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Title: Zonal flow sawteeth and the time period between edge-localized transport bursts in tokamaks

Abstract

The time period between particle and energy transport bursts in simulations of tokamak edge turbulence is determined by the magnitude of the diamagnetic drift parameter {alpha}{sub d}{identical_to}{omega}{sub *}/{gamma}{sub 0}, where the diamagnetic drift frequency {omega}{sub *}={rho}{sub s}c{sub s}/L{sub 0}L{sub n} and the characteristic ballooning mode growth rate {gamma}{sub 0}=c{sub s}/(RL{sub n}/2){sup 1/2}. Here, R is the major radius of the torus, L{sub n} is the density gradient scale length, {rho}{sub s} is the ion gyroradius, and c{sub s} is the ion acoustic speed. The scale length L{sub 0} is given by L{sub 0}=2{pi}qR {nu}{sub ei}{rho}{sub s}/2{omega}{sub e}R){sup 1/2}(2R/L{sub n}){sup 1/4}, where q is the safety factor, {nu}{sub ei} is the electron-ion collision frequency, and {omega}{sub e} is the electron cyclotron frequency. When the diamagnetic drift frequency becomes larger than the ballooning mode growth rate ({alpha}{sub d}>1), then the transport in the tokamak edge is characterized by regularly recurring bursts of particles and energy with a single well-defined frequency. As {alpha}{sub d} increases above unity, the time period between the bursts becomes much longer. The temporal dependence of the energy in the zonal flow generated nonlinearly has the appearance of sawteeth.

Authors:
;  [1]
  1. Institute for Research in Electronics and Applied Physics University of Maryland, College Park, Maryland 20742 (United States)
Publication Date:
OSTI Identifier:
20960091
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 1; Other Information: DOI: 10.1063/1.2424560; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BALLOONING INSTABILITY; BOUNDARY LAYERS; CYCLOTRON FREQUENCY; DENSITY; ELECTRON-ION COLLISIONS; ELECTRONS; IONS; NONLINEAR PROBLEMS; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA DIAMAGNETISM; PLASMA SIMULATION; SAFETY; SAWTOOTH OSCILLATIONS; TOKAMAK DEVICES; TURBULENCE; VELOCITY

