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

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.

Impact of microwave beam scattering by density fluctuations on the electron–cyclotron power deposition profile in tokamaks

“…For the ITER equatorial launchers in the baseline H-mode scenario, one can expect from 20% to 150% of beam broadening [21]. A multimodel approach has also been proposed to study NTM mitigation using EC waves in a turbulent plasma, concluding that, for ITER-relevant configurations, the damping time of the NTM is up to four times longer in the presence of edgelocalized density blobs because of the turbulence-induced beam refraction and diffraction [26]. Part of these studies focus mainly on the beam broadening but not on the resulting power deposition profile, which also depends on the wave injection configuration.…”