Impact of plasma triangularity and collisionality on electron heat transport in TCV L-mode plasmas

TLDR

Energy confinement time rises as plasma triangularity decreases, a trend not fully explained by temperature‑gradient changes. The study investigates how plasma shaping affects electron heat transport in TCV L‑mode plasmas. The authors varied triangularity from positive to negative while altering EC heating power and density, and used local gyro‑fluid and global gyro‑kinetic simulations to show that trapped electron modes dominate and their stability depends on triangularity. Electron heat diffusivity at mid‑radius drops as triangularity decreases, requiring only half the EC power at –0.4 versus +0.4, and the diffusivity’s dependence on temperature, density, and Z_eff collapses onto a single dependence on effective collisionality, leading to a continuous reduction in transport with lower triangularity and higher collisionality.

Abstract

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 in the energy confinement time with decreasing plasma triangularity which 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 triangularity and, for similar plasma conditions, only half of the EC power is required at a triangularity of −0.4 compared with +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.