Yuki Homma

Numerical modeling of the thermal force in a plasma for test-ion transport simulation based on a Monte Carlo Binary Collision Model (II) - Thermal forces due to temperature gradients parallel and perpendicular to the magnetic field

We have developed a numerical model of the thermal force for test-ion transport simulation in a magnetized background plasma, based on the Monte Carlo Binary Collision Model (BCM) [T. Takizuka, H. Abe, J. Comput. Phys. 25 (1977) 205]. The model is basically the same as presented in our previous paper [Y. Homma, A. Hatayama, J. Comput. Phys. 231 (2012) 3211-3227] for the case without magnetic field, but in the present paper, a more extended form of a distorted Maxwellian distribution is employed for the velocity distribution of background plasma ions to simulate the thermal force caused by parallel and perpendicular (with respect to the direction of magnetic field) temperature gradients. The model consists mainly of two steps: (i) choosing a background plasma ion velocity from a distorted Maxwellian distribution, and (ii) calculating a Coulomb collision between a test particle and the above chosen ion by using the BCM. In addition, equations of motion for charged test particle in the magnetic field are calculated by Buneman-Boris Algorithm. A series of test simulations has been done in a simple geometry with different temperature gradients and different strengths of magnetic field. Numerical results of the thermal force due to parallel and perpendicular temperature gradients agree well with the theoretical prediction for all test cases. Especially, it has been confirmed that the model reproduces the temperature screening effect of test particles (i.e. guiding center drift of test particle caused by the thermal force of perpendicular temperature gradient).

Numerical modeling of thermal force in a plasma for test-ion transport simulation based on Monte Carlo Binary Collision Model

We present a new numerical model of the thermal force in a plasma, based on the Monte Carlo Binary Collision Model (BCM) [T. Takizuka, H. Abe, J. Comput.Phys. 25 (1977) 205]. This model can be applied for the transport simulation of test ions. The model consists of two steps: (i) choosing a background plasma ion velocity from a distorted Maxwell distribution under the temperature gradient, and (ii) calculating a Coulomb collision between a test particle and the above chosen ion by using the BCM. For the step (i), we developed a velocity sampling method from a distorted Maxwellian, which enables the BCM to bring the thermal force on a test particle in the step (ii). A systematic series of simulations has been performed under various conditions to examine the model. The results of these simulations have been compared with the theoretical values, and it is shown that our model simulates the thermal force correctly for important characteristic features; dependences on the temperature gradient, the test particle velocity, and the background plasma density.

Kinetic modelling for neoclassical transport of high-Z impurity particles using a binary collision method

A new kinetic model for neoclassical impurity particle transport simulation has been developed. Our model is able to simulate the following two effects, which have been theoretically predicted, but neglected in most of the existing kinetic impurity transport simulations in the SOL (scrape-off layer)/Divertor plasmas of tokamak; (1) the neoclassical inward pinch (NC IWP) due to the density gradient of background plasmas and (2) the neoclassical temperature screening effect (NC TSE, outward transport) caused by the plasma temperature gradient. The IWP and TSE, both proportional to the impurity charge number Z, become especially important for higher-Z impurities such as tungsten. In this paper we focus on the case where background plasmas are in the Pfirsch–Schluter regime. The velocity distribution of background plasma ions is modelled by a distorted Maxwellian distribution, which includes the Pfirsch–Schluter flow velocity and the Pfirsch–Schluter heat flux density, in order to reproduce the NC IWP and NC TSE. A series of test simulations have been performed for a toroidal magnetic field geometry. Characteristics of the neoclassical transport, such as dependencies on the safety factor and on the impurity charge number, have been confirmed.