Real-time pedestal optimization and ELM control with 3D fields and gas flows on DIII-D. (27th May 2020)

• Record Type: Journal Article
• Authors: Laggner, F.M.; Eldon, D.; Nelson, A.O.; Paz-Soldan, C.; Bortolon, A.; Evans, T.E.; Fenstermacher, M.E.; Grierson, B.A.; Hu, Q.; Humphreys, D.A.; Hyatt, A.W.; Nazikian, R.; Meneghini, O.; Snyder, P.B.; Unterberg, E.A.; Kolemen, E.; DIII-D team, the
• Is Part Of: Nuclear fusion. Volume 60:Number 7(2020)
• Publication Date: 2020-05-27
• Subjects: tokamak -- super H-mode -- edge localized modes -- 3D fields; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/ab88e1
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: The capabilities of the DIII-D tokamak's plasma control system (PCS) were expanded to allow for pedestal optimization and edge localized mode (ELM) control. Three proof of principle control schemes are presented that were successfully implemented and tested. These use multiple inputs from real-time (RT) diagnostics like a D α based ELM monitor and edge profile measurements from Thomson scattering (TS) as well as 3D, i.e. non-axisymmetric, magnetic fields and gas puffs as actuators to regulate the density pedestal. The first scheme targets to optimize the access of ELM suppression induced by non-axisymmetric magnetic perturbations (MPs). The conducted set of experiments identifies a path dependence of plasma confinement on the applied MP amplitude. The controller aims to transition into ELM suppression at the minimum 3D field amplitude and reduces it further afterwards, allowing for partial confinement recovery. Another pedestal control scheme is deployed to compensate the density 'pump-out' in MP ELM suppression by regulating the gas puff. This uses RT TS diagnostic data, extracting the pedestal height from the electron density ( n e ) profiles and enables studies of the transition into and out of MP ELM suppression at constant density. A limit cycle behavior of edge rotation and MP amplitude persists under these conditions. The third control scheme combines MPs and gas puffs as actuators to perform pedestal density trajectory control to access Super high confinement mode (H-mode) and furthermore, allowing the integration of a radiative divertor in this regime. While MPs mainly impact the pedestal top density, the control scheme allows to loosen the tight coupling of pedestal top and separatrix density evolution. With respect to ITER, the achieved results emphasize the need for an advanced control system to keep MP amplitude close to but above the ELM suppression threshold at all times, enabling high confinement and, respectively, at high fusion energy gain factor ( Q ). Furthermore, pedestal control enables detailed physics studies in present-day tokamaks and allows the exploration of core-edge integrated plasma scenarios.