[en] Full text: The observed increase of energy confinement towards negative plasma triangularities, found in EC heated TCV L-mode plasmas and not explained by a change of the temperature gradient due solely to geometrical effects, has motivated a dedicated study of the impact of plasma shaping on local electron heat transport. In the present study, the plasma triangularity is varied over a wide range to investigate the effect of plasma shaping, and various plasmas conditions are explored by changing the total EC power and plasma density. When decreasing the plasma triangularity from +0.4 to -0.4, the mid-radius electron heat diffusivity is observed to decrease by 30%, and the central temperature to increase by the same amount, while the plasma density, electron heat flux, safety factor profiles and average effective charge are kept constant. Alternatively, only half of the EC power is required at this negative triangularity compared to the positive one to obtain the same temperature profile. In addition, the observed dependence of the electron heat diffusivity on the electron temperature, electron density and effective charge can be cast in a unique dependence on the plasma effective collisionality. In summary, the electron heat transport level exhibits a continuous decrease towards negative triangularities and high collisionalities, in EC heated plasmas of effective collisionality ranging from 0.2 to 1. Local gyro-fluid and global gyro-kinetic simulations predict the trapped electron modes to be the most unstable modes in these rather low collisionality plasmas. For higher effective collisionality, ranging from 1 to 2 and achieved in ohmic plasmas, the electron heat transport is no more observed to decrease towards negative triangularities. (author)

[en] Experimental results from the Tokamak a Configuration Variable (TCV) experiment have shown a heat transport coefficient χ_e two times greater with a triangularity δ= +0.4 than in a case with δ = -0.4 in L-mode plasma. These results were the motivation for a systematic study of shaping effects, and especially triangularity, on turbulent transport using the flux-tube gyrokinetic code GENE. In order to enable simulations of realistic tokamak plasma conditions and geometry, the code is extended from the s-alpha approximation to general axisymmetric geometry using an interface with an ideal MHD equilibrium code, CHEASE. In a second stage the code will be used to compare numerical results with experimental data from Electron Internal Transport Barriers (eITBs) studies conducted at TCV in a fully non-inductive discharge. The relative importance of Trapped Electron Modes and Electron Temperature Gradient modes will be investigated. The current status of this work will be presented. (Author)

[en] The role of the current profile and ion and electron temperature ratio in Tokamak a Configuration Variable (TCV) electron internal transport barrier (eITB) non-inductive discharges is inspected by means of the linear gyrokinetic global code LORB5 (Bottino et al 2004 Phys. Plasmas 11 198) and the Gyro-Landau fluid transport model GLF23 (Waltz et al 1997 Phys. Plasmas 4 2482). In this work we have compared two different MHD equilibria for a TCV discharge with an eITB, corresponding to a monotonic and a reversed safety factor profile. The latter has been obtained using the calculated electron-cyclotron driven density profile, the bootstrap current and the experimental pressure. Global simulations show that the most unstable electrostatic instabilities in the spectrum are trapped electron modes (TEM) for both equilibria. In the vicinity of the reverse shear region, where the magnetic shear s is positive and small (s << 1), modes tend to preserve the ballooning structure but their mixing length estimate for the thermal diffusion coefficient is significantly reduced as compared with the monotonic q equilibrium. This effect is further increased in the reverse shear region. Simulations performed with GLF23 show that the inclusion of the Shafranov shift in the model has an important stabilizing effect in all the plasma core and, in particular, in the barrier region. However, the Shafranov shift and reverse shear stabilization are not the only mechanisms through which the q profile affects the stability of the system. The local value of q in the barrier region also contributes to the stabilization. Global gyrokinetic and local Gyro-Landau fluid results suggest that the current profile modification alone, with the consequent negative shear and Shafranov shift stabilization on TEM modes, can explain the presence of the ITB in these TCV discharges

[en] A kinetic model addressing the destabilization of a current sheet by a microtearing mode is presented. For the first time, the magnetic drift, the electric potential fluctuation and collisions have been included together. As in reduced MHD, the evaluation of the current inside the resistive layer is obtained from a system of two equations linking the vector potential and the electric potential. When the electric field is neglected and using an effective magnetic drift, the magnetic drift is found to be destabilizing when combined with collisions. When both the electric potential and magnetic drift are kept, no analytical tractable solution has been found. (paper)

