Scrape-Off Layer physics in limited plasmas in TCV

Controlled nuclear fusion is the most promising candidate for being an inexhaustible, clean and intrinsically safe energy source. In the tokamak fusion reactor concept, a high temperature plasma is confined by magnetic fields. Turbulence diffuses the confined plasma to the tokamak outermost region, the Scrape-Off Layer (SOL), featuring open field lines. In the SOL, the plasma is convected along the field lines and deposited on the solid surfaces of the tokamak wall. The plasma-wall interaction through the SOL strongly affects the reactor performances, and the elevated heat loads on the solid surfaces are one of the most limiting factors for fusion. SOL physics is not completely understood, not even in the simple ``limited'' configuration, where the main plasma touches the reactor wall, and the contact point defines the Last Closed Flux Surface (LCFS). In this thesis, we advance the understanding of SOL physics in limited plasmas, combining experiments and numerical simulations. In particular, two topics are addressed. First, the separation between the ``near'' and ``far'' SOL is investigated. The near SOL extends typically a few mm from the LCFS, features steep radial profiles of parallel heat flux and is responsible for the peak of heat deposition on the tokamak first wall. The far SOL, typically a few cm wide, features flatter heat flux profiles, and accounts for the majority of the heat deposited on the first wall. Secondly, blob dynamics is investigated. Blobs are high density plasma filaments generated by turbulence, and are an ubiquitous feature of plasmas in open magnetic field lines. The blobs travel outwards to the reactor walls, increasing the cross field transport. Dedicated experiments have been performed on the TCV tokamak. Inboard-limited Deuterium (D) and Helium (He) plasma discharges are performed, varying the main plasma parameters (current, density and shaping). The parallel heat flux radial profiles are determined with infrared thermography. For the first time, the presence of a near SOL in TCV limited plasmas is reported, both for D and He discharges. The near SOL is found to disappear for high plasma resistivity. Non-ambipolar currents are measured to flow to the wall in the near SOL using Langmuir probes, and their presence is found to correlate with the strength of the near SOL heat fluxes. A simple interpretation is given. The heat fluxes and electric potentials are also measured on the low field side using a reciprocating Langmuir probe, and compared with the ones measured on the tokamak wall. A method for the mitigation and suppression of the near SOL heat fluxes through impurity seeding is proposed, and first experimental evidences are presented. The experiments are compared with numerical simulations of the TCV SOL, performed with the GBS code. The simulated parallel heat flux profiles qualitatively agree with the experimental ones, showing the presence of a near and far SOL. Also, non-ambipolar currents are observed to flow to the wall. The effect of resistivity is investigated through a second simulation with a 40 times higher resistivity. The blob dynamics in TCV is investigated using a conditional average sampling technique on the reciprocating Langmuir probe data. The results for two discharges, for low and high resistivity respectively, are discussed. A blob detection and tracking algorithm is applied to the numerical simulation outputs, and the results are discussed.


Advisor(s):
Furno, Ivo
Labit, Benoît
Year:
2017
Publisher:
Lausanne, EPFL
Keywords:
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis7475-0
Laboratories:




 Record created 2017-02-07, last modified 2019-10-04

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