Citation Formats

Kleva, Robert G., and Guzdar, Parvez N. Zonal flow sawteeth and the time period between edge-localized transport bursts in tokamaks. United States: N. p., 2007. Web. doi:10.1063/1.2424560.
Kleva, Robert G., & Guzdar, Parvez N. Zonal flow sawteeth and the time period between edge-localized transport bursts in tokamaks. United States. doi:10.1063/1.2424560.
Kleva, Robert G., and Guzdar, Parvez N. Mon . "Zonal flow sawteeth and the time period between edge-localized transport bursts in tokamaks". United States. doi:10.1063/1.2424560.
@article{osti_20960091,
title = {Zonal flow sawteeth and the time period between edge-localized transport bursts in tokamaks},
author = {Kleva, Robert G. and Guzdar, Parvez N.},
abstractNote = {The time period between particle and energy transport bursts in simulations of tokamak edge turbulence is determined by the magnitude of the diamagnetic drift parameter {alpha}{sub d}{identical_to}{omega}{sub *}/{gamma}{sub 0}, where the diamagnetic drift frequency {omega}{sub *}={rho}{sub s}c{sub s}/L{sub 0}L{sub n} and the characteristic ballooning mode growth rate {gamma}{sub 0}=c{sub s}/(RL{sub n}/2){sup 1/2}. Here, R is the major radius of the torus, L{sub n} is the density gradient scale length, {rho}{sub s} is the ion gyroradius, and c{sub s} is the ion acoustic speed. The scale length L{sub 0} is given by L{sub 0}=2{pi}qR {nu}{sub ei}{rho}{sub s}/2{omega}{sub e}R){sup 1/2}(2R/L{sub n}){sup 1/4}, where q is the safety factor, {nu}{sub ei} is the electron-ion collision frequency, and {omega}{sub e} is the electron cyclotron frequency. When the diamagnetic drift frequency becomes larger than the ballooning mode growth rate ({alpha}{sub d}>1), then the transport in the tokamak edge is characterized by regularly recurring bursts of particles and energy with a single well-defined frequency. As {alpha}{sub d} increases above unity, the time period between the bursts becomes much longer. The temporal dependence of the energy in the zonal flow generated nonlinearly has the appearance of sawteeth.},
doi = {10.1063/1.2424560},
journal = {Physics of Plasmas},
number = 1,
volume = 14,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2007},
month = {Mon Jan 15 00:00:00 EST 2007}
}
  • The character of particle and energy transport in simulations of tokamak edge turbulence is determined by the magnitude of the density gradient. Edge turbulence becomes increasingly intermittent as the edge density gradient increases. Beyond a critical limit in the edge density gradient, the transport is dominated by short, repetitive bursts of particles and energy outward toward the wall. These bursts are extremely ballooning in character, strongly localized on the large major radius side of the torus. The duration of a burst is given by the ballooning mode growth time t{sub 0}=(RL{sub n}){sup 1/2}/c{sub s}, where c{sub s} is the soundmore » speed, R is the major radius of the torus, and L{sub n} is the density gradient scale length. The bursts are coherent in structure with a poloidal scale size L{sub 0} that is proportional to the square root of the plasma resistivity {eta}. With further increases in the edge density gradient, the fluxes of energy and particles in the bursts become much larger in magnitude. The particle and energy bursts seen in the simulations are similar to the bursts in D{sub {alpha}} radiation seen during edge-localized modes in tokamaks.« less
  • Nondimensional parameter dependence of heat transport between edge localized modes (ELMs) is examined for H mode plasmas. The electron heat diffusivity between ELMs is reduced to the level of ion neoclassical transport in the plasma edge region which is affected by ELM burst. At lower edge collisionality, the heat flux assigned to the heat transport between ELMs is reduced and the ELM loss power is enhanced. During the inter-ELM phase, the energy confinement time becomes larger with decreasing the edge collisionality and poloidal Larmor radius.
  • The purpose of this paper is to investigate the possible relationships between transport properties such as thermal diffusivity and resistivity on the one hand, and the magnetic properties such as {ital q} profile and toroidal flux change on the other, in tokamaks that exist under macroscopically quasistationary conditions. It is experimentally well established that when the sources are held constant over times long compared with energy and particle confinement times, tokamak discharges can exist in a quasistationary state with or without periodic sawteeth superposed on the basic equilibrium.
  • Large scale transport events are studied in simulations of resistive ballooning turbulence in a tokamak plasma. The spatial structure of the turbulent flux is analyzed, indicating radially elongated structures (streamers) at the low field side which are distorted by magnetic shear at different toroidal positions. The interplay between self-generated zonal flows and transport events is investigated, resulting in significant modifications of the frequency and the amplitude of bursts. The propagation of bursts is studied in the presence of a transport barrier generated by a strong shear flow.
  • Turbulent energy transport in numerical simulations of the edge region of spherical tokamaks with small toroidal aspect ratio A=1.5 is directly compared to that in conventional tokamaks with larger aspect ratio A=3. Depending upon the aspect ratio of the torus, the energy flux can vary greatly. While a tokamak plasma with aspect ratio A=3 is in the high (H) mode where the transport rate is small, an otherwise identical tokamak plasma with small aspect ratio A=1.5 exhibits extremely poor confinement. The magnitude of the energy flux in these two cases differs by a multiplicative factor larger than 50. However, goodmore » H mode confinement in small aspect ratio spherical tokamaks can be obtained by a further increase in the edge density gradient. Confinement is then dominated by bursts of ballooning modes which periodically transport energy to large major radius. The periodic bursts in the numerical simulations are similar to the periodic radiation bursts that characterize edge-localized modes in tokamaks.« less