Transfer rates of toroidal angular momentum during neutral beam injection

Abstract

At the onset of neutral beam injection (NBI) in JET, the toroidal angular momentum is observed to rise rapidly in the outer regions of the plasma. The toroidal angular momentum in the plasma centre, where the fast ions are injected into passing orbits, and the thermal energy are found to rise on the slowing down time-scale of the fast ions. This behaviour can be explained by a model that incorporates three mechanisms for momentum transfer of fast ions to the bulk: (a) quasi-instantaneous, or first orbit, transfer which results mainly from particles that are injected into trapped orbits, (b) collisional transfer of momentum from passing ions during their slowing down process, (c) enhancement of the total angular momentum of the rotating plasma once the particles have thermalized. The model for the torque is applied to the study of toroidal angular momentum confinement in transient hot ion H mode plasmas in JET. In contrast to steady state conditions, such as L mode and ELMy H mode, where the toroidal angular momentum confinement time τL is approximately equal to the thermal energy confinement time τE, τL is found to be about a factor of 2 smaller than τE in the transient part of the ELM-free phase of the discharge.