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Author: Jianguo Chen
Requested Type: Poster
Submitted: 2017-03-17 12:31:31

Co-authors: X.Q. Xu, X. Chen, K.H. Burrell, T.M. Wilks

Contact Info:
Peking University
Room 543, #209, Rd. Chengfu
Beijing,   100871
P. R. China

Abstract Text:
As low-torque QH-mode discharge with improved pedestal conditions has been achieved on DIII-D recently, simulations are carried out to study the features and the mechanism of the QH-mode by using the 3-field and 6-field two-fluid BOUT++ code which is based on the Braginskii equations with non-ideal physics effects.
The results from 3-field two-fluid BOUT++ code with JET-like tokamak circular geometry equilibrium demonstrate the basic features about the edge harmonic oscillation (EHO): (1) the low-n peeling modes are mainly driven by the gradient of parallel current and large pressure gradient leads to high-n ballooning modes; (2) in low density cases, the low-n modes are more sensitive to the Er shear; (3) When increasing the ExB shear flow, the most unstable modes shift to lower-n and become much more unstable, the width of growth rate spectrum is narrower, (4) in the nonlinear simulations, the time-frequency analysis shows that there are dominant frequencies(n~1,2) in the saturated phase in case with higher net flow while the case without net flow shows the turbulence feature(no dominant frequencies). This trend is qualitatively consistent with the recent DIII-D experiments results in which the EHO state was replaced by broadband MHD turbulence with near-zero torque.
As the realistic situation is much more complex and the density, current profile and shaping play important roles in these low-torque QH-mode discharge and in the transition from EHO to broadband MHD state as well. The lower double null equilibrium from DIII-D low-torque QH-mode discharge #163518 in broadband MHD state is simulated using 6-field two-fluid code, where the experimental measured plasma profiles are smoothly extended to SOL region. The simulation results demonstrate that the ExB flow can destabilize the low-n modes which are in dominant during the QH phase. The analysis about heat and particle fluxes in nonlinear simulations is also presented.

*This work was performed under the auspices of the U.S. DoE by LLNL under Contract No. DE-AC52-07NA27344 and was supported by the ITER-China Program (2013GB111000, 2013GB112006), NSFC (11261140326).