Study Of Opposite Polarity Response And Internal Transport Barrier Phenomena Based On A Reduced Model

Tokamak is an experimental device for magnetic nuclear fusion.Many transport processes observed in tokamak magnetic confinment fusion are the non-local phenomena,such as opposite polarity response(spontaneous increase of core temperature associated with edge cooling),transport barrier advance and retreat,and confinement mode transition(L-H transition),which are characterized as being related to mesoscale phenomena and beyond the classical theory even new classical theory of collision transport.Mesoscale phenomena refer to the dynamics on the scales larger than a mode or integral scale eddy size,but smaller than the system size or profile scale length.In fusion plasma,turbulence spreading is one example of a mesoscale transport process.Thus,mesoscale phenomena all involve the collective interaction or cooperation of localized sites of turbulence.This paper considers the interaction of spatial propagation turbulence and background plasma.A two-field critical gradient model coupled turbulence intensity and temperature is used to study the mesoscale transport phenomena of tokamak.The structure of the paper is as follows.In chapter 1,an overview of opposite polarity response and internal transport barriers(ITB),mesoscale transport phenomena related to turbulence spreading,is introduced.In chapter 2,a two-field critical gradient model consisting of coupled equations of turbulence spreading transport and heat transport and its basic characteristics are introduced.In chapter 3,a two-field critical gradient model of turbulence intensity and temperature is used to study the opposite polarity response of edge cooling.It is found that a simplified model of turbulence-temperature interaction can explain the opposite polarity response.The injection of cold pulse causes the local temperature profile to collapse,which leads to local E×B shear enhancement.Then,the turbulence intensity decreases and the corresponding turbulent thermal diffusivity decreases due to the suppression effect of E×B shear on the turbulent growth and spreading.Thus,a transient increase of the temperature at core within 1 ms is observed.Once the cold pulse disappears,the system returns to the same equilibria as those before the cold pulse.The change of the core temperature is almost linearly proportional to the changes of edge temperature.The reversal position of opposite polarity increases linearly to E×B shear factor ck when the electric field shear factor is within a moderate range.When the E×B shear factor is large,the reverse position is almost unchanged.When the E×B shear factor is small,the opposite polarity response disappears.In this model,turbulence spreading plays important roles in the formation of opposite polarity.It is difficult to observe opposite polarity response when the turbulence spreading coefficient is smaller than a threshold.During the cold pulse injection,the stored energy instantaneously decreases at the beginning of the cold pulse,then increases and finally returns to the value before the cold pulse injection.In Chapter 4,the two-field critical gradient model with turbulence intensity and temperature considering the magnetic shear effect is used to simulate the internal transport barrier in the reversed magnetic shear.The results show that an internal transport barrier is easily observed in the central region of reversed magnetic shear,while the positive magnetic shear configuration in the same conditions has no internal transport barrier.The transport barrier of width about 0.12 a is located in the central region of reversed magnetic shear,and the position of outer edge of ITB is near the position of minq.In the strong reversed magnetic shear region,the temperature gradient threshold for stimulating turbulence is large,which results in the suppression of turbulence intensity and turbulent heat transport in this region.Then the temperature gradient becomes steeper in the ITB region,which leads to the enhancement of E×B shearing rate.And the turbulence intensity is further suppressed by the shear of E×B flow.Therefore,the reduction of heat transport,the increase of temperature gradient and the enhancement of plasma confinement in the ITB region are sustained by the synergetic effect of the magnetic shear and E×B flow shear.