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.
Highlights
These features take into account the discontinuous available particle flux values, the long response time due to pellet time-of-flight and the pellet-imposed strong local plasma perturbations resulting in distortion or even loss of many measurements employed for standard operational control [8]
The most relevant diag nostics are shown: the lines of sight (LOS) of the DCN laser interferometer, for the bremsstrahlung and the spectrometer observing the high-field-side high-density (HFSHD) front; particular locations where measurements are made by the Thomson scattering (TS) system and the charge eXchange recombination spectroscopy (CXRS) and the manometer measuring divertor neutral density
Shown in figure 2(g) is the evolution of the HFSHD front, a poloidally localised region of high density located in the high-field-side (HFS) scrape-off layer (SOL) and extending from the x-point towards the midplane
Summary
The operational scenario envisaged for ITER, the EU DEMO and future fusion power plants is the high-confinement mode (H-mode), preferably at high core densities close to or even. ASDEX Upgrade (AUG), with its reactor-relevant all-metal wall, versatile diagnostic set, flexible pellet system for high speed, inboard launching, and its powerful control system is especially suitable for these kinds of investigations. The investigations reported here new features were added to the DCS for the control of the density during pellet injection These features take into account the discontinuous available particle flux values, the long response time due to pellet time-of-flight and the pellet-imposed strong local plasma perturbations resulting in distortion or even loss of many measurements employed for standard operational control [8]. A ‘validated density’ representing the lineaveraged density ne = nedl/l, where ne is the local electron density and l is the chord length within plasma column, was derived from all available interferometer data and a suitable bremsstrahlung measurement This signal was applied in most investigations on controlling pellet actuation reported here. The most relevant diag nostics are shown: the lines of sight (LOS) of the DCN laser (continuous wave emission from deuterated HCN at 195 μm) interferometer, for the bremsstrahlung and the spectrometer observing the high-field-side high-density (HFSHD) front; particular locations (labelled ‘core’ and ‘edge’ in several figures) where measurements are made by the Thomson scattering (TS) system and the charge eXchange recombination spectroscopy (CXRS) and the manometer measuring divertor neutral density (small box below the divertor labelled ‘M’)
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