Impact of Plasma Shaping on Electron Heat Transport in TCV L-mode Plasmas at Various Collisionalities

The impact of plasma shaping on electron heat transport is investigated in TCV L-mode plasmas. The study is motivated by the observation of an increase of the energy confinement time with decreasing plasma triangularity that may not be explained by a change in the temperature gradient induced by changes in the geometry of the flux surfaces. The plasma triangularity is varied over a wide range, from positive to negative values, and various plasmas conditions are explored by changing the total electron cyclotron (EC) heating power and the plasma density. The mid-radius electron heat diffusivity is shown to significantly decrease with decreasing triangularities and, for similar plasma conditions, only half of the EC power is required at a triangularity of −0.4 compared to +0.4 to obtain the same temperature profile. Besides, the observed dependence of the electron heat diffusivity on the electron temperature, electron density and effective charge can be grouped in a unique dependence on the plasma effective collisionality. In summary, the electron heat transport level exhibits a continuous decrease with decreasing triangularity and increasing collisionality. Local gyro-fluid and global gyro-kinetic simulations predict that trapped electron modes are the most unstable modes in these EC heated plasmas with an effective collisionality ranging from 0.2 to 1. The modes stability dependence on the plasma triangularity is investigated. For higher effective collisionality, ranging from 1 to 2 and achieved in ohmic plasmas, the experimental electron heat transport is no longer observed to decrease with decreasing triangularity.