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Title: Large transport-induced operation limits of tokamak plasmas

Abstract

The two-dimensional phase space of tokamak edge plasmas identified in the numerical simulations by B. Rogers et al. [Phys. Rev. Lett. 81, 4396 (1998)] provides a unique prescription for the various regimes of operation of tokamak plasmas. Recent observations on Alcator C-Mod of these regimes, identified in terms of the above-mentioned phase-space parameters, is found to be in very good agreement with simulation results of Rogers et al. In this phase space, they identified a boundary at high collisionality that defines a region that is operationally inaccessible owing to very large transport in the edge region of the tokamaks. A second boundary at moderate to low collisionality is also indicated and associated with the transition between the low-confinement mode and the high-confinement mode. The high collisionality boundary is of particular interest since it appears to be fundamentally related to the empirical 'density limit' that is observed in tokamaks. In this Letter, we provide a theory that determines the conditions necessary for very high transport and hence the origin of the inaccessible 'density limit' in the two-dimensional phase space.

Authors:
; ; ; ; ;  [1];  [2];  [3]
  1. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 (United States)
  2. (India)
  3. (United States)
Publication Date:
OSTI Identifier:
20974814
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 2; Other Information: DOI: 10.1063/1.2436736; (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; ALCATOR DEVICE; BOUNDARY LAYERS; NUMERICAL ANALYSIS; PHASE SPACE; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA SIMULATION; TWO-DIMENSIONAL CALCULATIONS

