Research On Turbulence Behavior And Effect Of Flow Shear In Tokamak Core

The turbulence in tokamak plasmas is a key issue that results in the anomalous transport and determines the plasma confinement level.Up to now,the turbulence behaviors and related transport in the edge region of tokamaks have been studied extensively.However,there are relatively fewer researches on the core turbulence due to the shortage of diagnostic technology.In order to improve the understanding of the core turbulence as well as other related physical issues and to explore the operation mode with improved plasma confinement,experimental researches on turbulence and effect of flow shear in core region have been conducted on both SUNIST and HL-2A tokamaks.In this work,firstly,the power supply system of SUNIST was upgraded comprehensively to improve the operation level.As a result,more capacity for controlling plasma parameter was achieved.Moreover,the plasma stability significantly increased and the flat top of discharge was lengthened.Besides,a novel ultrafast reciprocating probe system was developed for plasma diagnosis,which achieved a maximum velocity of>20 m/s.Consequently,the behaviors of core turbulence in SUNIST was investigated.As the density increases,there are four regimes of the turbulence and magnetohydrodynamics(MHD)behaviors in SUNIST core plasma:(1)in low density(?1.3×1018 m-3),a low frequency(?10 kHz)MHD and a?50 kHz electrostatic quasi-coherent mode(QCM)co-exist;(2)in relatively low density(?1.6×1018 m-3),electrostatic turbulence is dominated by the QCM with weak MHD activities;(3)in relatively high density(?2.3×1018m-3),both of the MHD and turbulence are weak and the plasma is steady as whole;(4)in high density(?2.3×1018m-3),the strong MHD(?20 kHz)and broadband turbulence appear.This work partially focuses on the QCM in relatively low-density plasma.The features of the QCM including frequency,wavenumber,propagation direction and collisionality are consistent with the characteristics of dissipative trapped electron mode(DTEM),which is the first observation of the DTEM-like phenomena in spherical tokamak related experimental researches.In the plasma with high parameters,the phenomenon of the E×B staircase in core plsama was investigated on HL-2 A tokamak.An E×B staircase consists of a series of m/n=0/0 E×B shear layers formed at multiple radii via turbulent Reynolds stress.Up to now,most results about the E×B staircase are from simulation.In this work,the E×B staircase structure was idendified on HL-2A tokamak.Multiple features of the E×B staircase predicted by the theory and simulation were confirmed via the regulation effect of the E×B staircase on mean parameter profiles and turbulent transport.