Y. Camenen

Electron heat transport studies

Electron transport in fusion plasmas is intensively studied in coordinated experiments and great progress in physics understanding has been achieved during the past years. A threshold in normalized gradient explains most of the observations, both in steady-state and transient conditions. The results convincingly suggest that trapped electron modes (TEM) dominate electron transport at low and moderate collisionality, with electron heating. The stabilization of these modes at high collisionality predicted by theory is found in the experiments. Electron transport is then driven by the ion temperature gradient modes. At low collisionality, if TEM are stabilized by negative shear and Shafranov shift effects, electron internal transport barriers may develop.

Toroidal momentum transport in a tokamak due to profile shearing

The effect of profile shearing on toroidal momentum transport is studied in linear and non-linear gyro-kinetic simulations. Retaining the radial dependence of both plasma and geometry parameters leads to a momentum flux that has contributions both linear in the logarithmic gradients of density and temperature, as well as contributions linear in the derivatives of the logarithmic gradients. The effect of the turbulence intensity gradient on momentum transport is found to be small for the studied parameters. Linear simulations at fixed normalized toroidal wave number predict a weak dependence of the momentum flux on the normalized Larmor radius ρ*=ρ/R. Non-linear simulations, however, at sufficiently small ρ* show a linear scaling of the momentum flux with ρ*. The obtained stationary rotation gradients are in the range of, although perhaps smaller than, current experiments. For a reactor plasma, however, a rather small rotation gradient should result from profile shearing.

Gradient-driven flux-tube simulations of ion temperature gradient turbulence close to the non-linear threshold

It is shown that Ion Temperature Gradient turbulence close to the threshold exhibits a long time behaviour, with smaller heat fluxes at later times. This reduction is connected with the slow growth of long wave length zonal flows, and consequently, the numerical dissipation on these flows must be sufficiently small. Close to the nonlinear threshold for turbulence generation, a relatively small dissipation can maintain a turbulent state with a sizeable heat flux, through the damping of the zonal flow. Lowering the dissipation causes the turbulence, for temperature gradients close to the threshold, to be subdued. The heat flux then does not go smoothly to zero when the threshold is approached from above. Rather, a finite minimum heat flux is obtained below which no fully developed turbulent state exists. The threshold value of the temperature gradient length at which this finite heat flux is obtained is up to 30% larger compared with the threshold value obtained by extrapolating the heat flux to zero, and t...

Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

The experimental observation made on the TCV tokamak of a significant ă confinement improvement in plasmas with negative triangularity (delta < ă 0) compared to those with standard positive triangularity has been ă interpreted in terms of different degrees of profile stiffness (Sauter ă et al 2014 Phys. Plasmas 21 055906) and/or different critical gradients. ă Employing the Eulerian gyrokinetic code GENE (Jenko et al 2000 Phys. ă Plasmas 7 1904), profile stiffness and critical gradients are studied ă under TCV relevant conditions. For the considered experimental ă discharges, trapped electron modes (TEMs) and electron temperature ă gradient (ETG) modes are the dominant microinstabilities, with the ă latter providing a significant contribution to the non-linear electron ă heat fluxes near the plasma edge. Two series of simulations with ă different levels of realism are performed, addressing the question of ă profile stiffness at various radial locations. Retaining finite ă collisionality, impurities and electromagnetic effects, as well as the ă physical electron-to-ion mass ratio are all necessary in order to ă approach the experimental flux measurements. However, flux-tube ă simulations are unable to fully reproduce the TCV results, pointing ă towards the need to carry out radially nonlocal (global) simulations, ă i.e. retaining finite machine size effects, in a future study. Some ă conclusions about the effect of triangularity can nevertheless be drawn ă based on the flux-tube results. In particular, the importance of ă considering the sensitivity to both temperature and density gradient is ă shown. The flux tube results show an increase of the critical gradients ă towards the edge, further enhanced when d < 0, and they also appear to ă indicate a reduction of profile stiffness towards plasma edge.

