Collisional alpha transport in a weakly rippled magnetic field

To properly treat the collisional transport of alpha particles due to a weakly rippled tokamak magnetic field the tangential magnetic drift due to its gradient (the ∇B drift) and pitch angle scatter must be retained. Their combination gives rise to a narrow boundary layer in which collisions are able to match the finite trapped response to the ripple to the vanishing passing response of the alphas. Away from this boundary layer collisions are ineffective. There the much stronger ∇B drift of the alphas easily balances the small radial drift of the trapped alphas caused by the ripple. As this balance does not vanish as the trapped-passing boundary is approached, a narrow collisional boundary layer is necessary. The solution of this boundary layer problem allows the alpha transport fluxes to be evaluated in a self-consistent manner to obtain meaningful constraints on the ripple allowable in a tokamak fusion reactor. Furthermore, the analysis indicates that the alpha losses are most sensitive to the ripple near the equatorial plane on the high field side.

为准确处理弱波纹托卡马克磁场下α粒子的碰撞输运问题，必须保留由磁场梯度引发的切向磁漂移（∇B drift）以及螺距角散射。二者联合作用将形成一条狭窄边界层，在此区域内碰撞可将α粒子对波纹的有限俘获粒子响应与消逝的通行粒子响应相匹配。在该边界层以外区域，碰撞作用可忽略不计；此时α粒子更强的∇B漂移可轻易平衡波纹引发的俘获α粒子的微弱径向漂移。由于当趋近于俘获-通行边界时，该平衡并未消失，因此必须考虑狭窄的碰撞边界层。通过求解该边界层问题，可自洽地计算α粒子的输运通量，从而为托卡马克聚变反应堆中允许的波纹幅度获取有意义的约束条件。此外，分析结果表明，α粒子损失对高场侧赤道面附近的波纹扰动最为敏感。