Experimental study of hot ion modes on the HL-2A tokamak

Abstract Hot ion mode with a high ratio of ion temperature ( T i ) and electron temperature ( T e ) has been achieved on the HL-2A tokamak. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>e</mml:mi> </mml:msub> </mml:mrow> </mml:math> is higher than 3, and T i reaches 3.5 keV in the deuterium plasma with neutral beam injection power no more than 1.2 MW. A low density is observed to be favorable for the achievement of hot ion modes and high ion temperatures. In addition, core-localized electromagnetic instabilities are also unstable during the formation and evolution of the internal transport barrier (ITB). Numerical simulations for the hot ion scenario have been performed. Power balance analysis provided by the TRANSP code suggests the significant heating of ions in the core region but of electrons at the edge. The TRANSP code also reveals that the thermal ion heat diffusivity is comparable to the neoclassical level in the ITB region. Further analysis based on the GENE code indicates that the ion temperature gradient modes–induced heat transport is dominant in experiments, but kinetic ballooning mode–induced transport will play a more critical role when electron beta becomes higher. More interesting, the heat flux exhibits a large drop when fast ions are considered. These results may indicate that the fast ions have a stabilization effect on plasma turbulence, thus reducing the thermal ion transport and finally contributing to the formation of hot ion modes.