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Title: Dynamics and statistical features of coherent plasma structures in the SOL of a tokamak
Author: Higgins, David
ISNI:       0000 0004 2749 0492
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2012
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Understanding surface erosion in tokamaks due to contact with hot plasma is critical in designing new high power devices. The propagation of the plasma through the scrape off layer (SOL) ultimately defines the spatio-temporal characteristics of this erosion, hence modelling of this region is an important area of research. Transport in the SOL is attributed to advective motions of plasma blobs, for which the advective velocity is estimated in the literature. A new paradigm for comparing the theory of plasma blobs with experimental data is developed, which treats density and velocity data as sets of coherent structures via a peak detection algorithm. The phase difference of plasma density and radial velocity peaks take values depending on the dominating physics of the blob motion. Values of this phase difference are predicted in the interchange and drift wave cases for a strongly nonlinear plasma. Analysis of MAST data reveals interchange activity in the edge and SOL, and a phase structure typical of sheath limited models in the SOL. A further application of the paradigm examines the blob velocity-density scaling v ∝ nα. A new sheath limited model for blob advection with divertor density nt constant gives α = 1. Predictions in the zero parallel current case depend on the blob nonlinearity; we examine the dependency of α on the nonlinearity by solving the time independent equation of blob motion for a range of density profiles, finding α ∼ 0.3 for MAST nonlinearity strength. The α parameter is estimated statistically from MAST data, and it found to peak at α ∼ 1 near the last closed flux surface (LCFS) and fall to zero further from the plasma. The scaling behaviour is further examined using the TOKER code. A numerical model, hTOKER, is developed. A subgrid model is employed that terminates the plasma at a chosen scale with defined spectral properties, which allows a physically accurate way to reduce resolution and computational burden. We examine sheath potential drop (SPD) and finite ion temperature (FTI) effects on SOL transport in the cases of constant (CTI) and flute (SI) nt boundary conditions. For the advection of individual blobs, SPD effects that are stable in the SI case are found to be unstable in the CTI case, and FTI effects are found to be stabilising in all cases. SOL plasma simulations are used to examine the differences in particleenergy flux and peak phase difference using floating or plasma potential. Floating potential overestimates flux by a factor 2, and shifts phase differences from 0◦ to ∼ −30◦. FTI effects are without cancellation from the gyro-viscous counterparts.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics