Electron Heat Transport in JET from Ion to Electron scales: Experimental Investigation and Gyro-kinetic Simulations

Experimental studies and gyro-kinetic simulations of electron heat transport performed in JET C-wall L-mode plasmas with various combinations of NBI and ICRH heating have provided indications of a significant role of small-scale instabilities (ETGs, Electron Temperature Gradient). Comparison of the measured electron inverse critical gradient length with linear gyro-kinetic simulations using the GENE code is generally consistent with both TEM and ETG thresholds, but the rather high experimental electron stiffness level is not reproduced by non-linear gyro-kinetic simulations including only large-scale ITG/TEM instabilities. The fact that Te peaking is very sensitive to the value of ?=Zeff Te/Ti, which is a key player for ETG instabilities, suggests that ETG turbulence could account for the missing electron heat flux. A first study of the ETG contribution to the heat flux, using linear and non-linear local GENE simulations, was based on separate simulations of ion and electron scales. For the ETG saturation, either an ad hoc external flow shear or electron scale zonal flows were used. In both ICRH and ICRH+NBI cases it was found that a non-negligible electron heat flux can be carried by the ETG modes, explaining the observations. However, a high sensitivity of the results on multiple parameters was found. Following recent work showing that multi-scale simulations with real electron to ion mass ratio are needed for a proper ETG study, computationally heavy multi-scale simulations have then been started using GENE for these JET shots. First results indeed indicate a substantial fraction of ETG flux and a high stiffness level, consistent with experiment. These results are important in view of extrapolations to ITER scenarios, where the electron channel will be key for fusion performance, due to the dominance of electron heating.