Speaker
Rodrigo Saavedra
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1105.pdf
Monte-Carlo simulation of fast ion transport in magnetic island regions
Rodrigo Saavedra1 and Julio J. Martinell1
1 Instituto de Ciencias Nucleares, UNAM, A. Postal 70-543, México D.F., Mexico
There is theoretical evidence of the formation of magnetic islands in rational magnetic sur-
faces in toroidal fusion devices. Furthermore it has been shown that, in electron cyclotron reso-
nance heating (ECRH) experiments, such regions may act as transport barriers for suprathermal
electrons [1]. When the plasma heating is produced by neutral beam injection (NBI), a popula-
tion of fast ions arises which interacts with the magnetic islands altering its transport.
Here we study the transport of a population of fast particles in the presence of a magnetic is-
land configuration produced by collisions with a Maxwellian plasma background consisting of
electrons and a single species of ions, which are described by Lorentz scattering [2]. We obtain
the time evolution of the distribution function by solving Langevin equations for a large popu-
lation of particles corresponding to ECRH or NBI. Then, transport coefficients are calculated.
The equations of motion for the guiding center (GC) of a charged particle in a strong magnetic
are solved in 3 spatial dimensions (x, y, z) and 2D in velocity space represented by the normal-
ized kinetic energy v2 and the pitch angle λ . The set of equations which give the evolution of
phase space variables can be expressed in the form of Langevin equations which simulate the
effect of collisions of the test particle with the plasma background [3]. They include the energy
and pitch angle stochastic collision operators [2].
In order to determine transport coefficients the time evolution of a test distribution function of
N particles is obtained with Monte Carlo simulations, which is equivalent to solving the Fokker-
Planck equation. The equations are solved with a fourth order Runge-Kutta algorithm with a
random choice of the sign in the Lorentz collision operators at each time step. Additionally
a radial electric field was included, which enhances the transport of particles. The diffusion
1
coefficient was calculated from the standard expression D = 2tN ∑Nj=1 (x j (t) − x j (0))2 where
x j (t) is the position of a particle at time t.
The results show that the island acts as a transport barrier for electrons, and the ions experi-
ence a significant modification of their transport across the island.
References
[1] M. A. Ochando et. al., Plasma Phys. Control. Fusion 45, 221 (2003).
[2] A. H. Boozer and G. Kuo-Petravic, Physics Fluids 24, 851 (1981)
[3] A. De Bustos-Molina, PhD thesis, Universidad Complutense de Madrid (2013).
