ECRH Experiments in an Extended Power Regime on the Large Helical Device

摘要

The improvement of the ECRH system and obtained experimental results have been presented in this article. In the latest experimental campaign, seven gyrotron system was operable. One transmission line which was evacuated in whole system was installed in addition to the six existed 88.9 mm lines in atmospheric pressure. This line allows to transmit over 500 kW power. Flexible experiments could be performed by the inward shift of a scanning range of the existed antennas and by an installation of a new lower port antenna. Such improvement enabled the experiments in an extended power regime (injection power ≤ 1.8MW) of LHD. In special, on-axis heating opened new regime such as a high electron temperature state of over 10 keV and a formation of a transport barrier in the core. Such high temperature plasmas with the ITB are characterized by a high central temperature with its steep gradient around ρapprox=0.2―0.3. The formation of the ITB has a clear power threshold which increases with the electron density. The ECE signal analysis shows improved confinement of the electron thermal transport inside the ITB. The mechanism of the ITB formation is not still clear. The ambipolar condition of electron and ion fluxes predicts a strong positive radial electric field in the core (the electron root). This electric field can reduce the electron thermal diffusivity. This neoclassical depiction is not sufficient to explain the amount of X_e reduction. A sheared electric field may play an important role to suppress the anomalous transport. Direct measurement of E_r and turbulence behavior are the subjects of future work.