Collisionless pitch angle scattering of plasma ions at the edge region of an FRC

Motion of a plasma ion gyrating around the separatrix of a field-reversed configuration is studied. Numerical studies showed that action integral of a particle changes abruptly when a particle passes through the vicinity of a field null x-point. This phenomena is understood as collisionless stochastic scattering of pitch angle. In case of a particle with positive canonical angular momentum Pθ, resultant correlation coefficients of action integral between before and after the scattering appear to be stochastic in some cases. As action integral increases for a particle with negative Pθ, its motion tends to be adiabatic. If negative Pθ of a particle approaches to zero, a stochastic motion is observed. (author)