EffectiveCharge:

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   Computes the effective charge in a plasma.

   Type EffectiveCharge for a demo.
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   Form:
   zEff = EffectiveCharge( nI, zI )
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   ------
   Inputs
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   nI        (:)    Ion densities (#/m^3)
   zI        (:)    Ion atomic numbers

   -------
   Outputs
   -------
   zEff      (1,1)  Effective charge    

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   References: Bingren, S., "Core Plasma Characteristics of a Spherical
               Torus Fusion Reactor", Plasma Science & Technology, Vol.
               7 No. 2 April 2005. 
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