Impact of electron scale modes on electron heat transport in the JET tokamak

In JET L-mode plasmas with significant ion heating by Neutral Beam Injection (NBI), higher electron stiffness and lower values of R/LTe are observed than in plasmas with pure Ion Cyclotron Resonance Heating (ICRH), for the same normalized electron heat flux [1]. The main changes due to NBI heating are lower values of Te/Ti, higher values of R/LTi, the presence of additional fast ions and higher toroidal rotation. The mechanisms for this change in behavior in the electron heat channel are investigated with linear and nonlinear simulations using the GENE gyrokinetic code in the local limit [2]. Linear gyrokinetic simulations indicate that the higher values of R/LTi with NBI heating do not have a strong effect on the Trapped Electron Mode (TEM) growth rates [1]. Furthermore, it is known that Ion Temperature Gradient (ITG) modes are stabilized by nonlinear electromagnetic effects enhanced by the presence of fast ions [3-4]. Therefore the observations made on the electrons are not explained by effects due to higher R/LTi and ITG modes. A possible explanation is found in Electron Temperature Gradient (ETG) modes. Recent works indicate the possibility that ETG modes can be responsible for a significant amount of the electron heat transport in certain regimes [5-6]. Furthermore, ETGs are expected to be more unstable in discharges with lower values of Te/Ti [7]. Linear gyrokinetic simulations indicate that due to the differences in τ = Zeff*Te/Ti, the ETG threshold in the discharges with NBI heating is lowered, confirming the possibility of a greater influence of ETGs in plasmas with i ≥ e. In order to study the level of ETG electron heat flux, a series of nonlinear gyrokinetic simulations are carried out. Due to the high computational cost of multi-scale simulations, only the electron gyroradius scale and adiabatic ions are retained. The measured external flow shear is included in the simulations, and is the mechanism that leads to a saturation of the ETG streamers. This choice limits the direct comparison between these simulations and experiments but nevertheless gives a strong indication of the amount of electron heat flux predicted to be transported by ETG turbulence in the two experimental situations. --- [1] N. Bonanomi et al., accepted by Nuclear Fusion [2] F. Jenko et al., Phys. Plasmas 7, 1904 (2000) [3] P. Mantica et al., Phys. Rev.Lett. 107, 135004 (2011) [4] J. Citrin et al., Phys. Rev.Lett. 111, 155001 (2013) [5] N. T. Howard et al., Phys. Plasmas 21, 112510 (2000) [6] T. Gorler, F. Jenko, Phys. Rev.Lett. 100, 185002 (2008) [7] F. Jenko et al., Phys. Plasmas 8, 4096 (2001)