Interpretation of the Balmer series-limit recombination spectrum in the MAST-U divertor

The suite of spectrometers monitoring the lower divertor [Verhaegh et al., Nucl. Fusion 63, 016014 (2022)] on the MAST Upgrade tokamak has been extended before the start of the MU04 campaign. A new fiber bundle with good transmission in the near-Ultraviolet was installed to increase the number of instruments viewing the same radial profile in the divertor to a maximum of 4. This was done to accommodate a dedicated instrument to measure the Balmer series-limit recombination continuum around 365 nm. The ratio of the continuum above and below the recombination "edge," as well as the spectral shape of the free-bound continuum, allows us to derive the electron temperature in recombining plasmas independently of the excitation pathways [Lomanowski et al., Plasma Phys. Controlled Fusion 62, 065006 (2020)]. The electron temperature is key to understanding heat exhaust processes in the divertor, which is essential for future fusion power plant design. A good diagnosis has been achieved of deeply detached, electron impact recombining plasmas, confirming Te = 0.05-0.3 eV [Verhaegh et al., Nucl. Fusion 63, 126023 (2023)]. The electron temperature derived from the wavelength dependence of the continuum below 365 nm is in good agreement with the electron temperature derived from the intensity ratios of the high-n Balmer lines for ne = 1018-1019 m-3. At higher divertor temperatures, molecular assisted recombination mechanisms dominate, and the emission of CD and C2 molecules dominates the spectra, making it difficult to extract the temperature from the continuum.