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Experimental observation of the transition between resistive ballooning modes and ion temperature gradient modes in the edge of the HL-2A tokamak

The type of edge turbulence transition from resistive ballooning modes (RBM) to drift-type ion temperature gradient (ITG) modes has been observed in the gas puffing modulation experiments and also simulations at the HL-2A tokamak. The experimental evidence on RMB and ITG characteristics is given by Langmuir probe and Doppler reflectometry data. In the low temperature region (Te  <  80 eV), the variation of turbulence intensity is in phase with that of the parallel resistivity, consistent with RBM features. However, in the high temperature region (Te  >  80 eV), the turbulence intensity is enhanced with increasing temperature and reduction of the parallel resistivity, implying a drive by drift wave type ITG modes. By numerical simulations with drift resistive inertial ballooning model, the dependence of the growth rate on the electron temperature for the most unstable mode has been investigated based on experimental parameters. Both experiments and simulations have suggested that the dominating instability transits from RBMs in the low Te regime to ITG modes in the high Te regime.