Simulation of transport in ASDEX divertor discharges with neutral-injection heating

Abstract

Cross-field particle and energy transport in diverted ASDEX discharges with up to 2.2 MW neutral-beam heating is studied by testing transport code predictions against experimental data. With neutral injection, enhanced coefficients of electron heat diffusivity and diffusion are found which are independent of density, temperatures and poloidal beta over a large parameter range (χe = 2.9 × 104 cm2·s−1 and D = 0.2 χe). The ion heat diffusivity continues to be one to three times the neoclassical value. The period with degraded confinement can be considerably delayed in relation to the beams and is found to be correlated with a characteristic change in the shape of the electron temperature profile due to neutral injection and with the time behaviour of the sawtooth period. Energy and particle confinement do not deteriorate with increasing beam power. It is found that the sawtooth activity does not significantly impair the confinement outside the q = 1 surface. A possible connection between degradation of confinement and non-thermal electron or ion velocity distributions, plasma rotation and drift-wave turbulence is discussed.