Abstract
The L–H transition in magnetic confinement plasmas is investigated on the basis of concepts of two-field bifurcation and fixed-point stability. A set of heat and particle transport equations with both neoclassical and anomalous effects included is used to study ETB formation and also pedestal width and dynamics. It is found that plasmas can exhibit bifurcation where a sudden jump in the gradients can be achieved at the transition point corresponding to the critical flux. Furthermore, it is found that the transport barrier expands inward, whereby the radial growth of the pedestal initially appears to be superdiffusive but later slows down and stops. In addition, the time of barrier expansion is found to be much longer than the time that plasma takes to evolve from L-mode to H-mode. A sensitivity study is also performed, in which the barrier width is found to be sensitive to various parameters, e.g. heating, transport coefficients and suppression strength.
Highlights
Experimental observations in various magnetic confinement fusion devices have revealed that the formation of an edge transport barrier (ETB) results in a sudden transition from low-confinement mode (L-mode) to high-confinement mode (H-mode) with great improvement in plasma performance [1]
An analytical study based on bifurcation and stability of fixed points shows that an abrupt increase in the local gradients occurs at the onset of an L–H transition
This transition is found to depend on the direction of heat ramping, where a backward H–L transition can occur at lower fluxes than for a forward L–H transition, implying hysteresis phenomena
Summary
Experimental observations in various magnetic confinement fusion devices have revealed that the formation of an edge transport barrier (ETB) results in a sudden transition from low-confinement mode (L-mode) to high-confinement mode (H-mode) with great improvement in plasma performance [1]. This improvement is necessary for future nuclear fusion machines, such as ITER [2]. Some earlier research based on bistable s-curve bifurcation models [8,9,10,11,12,13,14,15] provided insights into qualitative aspects, and into L–H transition physics.
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