7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Disappearance of Dimits Shift in Realistic Fusion Reactor Plasmas withNegative Magnetic Shear
Yong Xiao1, Dingkun Yang1, Shengming Li1, Zhihong Lin2
1
Institute for Fusion Theory and Simulation, Zhejiang University
2
Physics and Astronomy, UC Irvine
e-mail (speaker):yxiao@zju.edu.cnTurbulent transport driven by drift wave instabilities iswidely observed in fusion plasmas. One of the common
causes of ion turbulent transport in tokamaks is the Ion
Temperature Gradient (ITG) instability, whichsignificantly reduces the efficiency of fusion reactors. The
profile stiffness propels fusion plasmas toward marginal
stability, resulting in the plasma temperature being
determined by the critical temperature gradient.
Consequently, even a slight elevation of the criticaltemperature could yield a remarkable increase in fusion
power.
The axisymmetric 𝐸 × 𝐵 rotation or zonal flow, selfconsistently generated by turbulence, is generally
accepted as a regulator of the saturation level of turbulent
transport in fusion plasmas. Near the marginal stability,
the zonal flow has been observed to completely suppress
turbulence and transport, causing a nonlinear up-shift ofthe critical temperature gradient for ITG, known as theDimits shift. Despite its significance in understanding
crucial dynamics in fusion plasmas, such as zonal flowturbulence interaction and L-H transition, the physics
mechanisms underlying the Dimits shift remain largely
elusive
Magnetic shear is another crucial factor that can regulate
turbulence in fusion plasmas. Experimental andsimulation analyses have shown that reverse magnetic
shear significantly affects particle and heat transport intoroidal plasmas through suppression of linear instabilities.
In this study, we employ global GTC (GyrokineticToroidal Code) simulations to investigate ITG turbulent
transport using plasma parameters based on the design of
the CFETR (China Fusion Engineering Experimental TestReactor) featuring reverse magnetic shear, aiming toinvestigate the nonlinear interaction between turbulence,zonal flow and magnetic shear. The weak negative
magnetic shear is observed to be more stable for the ITG
instability than strong positive shear in this equilibriumconfiguration, primarily stemming from the scarcity ofmode rational surfaces induced by the weak negative shear.
This superiority in suppression for the negative shear
persists in nonlinear turbulence with zonal flow
artificially eliminated, where the emergence of turbulence
solitons is observed and found associated with locallydense mode rational surfaces. However, the difference intransport levels among different magnetic shears
diminishes in the presence of self-consistently generated
zonal flow, accompanied by the disappearance ofturbulence solitons. The zonal flow is found to originatedfrom a force driven process by the primary instability,
instead of the conventional modulational instability. The
nonlinear generation of zonal flow is found to besignificantly affected by the magnetic shear. The study
further reveals a remarkable phenomenon that the Dimits
shift no longer exists for negative magnetic shear, which
are attributed to the weakness of the zonal flow generationnear the ITG marginal stability.