Progress of KSTAR 5-GHz lower hybrid current drive system

A 5-GHz steady-state lower hybrid (LH) current drive (LHCD) system is planned to support steady-state and advanced tokamak operation on the Korea Superconducting Tokamak Advanced Research (KSTAR) experiment. As an initial phase, a pulsed 5-GHz, 500-kW LHCD system has been installed in KSTAR for basic experimental studies of the LH coupling and flux saving in the plasma current ramp-up, prior to long-pulse noninductive operation in KSTAR. A Toshiba-made klystron developed in collaboration with Pohang University of Science and Technology in 2006 is utilized for the initial KSTAR LHCD system. The LH launcher is designed as a fully active waveguide grill type with a parallel refractive index n_IIvalue ranging from 1.8 to 4.3 and with high directivity. In the initial stage, the LH launcher consists of eight columns of four-way power splitters and two columns of dummy waveguides, one on each side. The operational n_IIvalue is fixed at 2.1 but can be adjusted by replacing waveguide components external to the vacuum vessel. Since the target operation pulse duration of the initial LHCD system is 2 s with an output power of 500 kW at the klystron window, the prototype klystron was recently successfully conditioned to a radio frequency power of 514 kW for a maximum pulse duration of 3 s using a matched dummy load (voltage standing wave ratio of 1.16:1). This paper presents the progress of the initial KSTAR LHCD system and the performance test results of the prototype klystron. The research plan aiming at steadystate LHCD operation in KSTAR is also described in this paper.