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Confined alpha particles near the plasma edge in tokamaks

The properties of classically confined high-energy alpha particles near the outer midplane of a tokamak torus are considered in order to provide the basic information required for theoretical studies of the superthermal ion cyclotron emission which has been observed in tokamak experiments. An analytical expression for the velocity distribution function of fusion reaction products has been derived, which generalizes previous results. The alpha-particle distribution function for test case data corresponding to typical parameters of the JET preliminary tritium experiment has been calculated using a three-dimensional Fokker-Planck code and it has been shown that this function can be well approximated by the derived analytical expression. The number of alpha particles produced in a pure deuterium plasma due to secondary fusion reactions has been evaluated. It is shown that, due to orbital effects, the local alpha-particle density in a deuterium plasma can be comparable to the density of protons which are produced directly by the DD reactions.