Abstract. This paper reviews transport and confinement in spherical tokamaks (STs) and our current physics understanding that is partly based on gyrokinetic simulations. We show that equilibrium flow shear can sometimes entirely suppress ion scale turbulence in today's STs. Advanced nonlinear simulations of electron temperature gradient (ETG) driven turbulence, including kinetic ion physics, collisions and equilibrium flow shear, support the model that ETG turbulence can explain electron heat transport in many ST discharges.

New transport experiments on JET indicate that ion stiffness mitigation in the core of a rotating plasma, as described by Mantica et al. [Phys. Rev. Lett. 102, 175002 (2009)] results from the combined effect of high rotational shear and low magnetic shear. The observations have important implications for the understanding of improved ion core confinement in advanced tokamak scenarios. Simulations using quasilinear fluid and gyrofluid models show features of stiffness mitigation, while nonlinear gyrokinetic simulations do not. The JET experiments indicate that advanced tokamak scenarios in future devices will require sufficient rotational shear and the capability of q profile manipulation.

Abstract. Isca is a framework for the idealized modelling of the global circulation of planetary atmospheres at varying levels of complexity and realism.

Beam Emission Spectroscopy (BES) measurements of ion-scale density fluctuations in the MAST tokamak are used to show that the turbulence correlation time, the drift time associated with ion temperature or density gradients, the particle (ion) streaming time along the magnetic field and the magnetic drift time are consistently comparable, suggesting a "critically balanced" turbulence determined by the local equilibrium. The resulting scalings of the poloidal and radial correlation lengths are derived and tested. The nonlinear time inferred from the density fluctuations is longer than the other times; its ratio to the correlation time scales as ν −0.8±0.1 * i , where ν * i = ion collision rate/streaming rate. This is consistent with turbulent decorrelation being controlled by a zonal component, invisible to the BES, with an amplitude exceeding the drift waves' by ∼ ν −0.8 * i .Introduction. Microscale turbulence hindering energy confinement in magnetically confined hot plasmas is driven by gradients of equilibrium quantities such as temperature and density. These gradients give rise to instabilities that inject energy into plasma fluctuations ("drift waves") at scales just above the ion Larmor scale. The most effective of these is believed to be the iontemperature-gradient (ITG) instability [1][2][3]. A turbulent state ensues, giving rise to "anomalous transport" of energy [4]. It is of interest, both for practical considerations of improving confinement and for the fundamental understanding of multiscale plasma dynamics, what the structure of this turbulence is and how its amplitude, scale(s) and resulting transport depend on the equilibrium parameters: ion and electron temperatures, density, angular velocity, magnetic geometry, etc.Fluctuations in a magnetized toroidal plasma are subject to a number of distinct physical effects, which can be thought about in terms of various time scales such as the drift times associated with the temperature and density gradients, the particle streaming time along the magnetic field as it takes them around the torus toroidally and poloidally, the magnetic (∇B and curvature) drift times of particles moving across the field, the nonlinear time of the fluctuations being advected across the field by the fluctuating E × B velocity, the time between collisions, the shear time associated with plasma rotation. Some of these time scales and, consequently, the corresponding physics may be irrelevant, while others play a crucial role for the saturation of the linearly unstable fluctuations. There has been a growing understanding [5], driven largely by theory [6][7][8][9], observations [10][11][12] and simulations of magnetohydrodynamic [13][14][15] and kinetic [7,16] plasma turbulence in space, that if a medium can

Detailed experimental studies of ion heat transport have been carried out in JET exploiting the upgrade of Active Charge Exchange Spectroscopy and the availability of multi-frequency ICRH with 3 He minority. The determination of ion threshold and stiffness offers unique opportunities for validation of the well-established theory of Ion Temperature Gradient driven modes. Ion stiffness is observed to decrease strongly in presence of toroidal rotation when the magnetic shear is sufficiently low. This effect is dominant with respect to the well-known w ExB threshold up-shift and plays a major role in enhancing core confinement in Hybrid regimes and Ion Internal Transport Barriers. The effects of T e /T i and s/q on ion threshold are found rather weak in the domain explored. Quasi-linear fluid/gyro-fluid and linear/non-linear gyro-kinetic simu lations have been carried out. Whilst threshold predictions show good match with experimental observations, some significant discrepancies are found on the stiffness behaviour.

The formation of internal transport barriers (ITBs) is investigated in MAST spherical tokamak (ST) plasmas. The roles of E×B flow shear, q-profile (magnetic shear) and MHD activity in their formation and evolution are studied using data from high-resolution kinetic-and q-profile diagnostics. In L-mode plasmas, with co-current directed NBI heating, ITBs in the momentum and ion thermal channels form in the negative shear region just inside q min. In the ITB region the anomalous ion thermal transport is suppressed, with ion thermal transport close to the neo-classical level, although the electron transport remains anomalous. Linear stability analysis with the gyro-kinetic code GS2 shows that all electrostatic micro-instabilities are stable in the negative magnetic shear region in the core, both with and without flow shear. Outside the ITB, in the region of positive magnetic shear and relatively weak flow shear, electrostatic micro-instabilities become unstable over a wide range of wave-numbers. At ITG length scales, flow shear reduces linear growth rates and narrows the spectrum of unstable modes, but flow shear suppression of ITG modes is incomplete. Flow shear has little impact on growth rates at ETG scales. This is consistent with the observed anomalous electron and ion transport in this region. With counter-NBI ITBs of greater radial extent form outside q min due to the broader profile of E×B flow shear produced by the greater prompt fast-ion loss torque.

In electrostatic simulations of MAST plasma at electron-gyroradius scales, using the local flux-tube gyrokinetic code GS2 with adiabatic ions, we find that the longtime saturated electron heat flux (the level most relevant to energy transport) decreases as the electron collisionality decreases. At early simulation times, the heat flux "quasisaturates" without any strong dependence on collisionality, and with the turbulence dominated by streamer-like radially elongated structures. However, the zonal fluctuation component continues to grow slowly until much later times, eventually leading to a new saturated state dominated by zonal modes and with the heat flux proportional to the collision rate, in approximate agreement with the experimentally observed collisionality scaling of the energy confinement in MAST. We outline an explanation of this effect based on a model of ETG turbulence dominated by zonal-nonzonal interactions and on an analytically derived scaling of the zonal-mode damping rate with the electron-ion collisionality. Improved energy confinement with decreasing collisionality is favourable towards the performance of future, hotter devices.

Abstract. Local and global electrostatic gyrokinetic simulations of MAST spherical tokamak for ITG modes with adiabatic electron response find good agreement in linear stability with growth rates below the experimental E × B shearing rate. The kinetic treatment of electrons is shown to increase the growth rates above the shearing rate. The collisionless simulations with kinetic electrons and experimental flow shear find unstable modes in the outer part of the plasma. In global simulations the flow shear stabilisation is found to be asymmetric with respect the direction of flow with a small destabilising effect with a modest flow in the co-direction.The global non-linear simulations show turbulence spreading from the outer part of the plasma into the linearly stable core region. With adiabatic electrons and experimental profiles the turbulent heat transport is at or below the neoclassical level even without the flow shear. Treating electrons kinetically and including the flow shear effects increases the turbulent transport well above the neoclassical level.