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Author:Kåll, Pia
Title:3D Simulations of Impurity Transport in the Divertor Region of ASDEX Upgrade Tokamak
3D Simuleringar av Orenheters Betende i ASDEX Upgrade Tokamakens Divertorområde
Publication type:Master's thesis
Publication year:2005
Pages:96      Language:   eng
Department/School:Teknillisen fysiikan ja matematiikan osasto
Main subject:Ydin- ja energiatekniikka   (Tfy-56)
Supervisor:Salomaa, Rainer
Instructor:Airila, Markus
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark TF80     | Archive
Keywords:tokamak
plasma-surface interactions
impurity transport
ERO
ASDEX Upgrade
plasma-vägg växelverkan
ASDEX Upgrade
orenheter
randplasma
Abstract (eng): The interactions between the hot fusion plasma and the surrounding vessel wallin magnetically confined fusion devices are not completely understood.
The plasma-surface interactions give rise to impurity particles, i.e. atoms or molecules from the surfaces that are dislodged and transported into the piasma.
The goal is to keep the impurity density in the main piasma as low as possible, since impurities that reach the central fusion-producing piasma will cool and dilute the piasma, thus reducing the power output.
On the other hand, in the edge region impurities are desirable to some extent, since the radiation they eause disperses the piasma power, which otherwise would cause high heat fluxes on Iocalized areas of the surfaces, and potentially damage the surfaces.
A balance between the wanted amount of impurities at the edge, and as few impurities as possible in the main piasma should be achieved, but to achieve this the transport of impurities from the surfaces, where they are produced, through the edge plasma to the main plasma have to be understood.

The production of impurities at the surfaces, and the transport of impurities inside the main piasma are quite weIl understood.
However, the impurity transport in the edge is stiil largely unexplored and it is not known how the amount of inipiirities produced at the walls is connected to the impurity density in the main piasma.
In this work explanations to experimental observations about the transport of carbon in the divertor region of ASDEX Upgrade tokamak were sought through simulations with the ERO code.
ERO is a Monte Carlo based Iocal impurity transport code that has been developed at Forschungszentrum Julich in Germany.
What distinguishes ERO froni other impunty transport codes is the detailed way m which plasma-surface interactions are taken into account and the possibility to simulate external sources.

As part of this work some improvements to ERO have been made and the previously 2D divertor version of ERO has been extended to a 3D version.
Benchmarking of the 3D divertor version has been made and the new code has been found to work welI.
Thereafter, the 3D divertor version has been used to simulate methane puffing experiments done at the outer divertor of ASDEX Upgrade during the 2003 campaign.
Differences in the long-term erosion and deposition patterns ofcarbons, and the deposition patterns found by puffing marker-carbon into the piasma, have earlier been found experimentally.
In this work an explanation to tbese differences were sought by investigating the influence of the puffed particles on the background plasma.
If the puffing would have significant impact on the background piasma, this could explain why the puffed carbon behaves differently than the carbon eroded from the piasma facing components.
It would also mean that ERO would not be suitable for simulating puffing experiments, since in ERO it is assumed that the impurities do not influence the background piasma.
This assumption ean be questionable especially with strong impurity sources as in puffing experiments.

From the simulations it can he concluded that the puffing does not influence neither the density nor the energy content of the background piasma with more than 10 %.
The area affected by the puffing is vety centralized around the puffing hole and quite small, in most cases it has a diameter snialler than one centimeter.
The simulations did not give an explanation to the differences in erosion and deposition patterns of carbon that have been found experimentally.
However, from the simulations it can be concluded that siniilar puffing experiments can he used in the future to determine the transport ofcarbon in the edge piasma without affecting the background piasma parameters.
Likewise the assumptions made in ERO are valid, and this type of simulations wiIl be continued.
Some further improvements to ERO are aiready under way.
ED:2006-01-16
INSSI record number: 30498
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