Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al. [Phys. Rev. Lett. 107, 135004 (2011)], this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.

The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-β JET L-mode discharge illustrates the relation between ITG stabilization, and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-β JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant, and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-β generalization of comparable observations found at low-β at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

Electron transport in tokamaks has many different features which are briefly reviewed. The paper is focused on electron heat transport in conventional tokamak plasmas. An inter-machine comparison indicates that the nondimensional gradient length of the electron temperature profiles R/L T e is almost independent of the devices and varies little with plasma parameters. This strongly suggests that electron heat transport is governed by turbulence with a threshold in R/L T e . This is confirmed by modulation experiments using electron cyclotron heating. Simulations with empirical and physics-based transport models confirm this assumption.

In the Rijnhuizen Tokamak Project plasmas, a transient rise of the core electron temperature is observed when hydrogen pellets are injected tangentially to induce fast cooling of the peripheral region. High-resolution Thomson scattering measurements show that the T e rise is associated with large temperature gradients in the region 1 , q , 2. This region acts as a layer of transiently increased thermal resistivity (transport barrier) when probed by fast heat pulses from modulated electron cyclotron heating.[S0031-9007(99)09366-7] PACS numbers: 52.55. Fa, 52.25.Fi, 52.50.Gj A widespread observation in transient transport experiments in tokamak plasmas is the so-called "nonlocal" response of the plasma core to a fast perturbation of the electron temperature ͑T e ͒ in the outer layers of tokamak plasmas [1]. Most commonly, a core T e rise is observed in response to fast edge cooling (cold pulse) by laser ablation or oblique pellet injection [1]. As reported in the literature, the central T e response contradicts local transport predictions in that (a) the core response is of opposite polarity and of larger magnitude than the edge perturbation; (b) the response takes place before the edge perturbation diffuses in. The phenomenon cannot be ascribed to Ohmic power redistribution according to [1][2][3]. Therefore it is interpreted in terms of a fast change of the core electron heat diffusivity ͑x e ͒ in response to a change in some plasma parameter elsewhere [1].A common feature is a density ͑n e ͒ dependence of the nonlocal response. At low n e , the nonlocality is strongest as the T e perturbation reverses its polarity in the plasma core. As n e is increased, the core T e variation is reduced and can be replaced by a fast response of the same polarity [1].Another common feature is a change, on the same time scale as the central T e variation, of the sawtooth period and amplitude, which has suggested a possible link between nonlocal transport and MHD behavior of the plasma [3].In this Letter we report new experimental results on the issue of nonlocal transport, obtained in the Rijnhuizen Tokamak Project (RTP). They are based on highresolution measurements of T e with Thomson scattering and on the use of modulated electron cyclotron heating (MECH) on top of cold pulses. These experiments show that the central T e rise following edge cooling takes place through the formation of a large temperature gradient (thermal barrier) in a radially localized region of the plasma. Such a barrier acts as a layer of transiently increased thermal resistivity when probed by MECH heat pulses.RTP (major radius R 0.72 m, minor radius a 0.16 m) is dedicated to transport studies and equipped with advanced diagnostics. Time resolved measurements of T e and n e are taken, respectively, with an electron cyclotron emission (ECE) radiometer covering the entire profile with 15 channels and with a 16 chord microwave interferometer. High spatial resolution is obtained with a double pulse Thomson scattering system measuring T e and n e on a vertic...