Spectroscopic investigations of divertor detachment in TCV

Abstract

The aim of this work is to provide an understanding of detachment at TCV with emphasis on analysis of the Balmer line emission. A new Divertor Spectroscopy System has been developed for this purpose. Further development of Balmer line analysis techniques has allowed detailed information to be extracted from the three-body recombination contribution to the n=7 Balmer line intensity. During density ramps, the plasma at the target detaches as inferred from a drop in ion current to the target. At the same time the Balmer $6\rightarrow2$ and $7\rightarrow2$ line emission near the target is dominated by recombination. As the core density increases further, the density and recombination rate are rising all along the outer leg to the x-point while remaining highest at the target. Even at the highest core densities accessed (Greenwald fraction 0.7) the peaks in recombination and density may have moved not more than a few cm poloidally away from the target which is different to other, higher density tokamaks, where both the peak in recombination and density continue to move towards the x-point as the core density is increased. The inferred magnitude of recombination is small compared to the target ion current at the time detachment (particle flux drop) starts at the target. However, recombination may be having more localized effects (to a flux tube) which we cannot discern at this time. Later, at the highest densities achieved, the total recombination does reach levels similar to the particle flux.Comment: Article accepted for publication in Journal of Nuclear Materials and Energ

Novel inferences of ionisation and recombination for particle/power balance during detached discharges using deuterium Balmer line spectroscopy

“…Partially due to that, volumetric recombination is often expected to play a central role in target ion flux reduction [21][22][23][24][25][26]. However, in previous work on TCV [6], the volumetric recombination rate was shown to be only a small fraction of the reduction of ion flux, which is in agreement with recent TCV simulations [27] as well as N2-seeded discharges in C-Mod [6]; further emphasizing the need for ionisation measurements. u Ionisation is the primary determinant of It (equation 2) during attached operation and at the detachment onset.…”

“…Although experimental indications for power limitation are available (either from inferred ion sources [9], or from qualitative spectroscopic 'indicators' based on Dα [30]), one weakness of previous results is that quantitative information on both divertor power/particle sinks/sources was not available. However, this study and other recent parallel studies [31,32] aim to provide quantitative information on ionisation during divertor detachment [31,33]. These recent parallel studies [31,32] have similar goals but differ in the solution method and require more diagnostic measurements, such as specialised VUV spectroscopy and recombination edge (365 nm) measurements.…”

Stark broadening of Balmer lines with low and moderate quantum number in dense divertor plasmas

“…Figure shows an example of a spectrum analysis. We have performed an adjustment of D δ and D ϵ observed in the divertor of TCV in the detached regime (shot # 52062 at t = 1.0 s, central line of sight). The real‐time application of the numerical simulation technique presented in Section 2 is not convenient, so in this framework we have constructed a database directly exploitable by an adjustment program through an interpolation routine (see Ref.…”

“…Values of 5 × 10 13 and 6 × 10 13 cm −3 were obtained for the electron density; the deviation between these values falls in the typical range of uncertainty expected from the accuracy of line‐shape model and experiment (in particular this includes the fitting method and the instrumental function characterization). Values of the same order of magnitude were obtained previously using a different Stark line‐shape model …”

“…The corresponding dipole moment is large enough for the atomic energy levels to be sensitive to the plasma's microscopic electric field (Stark effect). Accordingly, the emission lines observed in passive spectroscopy can be used as a probe of the electron density; recently, analyses have been performed on spectra observed in the divertor of ASDEX‐Upgrade, JET, and TCV . An issue that remains unclear concerns the validity of using a pure Stark broadening model, that is, not accounting for other line‐broadening mechanisms, on Balmer lines with a low or moderate quantum number such as D α and D β ( n = 3 → 2 and 4 → 2 transitions).…”