Experimental study of trapped-electron-mode properties in tokamaks: threshold and stabilization by collisions

Trapped electron modes are one of the candidates to explain turbulence driven electron heat transport observed in tokamaks. This instability has two characteristics: a threshold in normalized gradient and stabilization by collisions. Experiments using modulated electron cyclotron heating in the ASDEX Upgrade tokamak demonstrate explicitly the existence of the threshold. The stabilization with increasing collisionality is evidenced by a strong decrease of the propagation of heat pulses, explained by a transition to ion temperature gradient driven transport. These results are supported by linear gyrokinetic calculations.