New injection scheme and experimental results for improved heating efficiency of electron cyclotron heating in the LHD

We proposed a new electron cyclotron heating (ECH) system injected from a high field side in the Large Helical Device (LHD). We expect that a highly reliable and functional ECH system enhances plasma parameters and performs various physics experiments. The conventional EC beams cause the refraction in plasmas because of an oblique injection against magnetic fields [1], even if the electron density is below the cut-off density. The new method suppresses the refraction effect by directing the injected EC beam normal to the magnetic surfaces of the LHD. The suppression is confirmed by a ray-tracing calculation (TRAVIS code). We have designed and installed the new injection system in the LHD. From the plasma experiment, the straight propagation of the EC wave from the new injection system leads to the effective heating on the magnetic axis, even though the 77 GHz EC wave from the existing injection system refracts in the same shot. However, the new system suffers from the reflected wave to the gyrotron. In the present experiment, the gyrotron's output power and pulse length are limited so that the oscillation does not stop. We will discuss the solution for the next campaign.