Investigation of the effectiveness of 'multi-harmonic' electron cyclotron current drive in the non inductive EXL-50 ST

The fully non-inductive spherical tokamak EXL-50, built and operated by the ENN private limited company, has routinely achieved high current drive efficiency of ∼ 1 A/W in only ECRH powered experiments. We have numerically investigated the effectiveness of multiple electron cyclotron resonance (ECR) harmonics in generating such a high efficiency of electron cyclotron current drive (ECCD) in non-inductive plasma start-up. The Fokker-Planck equation is numerically solved to obtain the electron distribution function, under the steady state of relativistic nonlinear Coulomb collision and quasi-linear diffusion operators, for calculating plasma current driven by the injected EC waves. Multi-pass absorption simulations, done with the CQL3D code for extra-ordinary EC waves, demonstrate over 1 A/W efficiency in current for a relatively low density (∼ 2 × 1018m−3), and low temperature (∼ 100 eV) plasma, consistent with the experimental results observed on EXL-50. Systematic scanning of different ECR harmonics in simulation has revealed that the multi-harmonic resonance interaction in EXL-50 plays a pivotal role in generating the energetic electron tail responsible for the current.