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

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.

Plasma shaping effects on tokamak scrape-off layer turbulence

“…In fact, Refs. [22][23][24][25] pointed out that core turbulence is (i) usually stabilised by elongation, except for large positive triangularities, for which it is enhanced, (ii) stabilised by negative triangularity, and (iii) destabilised by large positive triangularity.…”

“…Despite the fact that the effects of plasma shaping on core turbulence have been extensively studied, both experimentally and numerically (see, e.g., Refs. [22][23][24][25][26]), the attemps to study the effects of plasma shaping on SOL turbulence in limited geometry are only experimental (see, e.g., Ref. [27]).…”

Overview of recent and current research on the TCV tokamak

“…This, too, has been investigated on TCV by analysis of integrated magnetic data. For both (3,2) and (2,1) mode excitation, it is found that a seed island forms within tens of μs of the sawtooth crash. The subsequent island development is consistent with the modified Rutherford equation.…”

“…4b) [16]. This is a form of ELM mitigation in an alternative configuration not available to ITER, although it is accompanied by a worsening of confinement -in marked contrast with the L-mode regime, where confinement is better for δ<0 than for δ>0 [2].…”

“…The strong observed dependence of confinement on triangularity and collisionality in TCV [2] has motivated inquiries into the behavior of the plasma turbulence that is thought to underlie anomalous transport. Preliminary investigations have been conducted using a 2-channel correlation ECE diagnostic, measuring electron temperature fluctuations.…”