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   Arc efficiency from spin rate and pulsewidth.
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   Form:
   eff = ArcEff( spinrate, pulsewidth )
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   Inputs
   ------
   spinrate         (1,1) Spin rate (rad/sec)
   pulsewidth       (1,:) Pulsewidth

   -------
   Outputs
   -------
   eff              (1,:) Efficiency

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