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Title: Investigations of radial transport in the scrape-off layer of JET-ILW
Author: Wynn, A. P.
ISNI:       0000 0004 7231 5247
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2017
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In the open magnetic field line boundary plasma (or Scrape-Off Layer (SOL)) of tokamaks, the stronger than diffusive transport of particles is to a large extent due to the convection of coherent, filamentary like structures when operating in divertor configuration. Reliable predictions of this transport are vital for the understanding of wall erosion, subsequent impurity release and the retention of fuel in future fusion devices. This thesis investigates factors which influence the quantity and properties of this cross-field transport, both from a steady state and time resolved perspective. Data have been analysed and interpreted from experiments conducted at the Joint European Torus with ITER-like wall (JET-ILW). The experiments used were designed either specifically, or for other purposes, but still provide relevant data. The magnitude of the main chamber radial particle flux density has been investigated via two methods. Firstly, a particle balance model has been used to infer the effective convective cross-field velocity v_eff for Ohmically heated L-mode plasmas performed at a range of plasma current, Ip, and density. The sensitivity to assumptions together with measurement uncertainties found v_eff to be reliable to within a factor three. Secondly, analysis of a flush mounted Langmuir probe on the outboard poloidal limiter suggests that the particle flux density scales proportional the to core density and inversely with the square of Ip for the plasmas studied, although there may also be some unidentified effect of the ion-gyroradius. The expected duration time of intermittent structures was also observed to scale with Ip^2 for a wide range of plasma density, consistent with an increase in size and/or a reduction of velocity of filaments perpendicular to the magnetic field. The collisionality was found not to be the general control parameter of the cross-field transport. Only in the Deuterium fuelled, horizontal target cases was cross field transport observed to increase with increasing collisionality. When using any other method to modify the collisionality, this correlation was not observed. This independence of the cross-field transport from collisionality is a key result of this thesis and contributes to similar conclusions of other recent studies. The plasma-neutral interaction of the fuel species in the divertor region, indicated by D_alpha emission, was found to more generally correlate with the main chamber cross-field transport than collisionality. Candidate plasma-neutral interactions include ionisation and or charge exchange (CX). Despite being unsure of the precise mechanism, undoubtedly plasma-neutral interaction of the fuel species in the divertor region plays a significant role in setting the main chamber cross field transport.
Supervisor: Lipschultz, Bruce Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available