Progress in Long Pulse ECH Discharges in LHD

Abstract This is a review paper as a part of the special issue dedicated for Large Helical Device (LHD). The topic of this paper is long pulse discharges performed by Electron Cyclotron Heating (ECH) in LHD, and it is described based on the previously published papers related to the topic. The ECH system on LHD has been upgraded step by step, applying state of the art gyrotrons at the steps. The first long pulse discharge was achieved by applying 84 GHz, 110 kW continuous wave gyrotron. The pulse width was 65 min, with rather low line average electron density n e_ave of 0.15 × 10 19 m − 3 and the central electron temperature T e0 of 1.7 keV. Gradually applied 5 gyrotrons (three 77 GHz and two 154 GHz) took the place of the power source for the long pulse ECH discharges. Owing to the improvement in the ECH system, a stable 39 min discharge with much improved parameters, n e_ave of 1.1 × 10 19 m − 3 , T e0 of over 2.5 keV, and a central ion temperature T i0 of 1.0 keV was successfully performed with ~ 350 kW ECH power. Also, long pulse experiments have made progress in sustainment of improved electron confinement states (e-ITB) at the plasma core region. Sustainment of a plasma having e-ITB with n e_ave of 1.1 × 10 19 m − 3 and T e0 of ∼3.5 keV for longer than 5 min with 340 kW ECH power was successfully demonstrated.