Modelling effects of edge density fluctuations on electron-cyclotron current drive used for neoclassical tearing mode stabilization

C Tsironis,K Hizanidis,F Bairaktaris,S I Valvis,P Papagiannis

doi:10.1017/s002237782000149x

Abstract

In this work we study the undesired effects of electron density fluctuations (in the form of blob structures which may exist in the edge region of tokamak plasmas) to the electron-cyclotron wave propagation and current drive in connection to the efficiency of neoclassical tearing mode stabilization. Our model involves the evaluation of the driven current in the presence of density perturbations, by using a combination of a wave solver based on the transfer matrix and electromagnetic homogenization methods for the propagation part prior to and inside the region of these structures (where standard asymptotic propagation methods may not be valid due to the short-wavelength limit breakdown), with a ray tracing code including island geometry effects and current drive computation for the propagation past the perturbed region. The computed driven current is input into the modified Rutherford equation in order to estimate the consequences of the wave deformation (driven by the density fluctuations) to the mode stabilization.

Highlights

In tokamak experiments, electron-cyclotron (EC) waves are launched into the plasma in the form of spatially narrow beams, and exchange energy with electrons passing the beam region when the specific resonance condition is satisfied (Brambilla 1998), ω − lhωc − k v ≈ 0, γ lh = ±1, ±2, . . . , (1.1)with ω, ωc the wave and cyclotron frequencies, respectively, γ the Lorentz factor and k, v the wavevector and particle velocity components parallel to the magnetic field
Since the tokamak magnetic field is non-uniform, under specific choices for the beam launching conditions, the EC resonance may be realized in a very narrow spatial region. In this respect, using EC waves gives the advantage of good localization in the power deposition, which is exploited in applications relevant to auxiliary plasma heating (Prater 2004), plasma ramp-up and breakdown assistance (Granucci et al 2012), sawtooth and neoclassical tearing mode (NTM) instability control (LaHaye 2006), as well as plasma diagnostics based on electromagnetic (EM) waves like, for example, reflectometry and interferometry (Hutchinson 2005)
This section is devoted to the numerical results of each physics model in our sequence, focusing on the assessment of the effect of the beam distortion by the density blobs to the NTM stabilization quality

Summary

Introduction

Since the tokamak magnetic field is non-uniform, under specific choices for the beam launching conditions, the EC resonance may be realized in a very narrow spatial region In this respect, using EC waves gives the advantage of good localization in the power deposition, which is exploited in applications relevant to auxiliary plasma heating (Prater 2004), plasma ramp-up and breakdown assistance (Granucci et al 2012), sawtooth and neoclassical tearing mode (NTM) instability control (LaHaye 2006), as well as plasma diagnostics based on electromagnetic (EM) waves like, for example, reflectometry and interferometry (Hutchinson 2005). We use a modelling loop that incorporates all the system physics: the propagation of the wave beam through the perturbed layer and up to the magnetic island, the power absorption and current drive on the island’s flux surfaces and, the ECCD-driven mode dynamic evolution.

Description of the physics models

Transfer matrix technique and EM homogenization method

Ray tracing algorithm

Set-up of the numerical simulations

Results and discussion

Conclusion