The confinement of alpha particles in a slightly asymmetric tokamak reactor

The effect of magnetic-field 'ripple' on alpha particles trapped in the major radial variation of the magnetic field in a tokamak reactor is to make their orbits stochastic so that they can escape from the plasma volume extremely rapidly. A guiding-centre orbit-following code is used to investigate the extent of the stochastic region in a reactor with INTOR parameters, and to compare the results so obtained with the predictions of an analytic map developed by Goldston et al. to model the banana orbits. Although the analytic map qualitatively reproduces the main trends observed in the guiding-centre orbits, differences in detail occur whose origins are discussed.