Hisanori Takamaru

Modelling of ion energy transport in perturbed magnetic field in collisionless toroidal plasma

Although all physical parameters of background plasma and magnetic field are fixed, it is not trivial that transport coefficients in an ergodic region bounded radially on both sides can be always evaluated as constants with respect to time because of non-Brownian motion of guiding centres in low-collisionality cases, as shown previously in mono-energetic test-particle simulations by Maluckov et al (2003 Physica A 322 13). Here the ergodic region consists of chaotic magnetic field lines caused by resonant magnetic perturbations (RMPs). In order to understand the fundamental properties of transport phenomena in the radially bounded ergodic region, a new computer simulation code based on the δf method solving the drift kinetic equation is developed and the energy transport of ions (protons) in the perturbed magnetic field is investigated in low-collisionality cases. We evaluate the ion thermal diffusivity as a constant with respect to time by using a quasi-steady-state solution of the guiding centre distribution function in five-dimensional phase space and find that the diffusivity depends on both the strength of the RMPs and the collision frequency. The diffusivity estimated by the δf simulation in the ergodic region is extremely small compared with the prediction of field-line diffusion theory. The radial transport is affected by the fact that the width of the ergodic region is finite.

Neoclassical effect on strike point patterns in local island divertor configuration of LHD

Monte Carlo simulation based on test particle representation is carried out in order to investigate the neoclassical effect on a strike point pattern of ions on the local island divertor (LID) head in the Large Helical Device configuration. According to collisionality, the particle orbits mainly contributing the particle flux from the core region to the LID head vary; in the present paper we call it the neoclassical effect on the edge transport phenomena. The pattern on the LID head is numerically observed by tracing the guiding centre orbits of the test particles under the effects of Coulomb collision and anomalous diffusion. We find that the strike point patterns on the LID head are varied according to collisionality. For the case of λmfp/Lc > 1, the pattern is characterized by the passing particle orbits along the field lines of the island separatrix, where λmfp is the mean free path and Lc ≈ 250 m is the connection length given as a length along a field line connecting the core region to the LID head. On the other hand, for the case of λmfp/Lc 1, the particles intensively strike the edge of the LID head, and the pattern is given by the trapped particle orbits. The trapped particle orbits mainly contribute the particle flux from the core region through the inside of the island, and finally the particles arrive at the edge of the LID head.

Dependence of radial thermal diffusivity on parameters of toroidal plasma affected by resonant magnetic perturbations

We investigate how the neoclassical thermal diffusivity of an axisymmetric toroidal plasma is modified by the effect of resonant magnetic perturbations (RMPs), using a drift-kinetic simulation code for calculating the radial thermal diffusivity of ion in the perturbed region under an assumption of zero electric field. Here, the perturbed region is assumed to be generated on and near the resonance surfaces, and is wedged in between the regular closed magnetic surfaces. We find that the dependence of the radial thermal diffusivity on parameters of the toroidal plasma is represented as , where is the neoclassical thermal diffusivity and c0 is a positive coefficient. Here, ωb is the bounce frequency, νeff is the effective collision frequency, Δb is the banana width, 〈‖δBr‖2〉1/2 is the strength of the RMPs in the radial directions, and |Bt0| is the strength of the magnetic field on the magnetic axis. The value of is significantly reduced to in the simulations because of the drift motion affected by the Coulomb collision, as contrasted with predicted by the so-called field-line diffusion theory, where q is the safety factor, Rax is the major radius of the magnetic axis, and ϵt is the inverse aspect ratio.

Development of three-dimensional neoclassical transport simulation code with high performance Fortran on a vector-parallel computer

A neoclassical transport simulation code (FORTEC-3D) applicable to three-dimensional configurations has been developed using High Performance Fortran (HPF). Adoption of computing techniques for parallelization and a hybrid simulation model to the df Monte-Carlo method transport simulation, including non-local transport effects in three-dimensional configurations, makes it possible to simulate the dynamism of global, non-local transport phenomena with a self-consistent radial electric field within a reasonable computation time. In this paper, development of the transport code using HPF is reported. Optimization techniques in order to achieve both high vectorization and parallelization efficiency, adoption of a parallel random number generator, and also benchmark results, are shown.

Path integral approach for electron transport in disturbed magnetic field lines

A path integral method is developed to investigate statistical property of an electron transport described as a Langevin equation in a statically disturbed magnetic field line structure; especially a transition probability of electrons strongly tied to field lines is considered. The path integral method has advantages that (1) it does not include intrinsically a growing numerical error of an orbit, which is caused by evolution of the Langevin equation under a finite calculation accuracy in a chaotic field line structure, and (2) it gives a method of understanding the qualitative content of the Langevin equation and assists to expect statistical property of the transport. Monte Carlo calculations of the electron distributions under both effects of chaotic field lines and collisions are demonstrated to comprehend the above advantages through some examples. The mathematical techniques are useful to study statistical properties of various phenomena described as Langevin equations in general. By using parallel generators of random numbers, the Monte Carlo scheme to calculate a transition probability can be suitable for a parallel computation.