Dokument: Hydrogen recycling and transport in the helical divertor of TEXTOR
| Titel: | Hydrogen recycling and transport in the helical divertor of TEXTOR | |||||||
| URL für Lesezeichen: | https://docserv.uni-duesseldorf.de/servlets/DocumentServlet?id=16587 | |||||||
| URN (NBN): | urn:nbn:de:hbz:061-20101217-095910-9 | |||||||
| Kollektion: | Dissertationen | |||||||
| Sprache: | Englisch | |||||||
| Dokumententyp: | Wissenschaftliche Abschlussarbeiten » Dissertation | |||||||
| Medientyp: | Text | |||||||
| Autor: | Clever, Meike [Autor] | |||||||
| Dateien: |
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| Beitragende: | Prof. Dr. Samm, Ulrich [Gutachter] Prof. Dr. Pretzler, Georg [Gutachter] | |||||||
| Stichwörter: | magnetic confinement fusion, plasma wall interaction, plasma surface interaction, helical divertor, ergodic divertor, resonant magnetic perturbations, TEXTOR, hydrogen recycling, stellarator, hydrogen spectroscopy, high recycling | |||||||
| Dewey Dezimal-Klassifikation: | 500 Naturwissenschaften und Mathematik » 530 Physik | |||||||
| Beschreibung: | Divertors with chaotic magnetic field structures are a field of major interest in todays fusion research not only for stellarators with their inherent complex magnetic field structures but also in tokamaks due to the application of resonant magnetic perturbations. This method is also used in the limiter tokamak TEXTOR for creating a helical divertor structure with a complex, three-dimensional (3D) magnetic field topology. The functionality of such divertors regarding control and efficient exhaust of particles and energy is still under investigation. Increasing the understanding in this respect has been the major motivation for this thesis. The accessibility of favourable divertor regimes with large temperature gradients between plasma core and divertor target, high neutral density in the divertor volume for efficient pumping and strong radiation for heat dissipation (high recycling regime) or additionally with reduced target particle and heat loads (detachment), is largely determined by the recycling of the hydrogen plasma at the divertor target. The aim of this thesis was therefore to investigate the hydrogen recycling at the target plates of the helical divertor in TEXTOR and by this the capability of this divertor configuration to access such favourable operational regimes.
In order to study the different divertor density regimes in TEXTOR, discharges were performed in which the total plasma density was increased continuously up to the density limit. The recycling was investigated in a fixed helical divertor structure where four helical strike points with a poloidal width of about 8 − 10 cm are created at the divertor target plates. The experimental investigation of the hydrogen recycling was carried out using mainly spectroscopic methods supplemented by Langmuir probe, interferometric and atomic beam measurements. In the framework of this thesis a spectroscopic multi camera system has been built that facilitates the simultaneous observation of four different spectral lines, recording images of the divertor target plates and the plasma volume close to the target. The system facilitates the simultaneous measurement of the poloidal and toroidal pattern of the recycling flux at the divertor target without the need for sweeping the plasma structure. The simultaneous observation of different spectral lines reduces the uncertainty in the analysis based on several lines, as the contribution from uncertainties in the reproducibility of plasma parameters in different discharges are eliminated and only the uncertainty of the measurement method limits the accuracy. The spatial resolution of the system in poloidal and toroidal direction (0.8 mm ± 0.01 mm) is small compared to the separation of the helical strike points, the capability of the measurement method to resolve these structures is therefore limited by the line-of-sight integration and the penetration depth of the light emitting species. The measurements showed that the recycling flux increases linearly with increasing plasma density, a high recycling regime is not observed. Its absence can be explained using an extended two point model including heat convection applied to the region dominated by parallel transport (laminar region). The radial penetration depth of the neutral hydrogen particles (3 − 4 cm) estimated from spectroscopic measurements was found to be often larger than the varying radial extent of this laminar region (few mm up to 6 cm) which finally leads to convective heat transport reducing parallel temperature gradients. Increasing the radial extent of the laminar region especially in front of the divertor strike points could lead to an improvement in this respect and provide access to a high recycling regime. The closing of the open divertor geometry of the helical divertor at LHD through the installation of baffles (which is currently under way) as well as the larger island size in the island divertor of the stellarator W7-X compared to its predecessor W7-AS is therefore expected to provide access to a high recycling regime there. The radiation instability developing at high plasma densities in the helical divertor in TEXTOR is preceded by a transient partial detachment of the plasma from the divertor target plates and leads to the formation of a poloidally structured and helically inclined radiating belt, a helical divertor MARFE. While typically leading to a density limit disruption, this MARFE has been stabilised using a feedback system and could provide some divertor functionality such as low target temperature, increased neutral density and increased radiation within the stochastic boundary. A detailed comparison of the experimental observations to results from modelling with the 3D numerical transport code EMC3-EIRENE showed agreement in the high density behaviour (in particular absence of a high recycling regime) as well as in the absolute values of the calculated and measured target particle fluxes. Simulations using two different cross-field transport coefficients showed, that this agreement is only found at a certain level of cross-field transport (D⊥ = 1 m^2 s^−1). The inclusion of carbon impurities in the simulations results in the experimentally observed reduction of the recycling flux. These findings support the use of the EMC3-EIRENE code for predictive modelling for other divertors with similar stochastic magnetic field structures. | |||||||
| Lizenz: |  Urheberrechtsschutz | |||||||
| Fachbereich / Einrichtung: | Mathematisch- Naturwissenschaftliche Fakultät » WE Physik » Laser- und Plasmaphysik | |||||||
| Dokument erstellt am: | 17.12.2010 | |||||||
| Dateien geändert am: | 05.11.2010 | |||||||
| Promotionsantrag am: | 25.06.2010 | |||||||
| Datum der Promotion: | 09.09.2010 |
