[Sneap] Focus resistor, Pd leak on AN2500

Fri Sep 28 12:57:55 EDT 2007

Stuart,
    Carbon film resistors do age in high voltage operation  the resistance
declining with time.  Finding the value
at 50% is very unusual and I suspect some damage from sparks.  I have
attached a drawing of our KN focus power supply
circuit made in 1997.  The components we used are identified on the drawing.
I also attach a sketch of the Focus output protection.
This helps prevent damage to the supply from tank sparks.
    Regarding the Palladium leak  yes we have gone through a number of them
over the years.  Sometimes they cold flow  which is more of a nuisance but
typically they will fail to pass hydrogen and have to be run hotter and
hotter until they no longer work at all.  There is a procedure that I have
seen for 
cleaning them and we have actually replaced the Palladium thimbles in the
past.  The last time I talked to HVEC about this  maybe ten or fifteen
years ago, I was told that they could no longer obtain quality thimbles and
were no longer making these leaks.  We switched to the TM leaks in our JN
and KN.  We tried the Palladium leaks made by Potentials Inc. but never got
satisfactory performance from them.
I  hope this is useful.
Regards,
- Chris
-- 

Chris R. Westerfeldt
Research Scientist / T.U.N.L. Radiation Safety Manager
Duke University Physics Department &
Triangle Universities Nuclear Laboratory
Science Drive, Box 90308
Durham, NC  27708-0308
Tel: (919) 660-2600
Fax: (919) 660-2634
Email: Cwest at Tunl.Duke.Edu

From: "Van Deusen, Stuart B" <sbvande at sandia.gov>
Reply-To: Symposium of Northeastern Accelerator Personnel
<sneap at tunl.duke.edu>
Date: Fri, 28 Sep 2007 09:08:25 -0600
To: <SNEAP at TUNL.DUKE.EDU>
Conversation: Focus resistor, Pd leak on AN2500
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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