Modelling Neutral Particle Analyzer Measurements of High Energy Fusion Alpha-Particle Distributions in JET

Measurements of high energy alpha-particle distribution functions using a neutral particle analyzer (NPA), in conjunction with measurements of the alpha-particle source profile, will be used to study the physics of fusion product generation and confinement in the forthcoming deuterium-tritium experiments in the Joint European Torus (JET). This paper addresses two key questions: the interpretation of the NPA measurements in terms of the time-evolving alpha-particle velocity distribution within the plasma, and the identification of the physical processes, additional to isotropic birth, confinement and classical slowing-down, which must be incorporated into theoretical models linking the measured alpha-particle source to the measured alpha-particle population. The NPA measurement can be identified with a vertical line integral through the plasma centre of the local alpha-particle velocity distribution. The latter quantity is computed rapidly by integrating along the characteristics of a simplified Fokker-Planck equation in which the alpha-particle source and plasma collisionality are empirical inputs. The source term is obtained from fusion reactivity profiles which are deduced in turn from tomographic measurements of 14 MeV neutron emission. Preliminary results, obtained using an idealized source term, indicate that the shape of the line-integrated alpha-particle energy spectrum can differ significantly from the local spectrum at the position of maximum fusion reactivity (the plasma centre), particularly when sufficient time has elapsed for a slowing-down distribution to become established. On the other hand, at energies close to the birth energy and at times (after the onset of alpha-particle production) shorter than the slowing-down time, the line-integrated and local energy spectra closely resemble each other. Application of the model is demonstrated for NPA measurements of fusion alpha-particle distribution functions in the Tokamak Fusion Test Reactor (TFTR).