Current Challenges in the First Principle Quantitative Modelling of the Lower Hybrid Current Drive in Tokamaks

Y Peysson,J Qian,J Decker,B J Ding,A Ekedahl,M Goniche,S Shiraiwa,M Li,X Zhai,A Garofalo,J Chen,P T Bonoli,J Hillairet

doi:10.1051/epjconf/201715702007

Abstract

The Lower Hybrid (LH) wave is widely used in existing tokamaks for tailoring current density profile or extending pulse duration to steady-state regimes. Its high efficiency makes it particularly attractive for a fusion reactor, leading to consider it for this purpose in ITER tokamak. Nevertheless, if basics of the LH wave in tokamak plasma are well known, quantitative modeling of experimental observations based on first principles remains a highly challenging exercise, despite considerable numerical efforts achieved so far. In this context, a rigorous methodology must be carried out in the simulations to identify the minimum number of physical mechanisms that must be considered to reproduce experimental shot to shot observations and also scalings (density, power spectrum). Based on recent simulations carried out for EAST, Alcator C-Mod and Tore Supra tokamaks, the state of the art in LH modeling is reviewed. The capability of fast electron bremsstrahlung, internal inductance l i and LH driven current at zero loop voltage to constrain all together LH simulations is discussed, as well as the needs of further improvements (diagnostics, codes, LH model), for robust interpretative and predictive simulations.

Highlights

With the highest known current drive (CD) efficiency in tokamaks and off-axis power absorption, the rf wave at the Lower Hybrid (LH) frequency is attractive for current profile shaping or steady-state operation [1]
For C3PO/LUKE, each lobe in the power spectrum is considered locally as a plane wave, even for the tail LH model, with a spectral width given by the Fourier transform of the initial wave electric field pattern
In order to recover a good agreement, the introduction of a fast fluctuating power spectrum with respect to the fast electron slowing-down time must be introduced in LH-driven Tore Supra plasmas [16]

Summary

1.Introduction

With the highest known current drive (CD) efficiency in tokamaks and off-axis power absorption, the rf wave at the Lower Hybrid (LH) frequency is attractive for current profile shaping or steady-state operation [1]. In this context, first principle modeling has started long time ago and is still active after more than 30 years of research. While modeling tools are able to reproduce quantitatively observations in low density plasmas, simulation becomes challenging when LH wave absorption becomes weak, i.e. for plasmas at high density and low temperature. The potential influence of edge plasma physics on core absorption is one the many challenges to be tackled concerning LH current drive in tokamak, in particular for ITER

Numerical tools used by LH current drive first principle modeling

A selection of results

Findings

Conclusions