Fig. 5: Increasing the divertor leg length does not alter the ionisation region after detachment, in agreement with simulations.

Synthetic D₂ Fulcher emission from SOLPS-ITER simulations for the Super-X (SXD) (a, blue), Elongated (ED) (b, green) and Conventional Divertors (CD (c, red), overlaid with 5 eV contours (dashed lines) and the separatrix (solid line). d–f Experimentally measured D₂ Fulcher band emission (595-605 nm) for a strike point scan with magnetic equilibrium shown, moving from CD to SXD at constant density and power, obtained through inverting Multi-Wavelength-Imaging (MWI) imaging data for # 46895⁵⁷. g–i 1D ion sources and sinks (ionisation - magenta, Molecular Activated Recombination (MAR) ion sink - magenta, Electron-Ion Recombination (EIR) ion sink - cyan), obtained from spectroscopic analysis integrated along the spectroscopic lines of sight (Fig. 1b) (part. m⁻² s⁻¹), compared against synthetic diagnostic results from SOLPS-ITER simulations (dotted lines). For the SXD (g) two SOLPS-ITER simulation results are shown: one with default rates and one with corrected molecular charge exchange (\({{{{\rm{D}}}}}_{{{{\rm{2}}}}}+{{{{\rm{D}}}}}^{+}\to {{{{{\rm{D}}}}}_{{{{\rm{2}}}}}}^{+}+{{{\rm{D}}}}\)) rates (‘Sim. Corr. Rate’), obtained from³⁴, which increases MAR. To guide the eye, a shaded magenta vertical line has been added at a radius of 0.95 m and a black arrow has been added at the strike point location (a–i). The experimental results in (g, h, i) are derived from a probabilistic sample obtained from a Bayesian spectroscopic analysis, showing the median (solid lines) and the 68% equal-tailed confidence interval (shaded region). See Methods section for more information about the analysis and uncertainty propagation.