Title: Identification of new turbulence contributions to plasma transport and confinement in spherical tokamak regime

Abstract

Highly distinct features of spherical tokamaks (ST), such as National Spherical Torus eXperiment (NSTX) and NSTX-U, result in a different fusion plasma regime with unique physics properties compared to conventional tokamaks. Nonlinear global gyrokinetic simulations critical for addressing turbulence and transport physics in the ST regime have led to new insights. The drift wave Kelvin-Helmholtz (KH) instability characterized by intrinsic mode asymmetry is identified in strongly rotating NSTX L-mode plasmas. While the strong E x B shear associated with the rotation leads to a reduction in KH/ion temperature gradient turbulence, the remaining fluctuations can produce a significant ion thermal transport that is comparable to the experimental level in the outer core region (with no "transport shortfall"). The other new, important turbulence source identified in NSTX is the dissipative trapped electron mode (DTEM), which is believed to play little role in conventional tokamak regime. Due to the high fraction of trapped electrons, long wavelength DTEMs peaking around k_θρs ~ 0.1 are destabilized in NSTX collisionality regime by electron density and temperature gradients achieved there. Surprisingly, the E x B shear stabilization effect on DTEM is remarkably weak, which makes it a major turbulence source in the ST regime dominant over collisionlessmore » TEM (CTEM). The latter, on the other hand, is subject to strong collisional and E x B shear suppression in NSTX. DTEM is shown to produce significant particle, energy and toroidal momentum transport, in agreement with experimental levels in NSTX H-modes. Furthermore, DTEM-driven transport in NSTX parametric regime is found to increase with electron collision frequency, providing one possible source for the scaling of confinement time observed in NSTX H-modes. Most interestingly, the existence of a turbulence-free regime in the collision-induced CTEM to DTEM transition, corresponding to a minimum plasma transport in advanced ST collisionality regime, is predicted.« less

Authors:

Wang, W. X. ^[1]; Ethier, S. ^[1]; Ren, Y.  ^[1];

• Search DOE PAGES for author "Ren, Y."
• Search DOE PAGES for ORCID "0000-0001-9831-6035"

Kaye, S. ^[1]; Chen, J. ^[1]; Startsev, E. ^[1]; Lu, Z. ^[2]; Li, Z. Q.  ^[3]

• Search DOE PAGES for author "Li, Z. Q."
• Search DOE PAGES for ORCID "0000-0003-0083-5844"

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+ Show Author Affiliations

1. Princeton Univ., Princeton, NJ (United States)
2. Univ. of California, San Diego, La Jolla, CA (United States)
3. Zhejiang Univ., Hangzhou (People's Republic of China)

Publication Date:

Thu Oct 15 00:00:00 EDT 2015

Research Org.:

Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Oak Ridge Leadership Computing Facility (OLCF); Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

Sponsoring Org.:

USDOE Office of Science (SC)

OSTI Identifier:

1256376

Alternate Identifier(s):

OSTI ID: 1223570

Report Number(s):

PPPL-5202
Journal ID: ISSN 1070-664X; PHPAEN

Grant/Contract Number:  

AC02-09CH11466

Resource Type:

Accepted Manuscript

Journal Name:

Physics of Plasmas

Additional Journal Information:

Journal Volume: 22; Journal Issue: 10; Journal ID: ISSN 1070-664X

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; particle simulation; toroidal plasmas; magnetic field; sheared flows; modes; instability; suppression; geometry; driven; edge; Kelvin Helmholtz instability; toroidal plasma confinement; ion temperature gradient mode; tokamaks; plasma transport properties