Critical gradient scale length measurements via variable location ECE channels in the EAST tokamak*

ECE radiometers that implement variable location channels using yttrium iron garnet (YIG) bandpass filters are capable of real-time ∇T_e and a/L_Te (where ais the minor radius and L_Te= T_e/∇T_e) measurements with sub-millisecond temporal resolution [Houshmandyar et al., Rev. Sci Instrum., 2018]. The key features of the YIG filters in these measurements are their narrow bandwidth and fast frequency switching at any given center frequency. These permit rapid relocation of the ECE channels, direct and calibration-free measurement of a/L_Te, and close spacing of the ECE channels, which can be done in real time during a plasma discharge. At the EAST tokamak, two YIG filters were substituted for the fixed filters in the IF section to permit gradient scale length measurements by fast slewing of their center frequencies. Electrons were heated via ECRH at different power levels and power deposition locations for the intent of varying the electron heat flux and simultaneously, gradually change the local gradient. The power balance analysis (Q/Q_GB) and the measured scale length (a/L_Te) resulting in the critical gradient scale length measurements at the two major radii, as well as turbulence measurements via CECE will be presented.