Feedback controlled, reactor relevant, high-density, high-confinement scenarios at ASDEX Upgrade. (12th January 2018)

• Record Type: Journal Article
• Authors: Lang, P.T.; Blanken, T.C.; Dunne, M.; McDermott, R.M.; Wolfrum, E.; Bobkov, V.; Felici, F.; Fischer, R.; Janky, F.; Kallenbach, A.; Kardaun, O.; Kudlacek, O.; Mertens, V.; Mlynek, A.; Ploeckl, B.; Stober, J.K.; Treutterer, W.; Zohm, H.
• Other Names: collab.
• Is Part Of: Nuclear fusion. Volume 58:Number 3(2018:Mar.)
• Publication Date: 2018-01-12
• Subjects: fuelling -- tokamak -- all metal wall -- pellet -- feedback control; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/aaa339
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: One main programme topic at the ASDEX Upgrade all-metal-wall tokamak is development of a high-density regime with central densities at reactor grade level while retaining high-confinement properties. This required development of appropriate control techniques capable of coping with the pellet tool, a powerful means of fuelling but one which presented challenges to the control system for handling of related perturbations. Real-time density profile control was demonstrated, raising the core density well above the Greenwald density while retaining the edge density in order to avoid confinement losses. Recently, a new model-based approach was implemented that allows direct control of the central density. Investigations focussed first on the N -seeding scenario owing to its proven potential to yield confinement enhancements. Combining pellets and N seeding was found to improve the divertor buffering further and enhance the operational range accessible. For core densities up to about the Greenwald density, a clear improvement with respect to the non-seeding reference was achieved; however, at higher densities this benefit is reduced. This behaviour is attributed to recurrence of an outward shift of the edge density profile, resulting in a reduced peeling-ballooning stability. This is similar to the shift seen during strong gas puffing, which is required to prevent impurity influx in ASDEX Upgrade. First tests indicate that highly-shaped plasma configurations like the ITER base-line scenario, respond very well to pellet injection, showing efficient fuelling with no measurable impact on the edge density profile.