[Sneap] terminal current surge problem

[Natko SKukan]

Hi Riyad,
I'd say it is a conditioning of your (LE) acceleration tubes..If you didn't go high with terminal voltage for a while, or you vented the tube, it appears. You could also see a change in vacuum readout and maybe some radiation as well.
What happens is leakage current and small sparks through the tube because some parts of the accelerator tube emit electrons and also probably ionise residual gas, because of high voltage gradient. The tube needs some time to "settle down". Check for example  http://cas.web.cern.ch/cas/Pruhonice/PDF/DC-accel-DB1.pdf


Be careful with increasing the voltage and follow the procedure for conditioning the tubes (HVEE instructed you how to do conditioning I guess). 
Basically you should go slowly up with terminal voltage and when you see conditioning started wait at that voltage until it decreases or completely disappears. Then go up for few tens of kilovolts.Observe HE and LE vacuum, HE and LE FC, downcharge currents and also ripple of the terminal with CPO. All those instruments could give you indication about conditioning. 
I don't know if you can switch your voltage regulator to some "upcharge control" mode, but in principle, for conditioning it is better to increase slowly upcharge current, without GVM feedback.
Be careful!
Natko


Natko Skukan
Laboratory for Ion Beam Interactions
Rudjer Boskovic Institute
Zagreb, Croatia
nskukan at irb.hr
http://www.irb.hr/en/str/zef/z3labs/liis/
  ----- Original Message ----- 
  From: riyad sha 
  To: sneap at tunl.duke.edu 
  Sent: Wednesday, August 20, 2008 8:54 AM
  Subject: [Sneap] terminal current surge problem


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        Hi sneaper's: 

        During the operation of the 3 Mv tandem accelerator, we experience the following: 

        When we apply the terminal voltage, the current highly fluctuate in the faraday cup before the accelerator tank (when it is closed or opened) even the source is off in the same time we noticed a decreasing in the law energy column current  . this phenomenon  disappears after a time  but as we go up with the terminal voltage value it appears again !!! 

        so any one has experience this problem or have an idea ,could help me.




        Riyad Shaban
        Electronic engineer 
        in AECS (atomic energy commission of syria)
        Accelerators Section
        mob:0963-932617224 




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