Validation of 2D Te and ne measurements made with Helium imaging spectroscopy in the volume of the TCV divertor

Abstract

Multi-spectral imaging of helium atomic emission (HeMSI) has been used to create 2D poloidal maps of Te and ne in TCV’s divertor. To achieve these measurements, TCV’s MANTIS multispectral cameras simultaneously imaged four He I lines (2 singlet and 2 triplet) and a He II line (468nm) from passively present He and He+. The images, which were absolutely calibrated and covered the whole divertor region, were inverted through the assumption of toroidal symmetry to create emissivity profiles and, consequently, line-ratio profiles. A collisional-radiative model (CRM) was applied to the line-ratio profiles to produce 2D poloidal maps of Te and ne. The collisional-radiative modeling was accomplished with the Goto helium CRM code which accounts for electron-impact excitation and deexcitation (EIE), and electron-ion recombination (EIR) with He+. The HeMSI Te and ne measurements were compared with co-local Thomson scattering measurements. The two sets of measurements exhibited good agreement for ionizing plasmas: (5 eV ≤ Te ≤ 60 eV, and 2 × 1018 m-3 ≤ ne ≤ 3 × 1019 m-3) in the case of majority helium plasmas, and (10 eV ≤ Te ≤ 40 eV, 2 × 1018 m-3 ≤ ne ≤ 3 × 1019 m-3) in the case of majority deuterium plasmas. However, there were instances where HeMSI measurements diverged from Thomson scattering. When Te ≤ 10 eV in majority deuterium plasmas, HeMSI deduced inaccurately high values of Te. This disagreement cannot be rectified within the CRM’s EIE and EIR framework. Second, on sporadic occasions within the private flux region, HeMSI produced erroneously high measurements of ne. Multi-spectral imaging of Helium emission has been demonstrated to produce accurate 2D poloidal maps of Te and ne within the divertor of a tokamak for plasma conditions relevant to contemporary divertor studies.

Characterisation of detachment in the MAST-U Super-X divertor using multi-wavelength imaging of 2D atomic and molecular emission processes

“…This approach was recently demonstrated using multispectral He-I imaging with the MANTIS system at TCV. Good agreement to co-local Thomson scattering measurements in majority deuterium plasmas was obtained in the operating space 10 eV ⩽ T e ⩽ 40 eV, 2×10 18 m −3 ⩽ n e ⩽ 3×10 19 m −3 [19]. Extension of this He-I emission analysis is required before application to MAST-U discharges, where temperatures in the detached phase are well bellow this threshold.…”

“…Neutral helium spectral emission was recently shown to provide additional avenues to inferring divertor density and temperature profiles [19]. The He content was too low for spectroscopic detection in the experiments discussed in this work, but He puffing for diagnostic purposes will be tested in upcoming campaigns.…”

“…In this process, additional constraints are provided by other divertor diagnostics such as interferometry, Thomson scattering, Langmuir probes and spectroscopy. Such methodologies have been developed and applied for various tokamaks such as TCV [17][18][19], JET [20][21][22][23] and MAST-U [24].…”

Estimating cross-field particle transport at the outer midplane of TCV by tracking filaments with machine learning

“…Moreover, specification of the training data provides intuitive means to control algorithm performance, and allows the algorithm to replicate the outcome of a human analyst. As a future work, the method can be improved by more accurately measuring the local blob density, for example with He I line-ratio methods [34,35] or by applying ML to GPI using He puffs [36], so that the density fluctuation in equation ( 7) can be directly measured without using the approximated relation with the GPI brightness in equation (6).…”

Comparison of electron temperature and density measured by helium line intensity ratio and Thomson scattering methods in ECH spherical tokamak plasma

“…With these as background, in this study, we aim to obtain experimental data on the performance of the helium line intensity ratio method in T e and n e measurements in a toroidal plasma, where T e and n e are the electron temperature and density, respectively. The technique can be used to evaluate the scrape-off layer (SOL) and divertor plasmas [9][10][11][12][13][14][15][16][17], and implemented only by passive emission spectroscopy, which is compatible with light collection through the ducts. Thus far, the method has been applied to various plasmas, and among them, recent results of comparisons with other methods [9][10][11][12][13][14] are plotted in figure 1.…”