RBS Instrumentation: Beam Line and Tank

Electrical isolation of the accelerator terminal obviously requires good insulators. Because air ionizes and becomes conducting in high electric fields, the accelerator components must be arranged to minimize electric fields. Thus, one guiding principle of accelerator design is elimination of any sharp points or edges that produce high electric fields. The high voltage components are housed in a pressure vessel filled with sulfur hexafluoride gas (SF6) because this gas resists electrical breakdown better than air.

The terminal and the beam line require mechanical support, a more demanding task because of the forces exerted by the pulley used as one end of a Van de Graaff or Pelletron voltage source. The beam line consists of alternating insulator and conductor sections connected to the terminal voltage along a resistor chain to linearize the electrical potentials and reduce electric fields between components. The beam line and the terminal are surrounded by a series of smooth conducting rings also connected along a resistor chain to the terminal voltage. These precautions serve to reduce voltage gradients between accelerator components and thus minimize electric fields. The precautions are necessary because the 750 kV terminal has the potential to produce very large sparks.

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