Investigation of high-n Balmer lines and D₂-Fulcher band during high-recycling and detached divertor DIII-D plasma

In this study, D₂ molecular band measurements are used to provide improved temperature constraints in a detached plasma by applying recent Bayesian-based Balmer spectroscopy analysis at DIII-D [1,2]. Combined D₂ Fulcher band and high-n Balmer line spectroscopic measurements are made on the DIII-D tokamak to investigate high-recycling/detached divertor plasma conditions. Earlier work [2] has shown that D₂ molecular spectroscopy can serve as a convenient diagnostic of background plasma temperature and that D₂ radiated power is a useful diagnostic for constraining neutral transport modeling. To understand the evolution of D₂-Fulcher band emission, the 598.5 to 632.5 nm wavelength range is monitored with four distinct wavelength intervals due to spectrometer bandwidth limitations: Q (0-0), Q (1-1) and Q (2-2) of Q-branch transitions are observed. Rotation temperatures of ~ 4500 K (Q (0-0) J=2 to 9) are measured. In addition, high-n Balmer line ratios measured using a low-resolution Avantes and Wide Spectral Emission spectrometer are compared with D_β/D_α ratios measured with the high-resolution Multichord Divertor Spectrometer. These ratios provide a convenient means to determine important information on the dominant atomic and molecular processes in cold, dense divertor plasma.



References:

[1] Verhaegh et al., Nucl. Fusion, 63 [2023]

[2] N Osborne et al., Plasma Phys. Control. Fusion, 66 [2024]

[3] Hollmann et al., Phys. Plasmas, 30 [2023]