Citation Formats

Guzdar, P. N., Kleva, R. G., Kaw, P. K., Singh, R., LaBombard, B., Greenwald, M., Institute for Plasma Research, Gandhinagar, Gujarat 78712, and Plasma Science and Fusion Center, Massachusetts Institute of Technology, 175 Albany Street, Cambridge, Massachusetts 02139. Large transport-induced operation limits of tokamak plasmas. United States: N. p., 2007. Web. doi:10.1063/1.2436736.
Guzdar, P. N., Kleva, R. G., Kaw, P. K., Singh, R., LaBombard, B., Greenwald, M., Institute for Plasma Research, Gandhinagar, Gujarat 78712, & Plasma Science and Fusion Center, Massachusetts Institute of Technology, 175 Albany Street, Cambridge, Massachusetts 02139. Large transport-induced operation limits of tokamak plasmas. United States. doi:10.1063/1.2436736.
Guzdar, P. N., Kleva, R. G., Kaw, P. K., Singh, R., LaBombard, B., Greenwald, M., Institute for Plasma Research, Gandhinagar, Gujarat 78712, and Plasma Science and Fusion Center, Massachusetts Institute of Technology, 175 Albany Street, Cambridge, Massachusetts 02139. Thu . "Large transport-induced operation limits of tokamak plasmas". United States. doi:10.1063/1.2436736.
@article{osti_20974814,
title = {Large transport-induced operation limits of tokamak plasmas},
author = {Guzdar, P. N. and Kleva, R. G. and Kaw, P. K. and Singh, R. and LaBombard, B. and Greenwald, M. and Institute for Plasma Research, Gandhinagar, Gujarat 78712 and Plasma Science and Fusion Center, Massachusetts Institute of Technology, 175 Albany Street, Cambridge, Massachusetts 02139},
abstractNote = {The two-dimensional phase space of tokamak edge plasmas identified in the numerical simulations by B. Rogers et al. [Phys. Rev. Lett. 81, 4396 (1998)] provides a unique prescription for the various regimes of operation of tokamak plasmas. Recent observations on Alcator C-Mod of these regimes, identified in terms of the above-mentioned phase-space parameters, is found to be in very good agreement with simulation results of Rogers et al. In this phase space, they identified a boundary at high collisionality that defines a region that is operationally inaccessible owing to very large transport in the edge region of the tokamaks. A second boundary at moderate to low collisionality is also indicated and associated with the transition between the low-confinement mode and the high-confinement mode. The high collisionality boundary is of particular interest since it appears to be fundamentally related to the empirical 'density limit' that is observed in tokamaks. In this Letter, we provide a theory that determines the conditions necessary for very high transport and hence the origin of the inaccessible 'density limit' in the two-dimensional phase space.},
doi = {10.1063/1.2436736},
journal = {Physics of Plasmas},
number = 2,
volume = 14,
place = {United States},
year = {Thu Feb 15 00:00:00 EST 2007},
month = {Thu Feb 15 00:00:00 EST 2007}
}
  • The neoclassical transport theory for tokamak plasmas in the plateau to Pfirsch--Schlueter regimes is extended to cases in which the parallel flow can become comparable to the ion thermal velocity. Important changes are found in the qualitative structure and the quantitative predictions of the theory. Solutions for both pure and impure plasmas in the plateau regime are obtained.
  • A consistent picture of sawteeth-induced impurity transport is presented based on a recently given model for plasma redistribution by sawtooth crashes. The redistribution model is based on ideal magnetohydrodynamic conservation laws and assumes that the magnetic flux and the volume of the layers that reconnect during the crash are conserved. The post-crash redistributed impurity density is found in terms of the impurity density before the crash and the profiles of the safety factor before and after the crash. Comparisons with experimental results show good qualitative agreement. {copyright} {ital 1996 American Institute of Physics.}
  • Wave-induced flows are calculated from high-resolution electromagnetic field calculations with either a compressible Reynolds stress or a second-order kinetic pressure model for the radio frequency forces. Results show that electron Landau damping and magnetic pumping, by themselves, do not lead to significant poloidal flow as long as there is little net input of momentum by the wave. But ion-cyclotron damping of either fast magnetosonic waves or ion-Bernstein waves can drive significant poloidal flows at power levels typical of plasma-heating experiments. {copyright} {ital 1999} {ital The American Physical Society}
  • Global gyrokinetic Vlasov simulations for trapped ion modes are performed by solving a Vlasov equation averaged over the cyclotron and bounce motions of trapped ions. The distribution function, for trapped ions, is then calculated in a two-dimensional phase space, parametrized by the longitudinal action (energy) and the magnetic moment in presence of magnetic shear. The physical mechanism of the saturation processes between streamerlike structures and zonal flows in relation to the suppression of turbulent transport is discussed. The magnetic shear is identified to play a key role in the dominant streamer-induced transport regime, which exhibits a Bohm-like scaling. The interactionmore » of streamerlike structures with plasma turbulence is shown to produce the inverse cascade that condenses onto long-wavelength trapped ion structures, on the basis on wave triad interactions.« less
  • First-principles numerical simulations are used to describe a transport bifurcation in a differentially rotating tokamak plasma. Such a bifurcation is more probable in a region of zero magnetic shear than one of finite magnetic shear, because in the former case the component of the sheared toroidal flow that is perpendicular to the magnetic field has the strongest suppressing effect on the turbulence. In the zero-magnetic-shear regime, there are no growing linear eigenmodes at any finite value of flow shear. However, subcritical turbulence can be sustained, owing to the existence of modes, driven by the ion temperature gradient and the parallelmore » velocity gradient, which grow transiently. Nonetheless, in a parameter space containing a wide range of temperature gradients and velocity shears, there is a sizeable window where all turbulence is suppressed. Combined with the relatively low transport of momentum by collisional (neoclassical) mechanisms, this produces the conditions for a bifurcation from low to high temperature and velocity gradients. A parametric model is constructed which accurately describes the combined effect of the temperature gradient and the flow gradient over a wide range of their values. Using this parametric model, it is shown that in the reduced-transport state, heat is transported almost neoclassically, while momentum transport is dominated by subcritical parallel-velocity-gradient-driven turbulence. It is further shown that for any given input of torque, there is an optimum input of heat which maximises the temperature gradient. The parametric model describes both the behaviour of the subcritical turbulence (which cannot be modelled by the quasi-linear methods used in current transport codes) and the complicated effect of the flow shear on the transport stiffness. It may prove useful for transport modelling of tokamaks with sheared flows.« less