[Sneap] FN tandem column/terminal electrostatics

[Tom Gray]

Tom:
    The optimum ratio of the radii for a concentric cylindrical set of electrodes with the smaller radius being Ri and the outer cylinder having radius Ro is Ro/Ri = e for a fixed potential difference between the two cylindrical electrodes. This gives minimum electric field strength at the surface of the smaller cylindrical electrode for a fixed voltage difference. If one plots the strength of the electric field at the surface of the inner electrode vs Ro/Ri it is a minimum at e and rises more rapidly with increasing Ro/Ri and not quite so rapidly as Ro/Ri as the ratio decreases below the value at the minimum of the curve.
    This is a good problem to ask your students to solve. I have used it at times in the past to get them to think about electrostatics in something that they can actually see when they look at the accelerator.
    We had an AN2000 HVEC single ended machine at Kansas State. The previous owner had upgraded the machine to operate at 2.5 MV. He increased the tank diameter as that seemed like the thing to do since he expected higher operating terminal voltages. This lowered the Ro/Ri value and actually made the situation worse. The real solution was to simply either add some SF6 to the insulating gas or increase the pressure of the CO2-N mixture if that was to remain the insulating gas.
    Physics is fun isn't it. Hope this answers your question.
                                                        Tom J. Gray
  ----- Original Message ----- 
  From: Miller, Thomas Edward 
  To: sneap at tunl.duke.edu 
  Sent: Monday, September 08, 2008 12:20 PM
  Subject: [Sneap] FN tandem column/terminal electrostatics


  The host for the 2008 SNEAP annual meeting is the University of Michigan at Ann Arbor.  The meeting dates are: October: 12 - 17, 2008.
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  SNEAPERs:

   

  I was wondering if anyone knows of any electrostatic calculations, diagrams, figures, etc. of the FN column and terminal structure. Just curious to know how they came up with the hoop diameter, spacing, and the terminal shell that we have on our FN.
   
  Thank you. Cheers!!!!  

   

           Tom Miller



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