The energetic ion distribution resulting from the injection of high-energy neutrals into a toroidal plasma has been derived. An appropriate kinetic equation which contains the angular scattering, friction, and diffusion of the energetic ions by the background particles, charge exchange on the background neutrals, and acceleration of the ions by the electric field has been solved analytically by use of the WKBJ method. Collisions of the energetic ions with the faster moving electrons results in some of the ions increasing their energy above the injection energy. The width of this ``high-energy tail'' is shown to depend upon the electron temperature and the electric field. An estimate of the effect upon the width of this tail of collisions between the energetic ions themselves is also given. Finally, illustrated examples of the various significant physical processes are presented.

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.

In the JET tokamak, ICRF driven fusion reactivity has been determined using measurements of 16.6 MeV γ-ray emission from d[3He, γ]5Li reactions during central RF heating in the (3He)d minority regime. Up to 1 MJ of fast minority ions in the plasma has been produced with the application of up to 15 MW of RF power. The maximum rate produced by d[3He, p]4He fusion reactions has been estimated as 2 × 10l6 s−1 (equivalent to 60 kW of fusion power in charged particle products). The reactivity increased strongly with coupled RF heating power (proportional to (PRF)5/3), with some evidence of a weakening of the dependence leading to a saturation in the energy gain Q at the highest coupled RF power levels (PRF > 8−12 MW). The experimentally measured anisotropic fast ion energies and fusion reaction rates have been simulated using a radially dependent Stix model for a wide variety of discharges. Analysis of the radial profile of fusion reactivity shows that when the RF power density is maximized on the magnetic axis of the discharge, the fusion reactivity is peaked away from the axis. This effect is caused by the minority ions near the centre of the discharge being driven to energies beyond the maximum in the fusion cross-section.

The measured D-D neutron rate of neutral beam heated JET baseline and hybrid H-modes in deuterium is found to be between approximately 50% and 100% of the neutron rate expected from the TRANSP code, depending on the plasma parameters. A number of candidate explanations for the shortfall, such as fuel dilution, errors in beam penetration and effectively available beam power have been excluded. As the neutron rate in JET is dominated by beamplasma interactions, the 'neutron deficit' may be caused by a yet unidentified form of fast particle redistribution. Modelling, which assumes fast particle transport to be responsible for the deficit, indicates that such redistribution would have to happen at time scales faster than both the slowing down time and the energy confinement time. Sawteeth and edge localised modes are found to make no significant contribution to the deficit. There is also no obvious correlation with magnetohydrodynamic activity measured using magnetic probes at the tokamak vessel walls. Modelling of fast particle orbits in the 3D fields of neoclassical tearing modes shows that realistically sized islands can only contribute a few percent to the deficit. In view of these results it appears unlikely that the neutron deficit results from a single physical process in the plasma.

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