Improved core transport triggered by off-axis ECRH switch-off on the HL-2A tokamak

The reduced core transport and the internal transport barrier (ITB) are observed after the ECRH is turned off in the HL-2A tokamak plasmas. It is found that the time delay of the central temperature afer ECRH switch-off increases as the ECRH deposition moves from center to edge. After the far off-axis ECRH switch-off (outside q=2 surface), the central temperature transiently increases for several tens of milliseconds before it starts to drop. A low-frequency, spectrally broad, poloidal structure that peaks near zero frequency is observed in the core region (near q=1 surface) by using the newly developed correlation reflectometer system. These structures have low poloidal mode number, high polodal correlation and short radial correlation comparable to the ambient turbulence decorrelation length. Observation shows that these structures play an important role in the suppression of the core turbulence, and in the improvements of the core transport after the off-axis ECRH is turned off.