Juul Rasmussen, J.

Madsen, J.

Naulin, V.

Nielsen, A. H.

Thrysøe, A. S.

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Mar 29, 2019The influence of blob intermittency on fuellingThrysøe, Alexander Simon; Madsen, J.; Naulin, Volker; Nielsen, Anders Henry; Rasmussen, Jens JuulPublication date:2017Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Thrysøe, A. S., Madsen, J., Naulin, V., Nielsen, A. H., & Juul Rasmussen, J. (2017). The influence of blobintermittency on fuelling. P5.158. Paper presented at 44th EPS Conference on Plasma Physics, Belfast, Ireland.The influence of blob intermittency on fuellingA.S. Thrysøe1, J. Madsen1, V. Naulin1, A.H. Nielsen1, J. Juul Rasmussen11PPFE, Department of Physics, DTU, DK-2800 Kgs. Lyngby, DenmarkIntroductionThe overall performance in a tokamak is strongly affected by the dynamics in the edge andScrape-Off Layer (SOL) regions. This region is characterized by the transition from closed toopen magnetic field lines, and the last-closed-flux-surface (LCFS) defines the boundary betweenthe confined plasma and the SOL region. The anomalous transport of heat and particles acrossthe LCFS is strongly intermittent, and the plasma mainly escapes the confined region in field-aligned filaments (blobs) that are formed near the LCFS and propagate radially outwards.The edge and SOL regions also contain a significant amount of neutral particles. The neutralscan either be puffed into the SOL for fuelling or diagnostic reasons, or originate from recyclingof plasma on material surfaces. The presence of neutrals is known to affect the edge and SOLplasma through various interactions such as dissociation and ionization reactions, or charge-exchange collisions.In this paper we present the results of a combined plasma-neutral model that allow for study-ing the effects of plasma turbulence and the associated transport on neutrals and vice versa.Both the plasma and neutral parts of the model are formulated in the fluid picture, which allowfor studying the interactions between neutrals and turbulence self-consistently. In particular itis investigated how blobs affect the fuelling rate during gas-puffing at the outboard-midplane ofa medium-sized tokamak.Combined plasma-neutral modelA slab perpendicular to the magnetic field lines at the outboard-midplane is simulated in amodel that describe the interactions between plasma and neutral particles in a self-consistentmanner. The evolution of plasma fields is described by the HESEL model [1], which is a four-field drift-fluid model for the density, electron and ion pressures, and the vorticity. The HESELmodel describes the edge and SOL dynamics, including the formation and propagation of fieldaligned filaments.The neutrals are described in a three-fluid neutral diffusive model similar to that of [2], butwith the difference that thermal molecules are now included. The three neutral fluids correspondto thermal deuterium molecules, deuterium atoms originating from dissociation of moleculesand a hotter species of atoms that are created in charge-exchange collisions with hot ions. The44th EPS Conference on Plasma Physics P5.158three species interact with the plasma and one another through the reactionse+D2 → e+2D ,e+D→ 2e+D+ ,e+D2 → 2e+D+2 → 2e+D+D+ ,D++D→ D+D+ ,(1)which are denoted dissociation and ionizing dissociation of molecules, and ionization andcharge-exchange collision of atoms respectively.Figure 1: Density (upper), and ionization rate (lower) event calculated by the HESEL code. The area tothe left of the LCFS (dashed vertical line) is the edge region, and that to the right of the LCFS is the SOLregion. The domain as a slab perpendicular to the magnetic field lines at the outboard-midplane.The interactions in (1) result in a set of fluid source terms. For the purpose of studying fuellingthe source term of most interest is that for the densitySn = ne (nn〈σizv〉+nN〈σIZv〉) , (2)where ne is the electron density, nn and 〈σizv〉 are the neutral atom density and ionization re-action rate, and nN and 〈σIZv〉 are the neutral molecule density and ionization reaction rate.The reaction rates are non-linear functions of the electron temperature obtained from [3]. Thedensity and the density source, i.e., the ionization rate, are shown for a blob event in Fig. 1.Blobs and fuellingThe combined HESEL-neutral model is suitable for studying fuelling and ionization in edgeand SOL regions with fluctuating plasma and neutral fields, since no approximations are made44th EPS Conference on Plasma Physics P5.158to simplify the products in (2). It is of interest to asses to what degree neutrals penetrate theSOL region and thus fuel the plasma, as well as understand the effects that blob filaments haveon the fuelling.Figure 2: Time-averaged density source profile (upper), and integrated ionization in the edge and SOLregions as a function of time (lower). It is observed that the ionization level in the two regions is compa-rable, and that the ionization in the edge region, i.e., the fuelling, increases significantly following a blobevent.In Figure 2 the density source profile reveals that a the edge and SOL regions experiencea comparable amount of ionization. The main part of the ionization in the edge region is dueto ionization of atoms, whereas the SOL ionization is dominated by ionizing dissociation ofmolecules.The spatially integrated density source is a measure of the amount of ions and electronscreated in ionization processesionization =∫dxdySn . (3)The ionization between the inner domain boundary and the LCFS is the main fuelling of theplasma, whereas electrons and ions created in SOL do not contribute directly to the fuelling.For the simulated domain the edge and SOL ionization is approximately equal.It is observed from Fig. 2 that the SOL ionization is significantly increased when a blobemerges into the SOL region. This is in good agreement with the positive effect of the increased44th EPS Conference on Plasma Physics P5.158pressure on the ionization reaction rate in (2). It is moreover observed that shortly after a blobevent, the fuelling is likewise increased.The increase in the fuelling rate can be explained by the neutral dynamics resulting from theincreased electron pressure of the blob. During a blob event the ionization of both neutral atomsand molecules increase. The dissociation of molecules into atoms, and the number of charge-exchange collisions likewise increase. The fuelling is effectively unaffected by the ionizationof molecules, since the edge is primarily fuelled by atoms, and since the density of moleculesis much larger than that of atoms in the SOL, the blob creates more atoms than it ionizes. Dueto their long mean-free path the atoms can penetrate the SOL and reach the edge region wherethey are ionized and thus contribute to fuelling.OutlookThe results on the effects of blobs on fuelling were obtained from a combined plasma-neutralfluid model that include hot ions and treats the interactions between plasma and neutrals in aself-consistent way. Those features are thought to be essential to ongoing research in edge andSOL relevant experimentally observed phenomena such as shoulder formation [4] and the fu-elling triggered H-mode density limit [5].This work has been carried out within the framework of the EUROfusion Consortium and hasreceived funding from the Euratom research and training programme 2014-2018 under grantagreement No 633053. The views and opinions expressed herein do not necessarily reflect thoseof the European Commission.References[1] J. Madsen et al., Physics of Plasmas 23, 032306 (2016)[2] A.S. Thrysøe et al., Plasma Phys. Control. Fusion, 58, 044010 (2016)[3] R.K. Janev, et al., 1987 Elementary Processes in Hydrogen-Helium Plasmas (Berlin: Springer-Verlag)[4] D. Carralero et al., Nuclear Materials and Energy (2016)[5] M. Bernert, et al., Plasma Phys. Control. Fusion 57 (2015) 01403844th EPS Conference on Plasma Physics P5.158

The influence of blob intermittency on fuelling

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The influence of blob intermittency on fuelling

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