F. Nabais

Modelling of JET hybrid scenarios with GLF23 transport model: E???B shear stabilization of anomalous transport

The E???B shear stabilization of anomalous transport in JET hybrid discharges is studied via self-consistent predictive modelling of electron and ion temperature, ion density and toroidal rotation velocity performed with the GLF23 model. The E???B shear stabilization factor (parameter ?E in the GLF23 model) is adjusted to predict accurately the four simulated quantities under different experimental conditions, and the uncertainty in ?E determined by 15% deviation between simulated and measured quantities is estimated. A correlation of ?E with toroidal rotation velocity and E???B shearing rate is found in the low density plasmas, suggesting that the turbulence quench rule may be more complicated than assumed in the GLF23 model with constant ?E. For the selected discharges the best predictive accuracy is obtained by using weak/no E???B shear stabilization (i.e. ?E???0) at low toroidal angular frequency (?? ?100?krad?s?1). Interestingly, a weak E???B shear stabilization of anomalous transport is found in the medium density strongly rotating discharge. An importance of linear ?e stabilization in this discharge is estimated and compared to the low density discharge with equally high ?e. The toroidal rotation velocity is well predicted here by assuming that the momentum diffusion coefficient is a fraction of thermal ion diffusivity. Taking into account the ?E and Prandtl number with their uncertainties determined in the modelling of JET hybrid discharges, the performance of ITER hybrid scenario with optimized heat mix (33?MW of NBI and 20?MW of ECCD) is estimated showing the importance of toroidal rotation for achieving Q?>?5.

Systematic linear-stability assessment of Alfvén eigenmodes in the presence of fusion α-particles for ITER-like equilibria

A systematic approach to evaluate the linear stability of Alfven eigenmodes in the presence of fusion-born -particles is reported. The techniques developed are particularly useful when dealing with scenarios where no experimental guidance is available about which eigenmodes are more easily destabilized by the supra-thermal population. The advantages and limitations of the underlying model chosen to describe the wave-particle interaction are discussed, along with the parallelization of the designed workflow in order to take advantage of massively parallel computer systems. This workflow is tested using a ITER baseline scenario, showing that it is able to routinely single out the most linearly unstable eigenmodes for a given equilibrium configuration. The eigenmodes with highest growth rate were found to be core-localised Toroidal Alfven Eigenmodes with toroidal mode number , placed near the maximum of the -particle density gradient and within a low magnetic-shear region.

Fast ion redistribution and losses in JET advanced tokamak scenario

The influence of Alfven eigenmodes (AEs) (core-localized toroidal Alfven eigenmode (TAE) and high-m global TAE) on redistribution and loss of fast ions in the advanced tokamak scenario has been investigated in JET dedicated experiments. Temporal evolution of the spatial structure of AEs present in the plasma during q-profile evolution has been calculated with the MISHKA code. On the other hand, fast ion losses with resolution in energy and pitch angle were measured with a scintillator probe, while the internal redistribution of the fast ions was analysed by measuring the temporal evolution of the radial profiles of γ-ray emission coming from the fast ions colliding with the main impurities, Be and C. Correlating the measurements above, it is possible to assess the effect of AEs on redistribution and loss of fast ions. It is found that the TAE localized in the core of the plasma causes a severe redistribution of the fast ions in the plasma centre, leading to a flattening of the vertical profile of γ-ray emission. In addition, the core-localized TAE also causes enhanced fast ion losses. As the core-localized TAE moves radially outwards, a transition to high-m global TAE occurs. In the transformation phase, a coupled mode with components both in the core and in the edge of the plasma exists. The maximum number of losses is normally measured when this coupled mode with a large radial extent is present in the plasma.

Comprehensive evaluation of the linear stability of Alfvén eigenmodes driven by alpha particles in an ITER baseline scenario

The linear stability of Alfven eigenmodes in the presence of fusion-born alpha particles is thoroughly assessed for two variants of an ITER baseline scenario, which differ significantly in their core and pedestal temperatures. A systematic approach based on CASTOR-K (Borba and Kerner 1999 J. Comput. Phys. 153 101; Nabais et al 2015 Plasma Sci. Technol. 17 89) is used that considers all possible eigenmodes for a given magnetic equilibrium and determines their growth rates due to alpha-particle drive and Landau damping on fuel ions, helium ashes and electrons. It is found that the fastest growing instabilities in the aforementioned ITER scenario are core-localized, low-shear toroidal Alfven eigenmodes. The largest growth-rates occur in the scenario variant with higher core temperatures, which has the highest alpha-particle density and density gradient, for eigenmodes with toroidal mode numbers . Although these eigenmodes suffer significant radiative damping, which is also evaluated, their growth rates remain larger than those of the most unstable eigenmodes found in the variant of the ITER baseline scenario with lower core temperatures, which have and are not affected by radiative damping.

Sensitivity of alpha-particle-driven Alfven eigenmodes to q-profile variation in ITER scenarios

A perturbative hybrid ideal-MHD/drift-kinetic approach to assess the stability of alpha-particle-driven Alfven eigenmodes in burning plasmas is used to show that certain foreseen ITER scenarios, namely the Ip = 15 MA baseline scenario with very low and broad core magnetic shear, are sensitive to small changes in the background magnetic equilibrium. Slight variations (of the order of 1%) of the safety-factor value on axis are seen to cause large changes in the growth rate, toroidal mode number, and radial location of the most unstable eigenmodes found. The observed sensitivity is shown to proceed from the very low magnetic shear values attained throughout the plasma core, raising issues about reliable predictions of alpha-particle transport in burning plasmas.

Pitch-angle distribution of TAE-induced losses of ICRH accelerated ions on JET

Experiments aiming to study the loss of energetic ions carried out in JET using low density plasmas and on-axis minority ion cyclotron resonance heating (ICRH) have been reported in Nabais et al (2010 Nucl. Fusion 50 115006). Though the fast ions accelerated by on-axis ICRH have a distribution in the normalized magnetic moment centred at ??=?1, experimental measurements have shown that the fast ions whose loss is triggered by tornado modes (toroidal Alfv?n eigenmodes (TAE) localized inside the q?=?1 surface) reach the scintillator cup with an average ? significantly higher than one (??=?1.1). On the other hand, the losses associated with the presence of TAE localized on the plasma boundary reach the scintillator cup with an average ? slightly below unity. Since ? increases while the fast ions drift radially outward due to resonant interaction with the Alfv?nic modes, the high values of ? characterizing the losses triggered by tornado modes indicate these ions have experienced a large radial displacement before being lost, i.e. they were originally localized in the plasma core. These experimental results corroborate the conclusion from Nabais et al (2012 Nucl. Fusion 52 083021) that the combined action of modes localized at different radial locations can lead to large radial transport and loss of ions originally localized in the plasma core. The results presented here also show that measurements of the pitch angle may be used to extract information about the original location of the ions reaching the scintillator cup.