Computation of EBW heating in the TJ-II stellarator

Abstract

The theoretical properties of electron Bernstein wave (EBW) plasma heating in the TJ-II stellarator are presented in this work. Previous studies carried out in this device have demonstrated that the O–X–B mode conversion at the fundamental electron cyclotron harmonic is the best scenario for plasma heating. This scheme presents high absorbed power for central densities above 1.2 × 1019 m−3 and has no upper density limit. In this paper, the ray tracing code TRUBA has been used in its non-relativistic modality to optimize the power injection position and to design a launching system that provides the optimum theoretical results. The main characteristics of the system, whose final design is based on the former calculations, are described. To explore the importance of relativistic effects and to compare them with the non-relativistic calculations, the weakly relativistic dispersion relation, valid for and thus suitable for EBW at low cyclotron harmonics, has been obtained and included in TRUBA. Although they are of little importance for the optimization calculations, the relativistic effects are shown to be not negligible both in the ray trajectories and in the power absorption estimations for temperatures above 1 keV.

Comparison of different models for EBCD calculation in the TJ-II Stellarator

“…As it is said above, the calculation has been performed considering the standard TJ-II magnetic configuration, and the optimum beam injection in terms of maximum O-X transmission efficiency [12]. A total injected power of 300 kW before O-X conversion is simulated.…”

“…[12]. Since the goal of the present paper is the calculation and comparison of the current drive obtained using different models, the non-relativistic dispersion tensor has been used.…”

“…It has been demonstrated [20] that the optimum O-X conversion efficiency is obtained when the beam curvature matches the curvature of the O mode cutoff layer. Moreover, if this condition is true, the simulation with rays perpendicularly distributed over the matched wavefront surface provides the best estimate of the O-X conversion problem [12]. For the optimum launching direction only the central ray has maximum conversion efficiency.…”

Grid Computing for Fusion Research

“…Another kinetic code that estimates collisional transport from a totally different point of view is DKES (Drift Kinetic Equation Solver) code, which solves the drift kinetic equation for the distribution function under several approximations [7]. Plasma heating can be performed experimentally by several methods, but there are two of them that can be modelled by means of grid technologies: electron heating by a microwave beam, which can be simulated by the estimate of a large number of rays as it is performed with the TRUBA code [8], and neutral beam injection (NBI) that can be simulated by means of the Monte Carlo code FAFNER [9]. The plasma wall interaction and the edge transport can be simulated by means of the use of a Monte Carlo code like EIRENE [10], a widely distributed code for plasma-wall interaction studies, or by a PIC code like BIT1 [11].…”

“…This type of waves is known as electron Bernstein waves, which are characterised by being approximately polarised along the propagation direction. The ray tracing TRUBA [8] has been used to simulate the behaviour of this type of waves in a complex system like TJ-II. The TRUBA code has been ported to the grid using the Gridway metascheduler [15] to perform massive ray tracing.…”