skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Ripple modifications to alpha transport in tokamaks

Abstract

Magnetic field ripple is inherent in tokamaks since the toroidal magnetic field is generated by a finite number of toroidal field coils. The field ripple results in departures from axisymmetry that cause radial transport losses of particles and heat. These ripple losses are a serious concern for alphas near their birth speed$$v_{0}$$since alpha heating of the background plasma is required to make fusion reactors into economical power plants. Ripple in tokamaks gives rise to at least two alpha transport regimes of concern. As the slowing down time$$\unicode[STIX]{x1D70F}_{s}$$is much larger than the time for an alpha just born to make a toroidal transit, a regime referred to as the$$1/\unicode[STIX]{x1D708}\propto \unicode[STIX]{x1D70F}_{s}$$regime can be encountered, with$$\unicode[STIX]{x1D708}$$the appropriate alpha collision frequency. In this regime the radial transport losses increase as$$v_{0}\unicode[STIX]{x1D70F}_{s}/R$$, with$$R$$the major radius of the tokamak. The deleterious effect of ripple transport is mitigated by electric and magnetic drifts within the flux surface. When drift tangent to the flux surface becomes significant another ripple regime, referred to as the$$\sqrt{\unicode[STIX]{x1D708}}$$regime, is encountered where a collisional boundary layer due to the drift plays a key role. We evaluate the alpha transport in both regimes, taking account of the alphas having a slowing down rather than amore » Maxwellian distribution function and their being collisionally scattered by a collision operator appropriate for alphas. Alpha ripple transport is found to be in the$$\sqrt{\unicode[STIX]{x1D708}}$regime where it will be a serious issue for typical tokamak reactors as it will be well above the axisymmetric neoclassical level and can be large enough to deplete the alpha slowing down distribution function unless toroidal rotation is strong.« less

Authors:
Publication Date:
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1610178
DOE Contract Number:  
FG02-91ER54109
Resource Type:
Journal Article
Journal Name:
Journal of Plasma Physics
Additional Journal Information:
Journal Volume: 84; Journal Issue: 5; Journal ID: ISSN 0022-3778
Publisher:
Cambridge University Press
Country of Publication:
United States
Language:
English
Subject:
Physics

Citation Formats

Catto, Peter J. Ripple modifications to alpha transport in tokamaks. United States: N. p., 2018. Web. doi:10.1017/s0022377818000715.
Catto, Peter J. Ripple modifications to alpha transport in tokamaks. United States. https://doi.org/10.1017/s0022377818000715
Catto, Peter J. Mon . "Ripple modifications to alpha transport in tokamaks". United States. https://doi.org/10.1017/s0022377818000715.
@article{osti_1610178,
title = {Ripple modifications to alpha transport in tokamaks},
author = {Catto, Peter J.},
abstractNote = {Magnetic field ripple is inherent in tokamaks since the toroidal magnetic field is generated by a finite number of toroidal field coils. The field ripple results in departures from axisymmetry that cause radial transport losses of particles and heat. These ripple losses are a serious concern for alphas near their birth speed$v_{0}$since alpha heating of the background plasma is required to make fusion reactors into economical power plants. Ripple in tokamaks gives rise to at least two alpha transport regimes of concern. As the slowing down time$\unicode[STIX]{x1D70F}_{s}$is much larger than the time for an alpha just born to make a toroidal transit, a regime referred to as the$1/\unicode[STIX]{x1D708}\propto \unicode[STIX]{x1D70F}_{s}$regime can be encountered, with$\unicode[STIX]{x1D708}$the appropriate alpha collision frequency. In this regime the radial transport losses increase as$v_{0}\unicode[STIX]{x1D70F}_{s}/R$, with$R$the major radius of the tokamak. The deleterious effect of ripple transport is mitigated by electric and magnetic drifts within the flux surface. When drift tangent to the flux surface becomes significant another ripple regime, referred to as the$\sqrt{\unicode[STIX]{x1D708}}$regime, is encountered where a collisional boundary layer due to the drift plays a key role. We evaluate the alpha transport in both regimes, taking account of the alphas having a slowing down rather than a Maxwellian distribution function and their being collisionally scattered by a collision operator appropriate for alphas. Alpha ripple transport is found to be in the$\sqrt{\unicode[STIX]{x1D708}}$regime where it will be a serious issue for typical tokamak reactors as it will be well above the axisymmetric neoclassical level and can be large enough to deplete the alpha slowing down distribution function unless toroidal rotation is strong.},
doi = {10.1017/s0022377818000715},
url = {https://www.osti.gov/biblio/1610178}, journal = {Journal of Plasma Physics},
issn = {0022-3778},
number = 5,
volume = 84,
place = {United States},
year = {2018},
month = {10}
}

Works referenced in this record:

Confinement of High-Energy Trapped Particles in Tokamaks
journal, August 1981


Theory of plasma confinement in non-axisymmetric magnetic fields
journal, July 2014


Recursive derivation of drift-kinetic equation
journal, January 1973


Plasma Diffusion in a Toroidal Stellarator
journal, March 1969


Limitations of gyrokinetics on transport time scales
journal, April 2008


Omnigenity as generalized quasisymmetry
journal, May 2012


Arbitrary poloidal gyroradius effects in tokamak pedestals and transport barriers
journal, June 2008


Banana drift transport in tokamaks with ripple
journal, January 1982


Rotation and neoclassical ripple transport in ITER
journal, August 2017


Guiding center drift equations
journal, January 1980


Transport of momentum in full f gyrokinetics
journal, May 2010


Neoclassical transport in stellarators
journal, January 1987


Plasma equilibrium with rational magnetic surfaces
journal, January 1981