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We have studied in this paper, the time evolution of a neutral
beam injected into a plasma. The relaxation time of neutral beam
having initially a $\delta-function$ shaped distribution function,
can be estimated by calculating the eigenvalues of Fokker-Plank
Operator(FPO). The coefficients of the FPO for our problem are
determined from Rosenbluth potentials. Stationary and
nonstationary solutions are found and discussed fot the
distribution function of neutral test particles.
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