[en] The collisional stabilisation via energy scattering and pitch-angle scattering of micro-instabilities in tokamak plasmas is investigated by means of gyrokinetic simulations with a special emphasis on the often neglected energy scattering operator. It is shown that in the linear regime energy scattering has a negligible effect on Ion Temperature Gradient (ITG) modes but enhances the stabilisation of Trapped Electron Modes (TEM) in presence of nonzero ion temperature and density gradients. This stabilisation is sensitive to the model used for the energy restoring term in the collision operator. The contributions of parallel and drift motion to the total growth rate in velocity space are used to characterize the complex stabilisation mechanisms behind pitch-angle and energy scattering for a range of relevant parameters such as the magnetic shear, the collisionality, the logarithmic density gradient, and the logarithmic ion temperature gradient. It is shown that depending on these parameters, energy scattering stabilisation of TEM can be either due to a decrease of the contribution from drifting trapped electrons or to an increase of the contribution from the parallel motion of passing electrons. Finally, for a standard ITG/TEM case, the effect of energy scattering on the nonlinear heat and particle fluxes is investigated

[en] Full text: Transport codes provide a classical way to infer the profile of transport coefficients in fusion plasmas: assuming given functionals for the transport coefficient profiles, the free parameters are iteratively adjusted to best reproduce the measurements. This work introduces a new technique, the matrix approach (MA), which avoids any a priori constraint of the profiles, and computes them by simply inverting a 2D matrix, which also provides the uncertainty on the reconstruction for the case of modulation experiments. This is done by a controllable smoothing of the experimental data, instead of the ad hoc regularization of the profile of transport coefficients operated by transport codes. As a preliminary check, the MA was applied to already published JET data of momentum transport corresponding to three discharges that share the same initial equilibrium state. While an analysis of the data by a transport code suggests that all three cases share nearly the same transport coefficients, the MA rules this out, since the three uncertainty domains do not overlap at the various measurement positions. This suggested performing a similar analysis involving a residual stress on top of the advective and diffusive contributions to the flux. Then a single set of transport coefficients was found to be compatible with all three cases. (author)

[en] 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, transferring only a few kW of ohmic power compared to 1.4MW of ECCD. The ohmic current increases (decreases) the eITB performance demonstrating the clear dependence of the eITB on the current profile

[en] Clear evidence is reported for the first time of a rapid localized reduction of core electron energy diffusivity during the formation of an electron internal-transport barrier. The transition occurs rapidly (≅3 ms), during a slow (≅200 ms) self-inductive evolution of the magnetic shear. This crucial observation, and the correlation of the transition with the time and location of the magnetic shear reversal, lend support to models attributing the reduced transport to the local properties of a zero-shear region, in contrast to models predicting a gradual reduction due to a weak or negative shear

[en] On the Tokamak a Configuration Variable (TCV), electron internal transport barriers (eITBs) can be formed during a gradual evolution from a centrally peaked to a hollow current profile while all external actuators are held constant. The formation occurs rapidly (<τ_eE) and locally and, according to ASTRA modelling, is consistent with the appearance of a local minimum in the safety factor (q) profile. The eITB is sustained by non-inductively driven currents (including the off-axis bootstrap current) for many current redistribution times while the current in the tokamak transformer is held constant. The maximum duration is limited by the pulse length of the gyrotrons. The transformer coil can be used as a counter (or co-) current source with negligible accompanying input power. In established eITBs the performance can be enhanced (degraded) by altering solely the central current or q-profile. New experiments show that the same stationary eITB performance can be reached starting from discharges with centrally peaked current. A fine scan in surface voltage shows a smooth increase in performance and no sudden improvement with voltage despite the fact that q_min must pass through several low-order rational values. The appearance, in some cases, of magnetic islands simultaneously with that of the eITB provides a new constraint on the simulation of the q-profiles

[en] The analytical treatment of plasma kinetic linear instabilities in toroidal geometry is commonly tackled employing a power series expansion of the resonant part of the dispersion relation. This expansion is valid under the assumption that the modulus of the mode frequency is smaller than the magnitude of the frequencies characterising the system (the drift, bounce and transit frequencies for example). We will refer to this approximation as high frequency approximation (HFA). In this paper the linear plasma dispersion relation is derived in the framework of the gyro-kinetic model, for the electrostatic case, in the local limit, in the absence of collisions, for a non rotating plasma, considering adiabatic electrons, in toroidal circular geometry, neglecting the parallel dynamics effect. A systematic analysis of the meaning and limitations of the HFA is performed. As already known, the HFA is not valid for tokamak relevant parameters. A new way to approximate the resonant part of the dispersion relation, called here Improved high frequency approximation (IHFA), is therefore proposed. A quantitative analysis of the ion temperature gradient (ITG) instability is presented. The IHFA is shown to be applicable to the treatment of the ITG instability in tokamaks. (paper)