Speaker
Xueqiao Xu
(Lawrence Livermore National Laboratory)
Description
Simulations with the BOUT++ code have been used to study the energy loss for edge localized modes (ELMs) at different collisionality & electric field Er shear and to investigate the controls of transition to different ELM and quiescent H-mode (QH-mode) regimes. The simulation results indicate that by development of a flexible Er control capabilities, it is conceivable that tokamak operation regime access can be achieved through a control of edge fluctuation spectrum via the radial electric field and its shear. By decreasing collisionality with a increasing Er for a narrow pedestal, nonlinear simulations show that (1) power spectrum becomes narrower and linear growth rate increases, the dominant mode decreasing from high-n ballooning modes to low-n peeling modes; (2) Bispectrum analysis shows that nonlinear mode coupling becomes weaker, resulting in the dominant filamentary structures and increasing ELM energy loss. The increasing Er shear at high collisionality with a narrow pedestal leads to strong nonlinear coupling and reduced ELM energy loss, In contrast, the increasing Er shear at low collisionality with a wide pedestal can modify the stability boundary to drive stable low-n peeling modes unstable, leading to enhanced pedestal transport due to either a saturated low-n peeling modes or a broadband turbulence, therefore leading to a reduced pedestal pressure gradient, allowing the development of a broader and thus higher transport barrier in QH mode without ELMs.
To validate BOUT++ simulations against experimental data for inter-ELM fluctuations, the BOUT++ simulations are performed based on a set of C-Mod and DIII-D experiment data, the overall signatures of simulation results for quasi-coherent fluctuations (QCF) show good agreement with C-Mod and DIIID measurements. QCFs are localized in the pedestal region having a predominant frequency at f ≈300−400kHz and poloidal wavenumber at k≈0.7/cm, and propagate in the electron diamagnetic direction in the laboratory frame. (2) The pedestal profiles giving rise to QCFs are near the marginal instability threshold for ideal peeling-ballooning (P-B) modes for both C-Mod and DIII-D. (3) Particle diffusivity is either smaller than the heat diffusivity for DIII-D or similar to the heat diffusivity for C-Mod.
This work was performed for USDOE by LLNL under DE-AC52-07NA27344.LLNL-ABS-680566.
| Country or International Organization | USA |
|---|---|
| Paper Number | TH/P6-18 |
Primary author
Xueqiao Xu
(Lawrence Livermore National Laboratory)
Co-authors
Dr
Ahmed Diallo
(PPPL)
Mr
Chenhao Ma
(Peking University)
Dr
Defeng Kong
(Institute of Plasma Physics Chinese Academy of Sciences)
Jerry Hughes
(MIT PSFC)
Mr
Jianguo Chen
(Peking University)
Dr
Ning Yan
(Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China)
Prof.
Patrick Diamond
(University of California San Diego)
Dr
Philip B. Snyder
(General Atomics)
Mr
R.J. Groebner
(General Atomics)
