Current ramps in tokamaks: from present experiments to ITER scenarios

In order to prepare adequate current ramp-up and ramp-down scenarios for ITER, present experiments from various tokamaks have been analysed by means of integrated modelling in view of determining relevant heat transport models for these operation phases. A set of empirical heat transport models for L-mode (namely, the Bohm-gyroBohm model and scaling based models with a specific fixed radial shape and energy confinement time factors of H(96-L) = 0.6 or H(IPB98) = 0.4) has been validated on a multi-machine experimental dataset for predicting the l(i) dynamics within +/- 0.15 accuracy during current ramp-up and ramp-down phases. Simulations using the Coppi-Tang or GLF23 models (applied up to the LCFS) overestimate or underestimate the internal inductance beyond this accuracy (more than +/- 0.2 discrepancy in some cases). The most accurate heat transport models are then applied to projections to ITER current ramp-up, focusing on the baseline inductive scenario (main heating plateau current of I(p) = 15 MA). These projections include a sensitivity study to various assumptions of the simulation. While the heat transport model is at the heart of such simulations (because of the intrinsic dependence of the plasma resistivity on electron temperature, among other parameters), more comprehensive simulations are required to test all operational aspects of the current ramp-up and ramp-down phases of ITER scenarios. Recent examples of such simulations, involving coupled core transport codes, free-boundary equilibrium solvers and a poloidal field (PF) systems controller are also described, focusing on ITER current ramp-down.</p

Predictive analysis of q-profile influence on transport in JET and ASDEX Upgrade hybrid scenarios

Hybrid scenarios in present machines are often characterized by improved confinement compared with the IPB98(y, 2) empirical scaling law expectations. This work concentrates on isolating the impact of increased s/q at outer radii (where s is the magnetic shear) on core confinement in low-triangularity JET and ASDEX Upgrade (AUG) experiments. This is carried out by predictive heat and particle transport modelling using the integrated modelling code CRONOS coupled to the GLF23 turbulent transport model. For both machines, discharge pairs were analysed displaying similar pedestal confinement yet significant differences in core confinement. From these comparisons, it is found that s/q shaping at outer radii may be responsible for up to similar to 50% of the relative core confinement improvement observed in these specific discharges. This relative improvement is independent of the degree of rotational shear turbulence suppression assumed in the GLF23 model. However, employing the full GLF23 rotational shear model leads to an overprediction of the ion temperatures in all discharges analysed. Additional mechanisms for core confinement improvement are discussed and estimated. Further linear threshold analysis with QuaLiKiz is carried out on both pairs of discharges. This work aims to validate recent predictions of the ITER hybrid scenario also employing CRONOS/GLF23, where a high level of confinement and resultant fusion power sensitivity to the s/q profile was found

Predictive analysis of q-profile influence on transport in JET and ASDEX Upgrade hybrid scenarios

Hybrid scenarios in present machines are often characterized by improved confinement compared with the IPB98(y,2) empirical scaling law expectations. This work concentrates on isolating the impact of increased s/q at outer radii (where s is the magnetic shear) on core confinement in low-triangularity JET and ASDEX Upgrade (AUG) experiments. This is carried out by predictive heat and particle transport modelling using the integrated modelling code CRONOS coupled to the GLF23 turbulent transport model. For both machines, discharge pairs were analysed displaying similar pedestal confinement yet significant differences in core confinement. From these comparisons, it is found that s/q shaping at outer radii may be responsible for up to ∼50% of the relative core confinement improvement observed in these specific discharges. This relative improvement is independent of the degree of rotational shear turbulence suppression assumed in the GLF23 model. However, employing the full GLF23 rotational shear model leads to an overprediction of the ion temperatures in all discharges analysed. Additional mechanisms for core confinement improvement are discussed and estimated. Further linear threshold analysis with QuaLiKiz is carried out on both pairs of discharges. This work aims to validate recent predictions of the ITER hybrid scenario also employing CRONOS/GLF23, where a high level of confinement and resultant fusion power sensitivity to the s/q profile was found