Charge-Exchange Spectra Near the Injection Energy in Tokamaks Equipped With Tangential Neutral Beams Experiment and Theory

A relatively simple theory is developed to predict the energy spectrum of charge-exchange neutrals emitted from a tokamak at energies near that of a tangentially injecting neutral beam. The theory is based on a solution to the Fokker-Planck equation which is valid near the injection energy and angle. It is compared with detailed charge-exchange measurements of ion bunches slowing down in the Atc plasma, with steady-state spectrum measurements, and with rise-time measurements of the spectrum near the injection energy after the beam is turned on. Each of these is a test of the classical nature of the slowing-down process, and each gives excellent agreement with theory. On the basis of these results, the theory is used to show how a collimateci neutral-beam probe combined with a multi-sight-line detector can be used to measure radial profiles of background neutral density, and of Zeff.