inner circle of the toroid

例句：

1. 1. The toroid is a type of rotating structure that resembles a doughnut.
   环形结构是一种类似于甜甜圈的旋转结构。
2. 2. The toroid was used as a protective cover for the spacecraft during its launch.
   环形结构用作航天器发射时的防护罩。
3. 3. The toroid was designed to reduce the weight and increase the efficiency of the spacecraft.
   环形结构的设计旨在减轻重量并提高航天器的效能。