ETD

• Plasma Turbulence
• Tokamak
• Zonal Flows

Turbulent heat transport in tokamaks is dominated by the ion-temperature-gradient (ITG) instability, but its losses are kept low by self-generated zonal shear flows. Transport is then set by the perturbations that survive inside those flows. Both a reduced two-field fluid model and gyrokinetic simulations of the MAST tokamak show the same survivor: a long-lived, soliton-like structure drifting radially across the shear, which we call the ferdinon. In both cases it appeared only blurred within fully developed turbulence. This thesis introduces a two-dimensional, two-field model retaining only ITG coupling and an imposed background shear, governed by a single shear parameter, in which the ferdinon can be excited and studied in isolation. We characterise its structure, propagation and drive; give numerical evidence that it is an exact, stable travelling solution acting as a nonlinear attractor; and identify three regimes separated by two shear thresholds. Implications for a ceiling on zonal-flow amplitude and for non-local, ballistic heat transport are discussed.