Influence of trapped electrons on ECCD in Heliotron J

Second-harmonic electron cyclotron current drive (ECCD) experiments have been performed in the stellarator/heliotron (S/H) device, Heliotron J. A focused Gaussian beam is injected with the parallel refractive index, N||, ranging from −0.05 to 0.6. The EC driven current is estimated by excluding the bootstrap current from the total current. The experimental results show that the EC driven current is determined by the local magnetic field structure where the EC power is deposited. The maximum EC driven current is attained around N|| = 0.5 when the EC power is deposited at nearly the top of the magnetic ripple. A large increase in electron cyclotron emission (ECE) signals is observed when the EC current is driven, indicating an important role for high-energy electrons in the ECCD. The experimental results for N|| and B dependence agree with a ray-tracing simulation using a parallel momentum conservation model in which the trapped particle effect is included