New assessment of the fast ion energy in ASDEX upgrade H-mode discharges

Abstract

Confinement scaling laws such as IPB98(y, 2) are widely used to extrapolate the performance of present tokamaks to next-step devices such as ITER or DEMO. The thermal energy of the plasma (W th), which is used to determine the energy confinement time for most scaling laws, is difficult to measure, due to the sizeable uncertainties in the experimental kinetic profiles. The common approach in the tokamak community is to derive W th as the difference between the measured magnetohydrodynamic (MHD) energy and some simulation-based estimate of the fast ion energy W fi. In H-mode plasmas W fi can be as high as W th, in presence of neutral beam injection (NBI) or ion cyclotron radio frequency heating (ICRF), therefore an accurate assessment of W fi is crucial to have a somewhat reliable H-factor, regardless of the power-scaling of a given scaling law. In this paper we aim at evaluating the current approach to estimate W fi, by comparing its predictions with a wide database of calculations using validated NBI codes. Systematic deviations and trends, as well as statistical scatter are discussed. We use a comprehensive database of AUG H-mode deuterium plasmas, with significant variations of plasma current, NBI power and plasma density. We neglect thereby the fast-ion losses caused by MHD modes and the synergy effect between NBI and ICRF. A new approach is proposed based on the newly developed fast NBI code RABBIT.