Impurity holes in tokamaks with electron cyclotron resonance heating of the helical core

Long-living internal kink mode, which is often observed between the sawtooth crashes in tokamaks, leads to the formation of a new kind of fast particle orbit, namely, the toroidally trapped superbanana orbits with width Delta r(sb) similar to(xi(0)r)(1/2), where xi 0(r) is the amplitude of a kink displacement (flux surface radius). In the velocity space, such orbits are formed around resonant pitch-angles corresponding to zero toroidal precession of the banana guiding center. High-field side (HFS) electron cyclotron resonance heating (ECRH) applied to such helical cores generates a significant population of fast electrons with resonant pitch angles, which results in the strong non-ambipolar electron flux greatly exceeding the flux of non-resonant thermal ions. Ambipolarity is restored by the strong positive radial electric field (E-r), which annihilates this flux. The height of this central "hill" with E-r>0 can be comparable in magnitude with the depth of the edge E-r<0 "well" supporting H-mode pedestal. This effect can be responsible for the tungsten "hole" observed between sawtooth crashes during HFS ECRH in the ASDEX Upgrade tokamak [Sertoli et al. Nucl. Fusion 55, 113029 (2015)]. Published under license by AIP Publishing.