A time-of-flight system using a novel mechanical chopper has been developed to measure the energy spectrum of neutral deuterium atoms emitted from a tokamak plasma. The chopper system consists of a motor, a magnetically levitated shaft, and a chopper disk. The disk is rigidly attached to a 159 g shaft assembly and the set is supported against gravity in vacuum by permanent magnets and a stabilizing servosystem. All components are UHV compatible to avoid contamination to the tokamak and to the electron multiplier detector. The 25. 4-cm-o. d. , 0. 005-cm-thick, 20 g, 301 stainless-steel chopper disk has 24 0. 025-cm-wide slots photoetched at an 11. 4-cm radius. An effective aperture time of 0. 55 mu s is achieved during typical steady-state operation at 22500 rpm with a vacuum pressure of 2 multiplied by 10** minus **8 Torr.
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