Entropy production rate in tokamaks with nonaxisymmetric magnetic fields

Abstract

International audienceA variational principle based on an extremum of entropy production rate is derived for an integrable Hamiltonian system in the presence of two perturbations, which are resonant on neighboring or identical resonant surfaces. This method can be used to calculate the neoclassical transport in a magnetic fusion device with a nonaxisymmetric magnetic field. When applied to a tokamak with ripple, it provides an efficient means to discriminate between various regimes, depending on collisionality and ripple amplitude. This technique provides a means to test simplified collision operators suitable for gyrokinetic codes. It is found that these operators need only satisfy modest requirements in order to recover the correct neoclassical equilibrium, at least in the large aspect ratio limit. Finally, it appears that the toroidal viscous damping rate due to ripple is usually not negligible, thus removing the degeneracy between the radial electric field and the toroidal velocity

Residual parallel Reynolds stress due to turbulence intensity gradient in tokamak plasmas

“…4 More generally, the presence of any non-axisymmetry in the magnetic field configuration, be it due to MHD modes, magnetic islands, or external error fields can produce a neoclassical toroidal viscous force that set the offset value of toroidal rotation. [15][16][17][18] This was observed in various tokamaks including NSTX 19,20 and D-IIID. 21,22 While there are different mechanisms, which give rise to "intrinsic" rotation ͑i.e., rotation with no applied torque͒ that may differently scale with different parameters dominant in different machines and in different modes of operation, there seems to be a universal part of this rotation that is associated with the self-organization process that leads to the formation of the H-mode.…”

The Mistral base case to validate kinetic and fluid turbulence transport codes of the edge and SOL plasmas

“…Last and not least, Tore Supra is characterised by significant ripple that can be varied with a proper positioning of the plasma within the chamber. This provides a means to modify the plasma toroidal rotation while maintaining most other parameters constant [3,4]. Together with appropriate diagnostics, this offers a unique opportunity to investigate plasma rotation and validate the modelling effort.…”

Turbulent momentum transport in core tokamak plasmas and penetration of scrape-off layer flows

“…Generally speaking, any breaking of the axisymmetry in a tokamak can lead to the generation of toroidal rotation. This symmetry can be broken by a non-axisymmetric magnetic field, which acts trough collisional processes as a friction term on the toroidal velocity (see for example [2,3,4]). For instance, modifications of the toroidal rotation profile have been reported experimentally in the presence of toroidal field ripple [5,6,7] or externally applied magnetic perturbations [8,9].…”