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Initial observations in X-point target divertor discharges on MAST-U

N. Lonigro, K. Verhaegh, J. Harrison, B. Lipschultz, C. Bowman, F. Federici, J. Flanagan, D. Greenhouse, D. Moulton, P. Ryan, R. Scannell, S. Silburn, T. Wijkamp, D. Brida, C. Theiler, the EUROfusion Tokamak Exploitation Team, the MAST Upgrade Team

TL;DR

Addresses divertor exhaust challenges for future reactors by testing the X-point target (XPT) divertor on MAST-U, a configuration that combines a SXD-like large strike-point radius with a secondary X-point near the separatrix. Uses high-power H-mode discharges and diagnostics (imaging, coherence imaging spectroscopy, neural-net analysis of molecular processes, and Langmuir probes) to compare XPT against SXD and quantify volumetric sinks and target conditions. Finds that XPT broadens cross-field density profiles and increases plasma-neutral interactions, with stronger MAR/MAD and EIR emissions, resulting in lower target electron temperatures and reduced peak heat/particle fluxes; heat-flux estimates follow $q_{||} = (\

Abstract

The first high-power (> 3 MW) H-mode experiments using a double-null X-point-target (XPT) divertor configuration have been performed on MAST-U. The XPT geometry is obtained by combining a large strike point radius, similar to the Super-X divertor (SXD), with an additional X-point near the separatrix in the baffled outer divertor chambers and leads to additional exhaust benefits over the SXD. The broader electron density profile near the secondary X-point leads to additional plasma-neutral interactions, evidenced by a broader hydrogenic emission profile, and resulting in larger power and ion sinks. The increase in plasma-neutral interactions also leads to lower target electron temperatures and heat fluxes. These benefits appear to extend to transients, and preliminary evidence of improved ELM buffering in the XPT is presented. These results showcase how multiple alternative divertor configuration strategies can be combined to improve momentum, power, and particle losses, which may be required for the challenging exhaust conditions of future reactors.

Initial observations in X-point target divertor discharges on MAST-U

TL;DR

Addresses divertor exhaust challenges for future reactors by testing the X-point target (XPT) divertor on MAST-U, a configuration that combines a SXD-like large strike-point radius with a secondary X-point near the separatrix. Uses high-power H-mode discharges and diagnostics (imaging, coherence imaging spectroscopy, neural-net analysis of molecular processes, and Langmuir probes) to compare XPT against SXD and quantify volumetric sinks and target conditions. Finds that XPT broadens cross-field density profiles and increases plasma-neutral interactions, with stronger MAR/MAD and EIR emissions, resulting in lower target electron temperatures and reduced peak heat/particle fluxes; heat-flux estimates follow $q_{||} = (\

Abstract

The first high-power (> 3 MW) H-mode experiments using a double-null X-point-target (XPT) divertor configuration have been performed on MAST-U. The XPT geometry is obtained by combining a large strike point radius, similar to the Super-X divertor (SXD), with an additional X-point near the separatrix in the baffled outer divertor chambers and leads to additional exhaust benefits over the SXD. The broader electron density profile near the secondary X-point leads to additional plasma-neutral interactions, evidenced by a broader hydrogenic emission profile, and resulting in larger power and ion sinks. The increase in plasma-neutral interactions also leads to lower target electron temperatures and heat fluxes. These benefits appear to extend to transients, and preliminary evidence of improved ELM buffering in the XPT is presented. These results showcase how multiple alternative divertor configuration strategies can be combined to improve momentum, power, and particle losses, which may be required for the challenging exhaust conditions of future reactors.
Paper Structure (16 sections, 3 equations, 16 figures)

This paper contains 16 sections, 3 equations, 16 figures.

Table of Contents

  1. Introduction
  2. XPT discharge overview
  3. Evidence of 2nd null action in experimental discharges
  4. Comparing the exhaust performance of the Super-X and X-point target divertor configuration
  5. Broadening of the plasma-neutral interaction region
  6. Volumetric processes
  7. Target quantities
  8. ELM buffering
  9. Discussion
  10. The effect of increasing connection length
  11. Comparison of detachment location sensitivity
  12. Extrapolations with the detachment location sensitivity model
  13. Limits in the experimental scenarios
  14. Implications for reactors and ADC research
  15. Conclusions
  16. ...and 1 more sections

Figures (16)

  • Figure 1: Comparison of experimental EFIT magnetic reconstructions of conventional (CD, #49139),Super-X (SXD, #49323), and X-point target (XPT, #49320) divertor configurations.
  • Figure 2: (a) EFIT magnetic reconstruction of XPT discharge #49320 at 0.5s with the separatrix plotted as a solid line and (b) comparison of poloidal field coil currents used to achieve the SXD and XPT configurations. The coil current limits are shown in black.
  • Figure 3: Overview of SXD and XPT discharges, showing the line-averaged density, power crossing the separatrix ($P_{SOL}$), stored energy, distance between the separatrices, midplane $D_\alpha$ signal, and divertor neutral pressure measured by the pressure gauge.
  • Figure 4: Balmer alpha inter-ELM camera images in the divertor chamber overlaid on a wireframe of the machine in (a) SXD and (b) XPT configuration. Corresponding 2D emissivity inversions in (c) SXD and (d) XPT configurations with flux surfaces overlaid in white. The emission near the baffle and the discontinuity in the emission near the divertor entrance are considered inversion artefacts due to the poor diagnostic coverage of the region. The region that can be affected by these artifacts is shaded in gray.
  • Figure 5: $n_e$ profiles inferred via coherence imaging spectroscopy in (a) SXD and (b) XPT configurations. Overlaid are the separatrix (black) and flux surfaces (red) from the EFIT magnetic reconstruction. The peak density is observed in the PFR and the $\Bar{\psi} = 0.975$ surface is shown in orange for reference. The cross-field line at 15 cm from the target (L = 0.15) along the separatrix is also shown, and the profiles along this line are compared in figure \ref{['fig:fig7_cross_field']}.
  • ...and 11 more figures