X-point position dependence of edge intrinsic toroidal rotation on the Tokamak à Configuration Variablea)

Recent theoretical work predicts intrinsic toroidal rotation in the tokamak edge to depend strongly on the normalized major radial position of the X-point. With this motivation, we conducted a series of Ohmic L-mode shots on the Tokamak a Configuration Variable, moving the X-point from the inboard to the outboard edge of the last closed flux surface in both lower and upper single null configurations. The edge toroidal rotation evolved from strongly co-current for an inboard X-point to either vanishing or counter-current for an outboard X-point, in agreement with the theoretical expectations. The whole rotation profile shifted roughly rigidly with the edge rotation, resulting in variation of the peak core rotation by more than a factor of two. Core rotation reversals had little effect on the edge rotation. Edge rotation was slightly more counter-current for unfavorable than favorable ∇B drift discharges.

Linear multispecies gyrokinetic flux tube benchmarks in shaped tokamak plasmas

Verification is the fundamental step that any turbulence simulation code has to be submitted in order to assess the proper implementation of the underlying equations. We have carried out a cross comparison of three flux tube gyrokinetic codes, GENE [F. Jenko et al., Phys. Plasmas 7, 1904 (2000)], GKW [A. G. Peeters et al., Comput. Phys. Commun. 180, 2650 (2009)], and GS2 [W. Dorland et al., Phys. Rev. Lett. 85, 5579 (2000)], focusing our attention on the effect of realistic geometries described by a series of MHD equilibria with increasing shaping complexity. To simplify the effort, the benchmark has been limited to the electrostatic collisionless linear behaviour of the system. A fully gyrokinetic model has been used to describe the dynamics of both ions and electrons. Several tests have been carried out looking at linear stability at ion and electron scales, where for the assumed profiles Ion Temperature Gradient (ITG)/Trapped Electron Modes and Electron Temperature Gradient modes are unstable. The capa...

Experimental observations and modelling of intrinsic rotation reversals in tokamaks

The progress made in understanding spontaneous toroidal rotation reversals in tokamaks is reviewed and current ideas to solve this ten-year-old puzzle are explored. The paper includes a summarial synthesis of the experimental observations in AUG, C-Mod, KSTAR, MAST and TCV tokamaks, reasons why turbulent momentum transport is thought to be responsible for the reversals, a review of the theory of turbulent momentum transport and suggestions for future investigations.

Current profile tailoring with far off-axis ECH power deposition in TCV elongation experiments

Note: Proc. 12th Joint Workshop in Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC-12), Aix-en-Provence, May 2002, 407 - 412 (2003) ISBN 981-238-189-9 Reference CRPP-CONF-2003-011 Record created on 2008-05-13, modified on 2017-05-12

Summary of 21st joint EU-US transport task force workshop (Leysin, September 5–8, 2016)

This conference report summarizes the contributions to, and discussions at, the 21st Joint EU-US Transport Task Force workshop, held in Leysin, Switzerland, during 5-8 September 2016. The workshop was organized under 8 topics: progress towards full-F kinetic turbulence simulation; high and low Z impurity transport, control and effects on plasma confinement; 3D effects on core and edge transport (including MHD, external fields and stellarators); predictive experimental design; electron heat transport and multi-scale integration; understanding power decay length in the SOL; role of the SOL in the L-H transition; validation of fundamental turbulence properties against turbulence measurements. This report follows the same structure.

Control of the eITB formation and performance in fully non-inductively sustained ECCD discharges in TCV

The X2 ECH antennas on TCV are used to sustain and tailor the plasma current profile, forming either a centrally peaked or hollow profile. During the transition from peaked to hollow profile, an eITB is observed to form rapidly and in a localized region, which correlates with the appearance of a zero shear flux surface off‐axis according to the ASTRA transport code. The barrier position can be controlled via the co‐ECCD off‐axis deposition location, and the barrier strength with central heating or counter‐ECCD (increasing the depth of the hollow current profile), achieving H‐factors of up to ⩽6. In these discharges, the current in the ohmic transformer coil is held constant to avoid an inductively driven current contribution. After the eITB is created, a small amount of ohmically driven counter (co‐) current has been added as a perturbative current source, transfering only a few kW of ohmic power compared to 1.4MW of ECCD. The ohmic current increases (decreases) the eITB performance demonstrating the clea...