Investigations of atomic and molecular processes of NBI-heated discharges in the MAST Upgrade Super-X divertor with implications for reactors
K. Verhaegh, James Harrison, Bruce Lipschultz, Nicola Lonigro, Stijn Kobussen, David Moulton, Nick Osborne, Peter Ryan, Christian Theiler, T. Wijkamp, Dominik Brida, Gijs Derks, R.S. Doyle, F. Federici, A. Hakola, S. Henderson, Bob Kool, Sarah Newton, Ryoko Osawa, Xander Pope, Holger Reimerdes, Nicola Vianello, Marco Wischmeier, the EUROfusion Tokamak Exploitation Team, the MAST-U Team
Abstract
Abstract This experimental study presents an in-depth investigation of the performance of the MAST-U Super-X divertor during NBI-heated operation (up to 2.5 MW) focussing on volumetric ion sources and sinks as well as power losses during detachment. The particle balance and power loss analysis revealed the crucial role of Molecular Activated Recombination and Dissociation (MAR and MAD) ion sinks in divertor particle and power balance, which remain pronounced in the change from ohmic to higher power (NBI heated) L-mode conditions. The importance of MAR and MAD remains with double the absorbed NBI heating. MAD results in significant power dissipation (up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:mrow><mml:mo>∼</mml:mo></mml:mrow><mml:mn>20</mml:mn><mml:mi mathvariant="normal">%</mml:mi></mml:mrow></mml:math> of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:msub><mml:mi>P</mml:mi><mml:mtext>SOL</mml:mtext></mml:msub></mml:mrow></mml:math> ), mostly in the cold ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:msub><mml:mi>T</mml:mi><mml:mi>e</mml:mi></mml:msub><mml:mo><</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:math> eV) detached region. Theoretical and experimental evidence is found for the potential contribution of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:msup><mml:mi>D</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:mrow></mml:math> to MAR and MAD, which warrants further study. These results suggest that MAR and MAD can be relevant in higher power conditions than the ohmic conditions studied previously. Post-processing reactor-scale simulations suggests that MAR and MAD can play a significant role in divertor physics and synthetic diagnostic signals of reactor-scale devices, which are currently underestimated in exhaust simulations. This raises implications for the accuracy of reactor-scale divertor simulations of particularly tightly baffled (alternative) divertor configurations.