The filament is hidden below and out of the line-of-sight of the crucible. This prevents ionerosion and provides maximum protection from shorting caused by stray particles or condensate.
灯丝隐藏在下面，在坩埚的视线外这样可以预防离子腐蚀，最大程度的保护其不受游离的碎料和冷凝物的侵袭。
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The problem is, ions that bump into the grid instead of passing through it cause erosion, which limits the average NASA ion engine's lifespan to about 3 years.