[Sneap] Focus resistor, Pd leak on AN2500

[Van Deusen, Stuart B]

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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