l-mode investigation on the Experimental Advanced Superconducting Tokamak

摘要

By analyzing large quantities of discharges in the unfavorable ion (B) over right arrow x del B drift direction, I-mode operation has been confirmed in the Experimental Advanced Superconducting Tokamak. During the L-mode to I-mode transition, the energy confinement is improvement substantially by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with I-mode observations on other devices, the E-r profiles obtained by the eight-channel Doppler backscattering system (DBS8) (Hu et al 2017 Rev. Sci. Instrum. 88 073504) show a deeper edge E-r well in the I-mode than that in the L-mode. Additionally a weak coherent mode (WCM) with the frequency range of 40-150 kHz is observed at the edge plasma with the radial extent of about 2-3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the E-r fluctuation and the caused flow shear from WCM should play an important role during I-mode operation. In addition, a low-frequency oscillation with a frequency range of 5-10 kHz is always accompanied with WCM, where the geodesic acoustic model intensity decreases or disappears. The evidence shows that the a low-frequency oscillation may be a novel kind of limited cycle oscillation but further investigation is needed to explain the new properties such as the harmonics and obvious magnetic perturbations.