Extension of high Te regime with upgraded ECRH system in the LHD

Abstract

Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012 and the total injection power of ECRH reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature Te0 = 13.5 keV with ne = 1×1019m−3. In the LHD, an electron-internal-transport barrier (e-ITB) related to the production of high Te plasmas has been realized by strongly centre-focused ECRH. The electron thermal confinement clearly improved inside the e-ITB. The radial electric field was measured using the heavy ion beam probe. The formation of the positive Er was observed in the core region, which well agreed with the prediction of the neoclassical transport theory. The energy confinement characteristics have been investigated in the ECRH plasmas. It was found that higher plasma stored energy and lower radiation power was realized in the outward configuration. The plasma stored energy of 530 kJ with ne = 3.2×1019m−3, which is the 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.