadening is proportional to the square root of the emitter temperature, the Doppler width increases by a factor of about 3 when T varies from 1 to 10 eV. On the other hand, the Stark width due to the electron contribution changes drastically (as it is proportional to the electron density) when Ne varies from 1019 to 1021 m−3, the dependence of Stark broadening on the electron and ion temperatures being weak. Therefore
Conclusion
Line broadening mechanisms (Doppler, Stark and Zeeman effects) affecting the profiles of the C IV n = 5–6 and n = 6–7 lines have been examined for plasma conditions relevant to detached divertor plasmas. A line shape code has been used to calculate profiles with the aim of comparing them with experimental spectra from JT-60U and other fusion devices. Preliminary comparisons with published spectra from JT-60U show that the electron density in the JT-60U divertor around the X-point should not exceed 2
Acknowledgements
This work is supported by the project “Radiation absorption effects” of the “Fédération de Recherche sur la Fusion par Confinement Magnétique” FR-FCM in the frame of the LRC DSM 99-14 (PIIM Laboratory/CEA Cadarache) and by the ANR PHOTONITER contract (ANR-07-BLAN-0187-01).
References (10)
Nucl. Fus.
(2007)J. Plasma Fus. Res.
(2004)Plasma Phys. Control. Fus.
(2000)Phys. Rev. A
(1995)Plasma Spectroscopy
(1964)
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Diagnostics of JT-60U divertor plasmas by Stark-Doppler broadening of carbon spectral lines
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