[Sneap] Focus resistor, Pd leak on AN2500

[jim]

Focus resistor, Pd leak on AN2500  Hi Stuart
  Well going back in time for me to palladium leaks, the common failure was poisoning of the palladium from bad gas or leaks to the palladium  when switched off SF6 for us leaking into the source then back to leak I suspect, which gives you no leak rate, or cracking the palladium which can leave you with a small leak until heated which rises rapidly or flooding your source with a small amount of heat applied.As to the leak rate when not heated palladium or T.M. leaks, the leak detectors we had at the time would not show much of anything not to say there wasn't gas coming through, as we could see in the colour of the strike while in the machine. Not much help with that.  To the resisitor. We used old carbon resistors and never found them to devalue to my knowledge. Years ago people had lots of old parts around, they are not as readily availble anymore. Anyone who has old SS White resistors should list them with SNEAP they are always useful.
   Good Luck
  ----- 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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