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.