Recently, stochastic motion of 3.5-MeV alpha particles with orbits that vary between locally trapped and locally passing states (transitioning particles) in a Helias reactor was observed numerically. This validated the theoretical predictions that (i) the stochastic diffusion represents a mechanism of considerable delayed loss of fast ions in quasi-isodynamic stellarators and (ii) it is possible to prevent the escape of particles to the wall by closing the separatrix between the locally trapped and passing states. It was concluded that, in principle, the separatrix could be made closed, resulting in a reduction of fast ion losses, by compensating for the effect of the helical component of the magnetic field $(1, 1)$ with an enhanced ‘anti-helical’ harmonic $(1, -1)$ ; the enlargement of this harmonic was proposed previously for other reasons. This possibility is explored in this work. Equations of previous relevant works were generalised to include the $(1, -1)$ harmonic. Calculations were carried out for several magnitudes of the ratio of anti-helical to helical magnetic field harmonics. Positive effects were found already at the smallest anti-helical harmonic considered: when the latter ratio is 0.25, transitioning particles with the smallest and intermediate pitch parameters are confined and, moreover, their fraction decreased. When the ratio is 0.85, almost all transitioning particles are confined and their fraction is minimal; well-confined localised orbits dominate at the smallest pitch parameters.

中文翻译：

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双螺旋 Wendelstein 型构型中过渡粒子的限制

最近，在 Helias 反应堆中对 3.5 MeV α 粒子的随机运动进行了数值观察，其轨道在局部捕获态和局部通过态（跃迁粒子）之间变化。这验证了理论预测：（i）随机扩散代表了准等动力仿星器中快离子大量延迟损失的机制，以及（ii）可以通过关闭局部粒子之间的分界线来防止粒子逃逸到壁上。被困状态和通过状态。结论是，原则上，可以通过补偿磁场螺旋分量的影响，使分界线闭合，从而减少快离子损失 $(1, 1)$ 具有增强的“反螺旋”谐波 $(1, -1)$ ; 之前出于其他原因提出了扩大该谐波。这项工作探讨了这种可能性。先前相关工作的方程被概括为包括 $(1, -1)$ 谐波。对反螺旋磁场谐波与螺旋磁场谐波的比率的几个幅度进行了计算。在所考虑的最小反螺旋谐波中已经发现了积极的影响：当后一个比率为 0.25 时，具有最小和中间螺距参数的过渡粒子被限制，而且，它们的分数减少。当比率为0.85时，几乎所有过渡粒子都被限制，并且它们的比例最小；良好限制的局域轨道在最小螺距参数下占主导地位。