Sherwood 2015

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The Characteristics of the micro-turbulence in the pedestal region during the inter-ELM phase on DIII-D

Author: Jingfei Ma
Requested Type: Consider for Invited
Submitted: 2015-01-19 13:09:51

Co-authors: X. Q. Xu, R. J. Groebner

Contact Info:
University of Texas as Austin
2515 speedway, C1600
Austin, TX   78712
United States

Abstract Text:
We present here the global analysis of the electromagnetic pedestal turbulence on DIII-D using BOUT++ codes. Two types of electromagnetic micro-instabilities have been identified in the pedestal region during the inter-ELM phase on DIII-D numerically, using a six-field Braginskii fluid model with Landau-Fluid closure, under the BOUT++ framework. One is a ballooning-type instability, which is driven by the ion temperature gradient and curvature and localizes at the outboard midplane. Another is the drift-Alfven instability, which is driven by the electron pressure gradient and localizes at the top of the Tokamak, as well as near the X-point. The drift-Alfven modes have larger growth rate than the ballooning-type modes. The free energy for both instabilities comes from the background profile gradients.

A set of pressure scan equilibria based on DIII-D H-mode discharge 132016 is generated using the VARYPED tool, which represents the 75%~99% portion of ELM cycle. The reference discharge (132016) has a plasma current of 1.5 MA, toroidal field of 2.13T and average triangularity of 0.55. The pressure scan equilibria include pressure profiles from 50% to 250% of the original one.

The ballooning-type instability, which appears to be the micro-instabilities below the ideal ballooning threshold, is driven by the ion temperature gradient and the finite larmor radius effects. Driven by the toroidicity, the linear mode structure mainly localizes at the outboard midplane, which is recognized as the ‘bad curvature’ region. In contrast, the Drift-Alfven instability occurs when the electron temperature gradient reaches a threshold and appears along the whole poloidal torus, especially at the top and near the X-point of the Tokamak. The linear characteristics of the Drift-Alfven modes calculated from BOUT++ are consistent with the theoretical results: (1) The wave propagates along the electron diamagnetic direction and the real frequencies are proportional to the perpendicular wave number (2) The growth rate increases as the background electron pressure gradient increases, and decreases as the electron parallel collisionality increases.

Besides the linear analysis, nonlinear simulations are conducted in the pedestal region to study the wave coupling, energy transport and the impact on the micro-turbulence on the ELM bursts.
(This work was supported in part by the US DOE under DE-AC52-07NA27344 and DE-FC02-04ER54698.)


March 16-18, 2015
The Courant Institute, New York University