[Sneap] Focus resistor, Pd leak on AN2500

[Walter Augustyniak]

Focus resistor, Pd leak on AN2500Stuart,
    With our AN-2500 at Bell Labs and now at Vanderbilt, we too, scrapped the Pd leak for Hydrogen isotopes, and switched to conventional TM leaks. These have served us very well over the years.
We also scrapped the use of carbon resistors in high voltage, current limiting situations. They are notorious for breaking down after extended use. We now use metal oxide type resistors, which I believe are the type now incorporated in the string of column resistors on machines such as the AN 2500.

    Walt Augustyniak
         retired but still interested....
  
  ----- Original Message ----- 
  From: Van Deusen, Stuart B 
  To: SNEAP at TUNL.DUKE.EDU 
  Sent: Friday, September 28, 2007 11:08 AM
  Subject: [Sneap] Focus resistor, Pd leak on AN2500


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  Hello all 

  The output from the focus power supply on an AN2500 goes through a resistor on its way to being clipped to the focus plane on the column.  The HVEC circuit diagram shows this as a 2 Mohm resistor.  When I started operating my machine 11 years ago, it was equipped with a string of 2W carbon composition resistors totaling about 11Mohm.  I recently managed to blow these resistors up (probably by arcing over) by running the focus voltage too high while the machine was in normal operation.  I replaced the string of carbon composition resistors with a single thick carbon film resistor, rated at 5W and 20 KV.  I find that after some use, the value of the resistor drops, in the most recent case by almost half.  I don't really understand what is happening to the resistor, and if it will ultimately fail open or not.

  My question is, what do you all put in when you replace this resistor? 

  A second question concerns the palladium leak used for hydrogen in the AN2500 (and I'm sure other machines as well).  Does anyone have experience with these leaks failing, and if so, what is the failure mode?  On both this leak and the thermo mechanical leak used for He, what is the expected leak rate when a good leak is fully "closed"?

  Thank you for your help. 

  Stuart Van Deusen 



  ----------------------------------------------------------- 
  Stuart B. Van Deusen 
  Radiation Solid Interactions and Processing 
  Dept. 1111, M/S 1056 
  Sandia National Laboratories 
  P.O. Box 5800 
  Albuquerque, NM  87185-1056 
  USA 
  PH (505)844-7782 
  FAX (505)844-7775 
  e-mail: sbvande at sandia.gov 

  address for package delivery: 

  US NNSA c/o Sandia Natl. Labs 
  ATTN: Stuart Van Deusen 
  Bldg. 884, Rm. 11 
  1515 Eubank S.E. 
  Albuquerque, NM  87123 
  USA 



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