[Sneap] electron Van de Graaff source troubles

[Walter Augustyniak]

Fred,
    It has been decades since I played with an electron VdG (at Bell Labs).
You have checked the pentode circuit, but as I recall, the cathode filament 
in the head
of the accelerator tube, is run at a high current to give steady maximum 
emission.
    Could there be some elec component in the cathode heating circuit that 
unknowingly
heats up over time, changes value, starts limiting the cathode current, and 
thereby
lowers the electron emission which is such a strong function of the cathode 
temperature?

    Good luck

            Walt Augustyniak

----- Original Message ----- 
From: "Fred B. Bateman" <fbb at nist.gov>
To: <sneap at daytona.tunl.duke.edu>
Sent: Friday, November 17, 2006 11:39 AM
Subject: [Sneap] electron Van de Graaff source troubles


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> Dear SNEAP:
>
> We have a KS-4000 HVEC electron Van de Graaff accelerator here at NIST. 
> Recently, we
> have been having serious trouble with the source emission.  The electron 
> source uses a
> DC cathode emission control unit supplied by HVEC.  The electron beam 
> emission is
> controlled by varying the grid bias applied to a pentode tube.  For many 
> years the beam
> output was in the range of 100-200 uA. Recently, we have only been able to 
> get about 70
> uA out for a brief period, and then the current drops to   about 25 uA 
> after about 20
> mins. running time.  Eventually, if the machine continues to operate, the 
> output is
> reduced to about 5 uA.  The first time this happened we suspected a 
> problem with the
> pentode tube or the cathode filament.  Emission testing performed with the 
> tank opened
> revealed no problems with the pentode tube or emission control unit.  The 
> filament
> appeared okay but was replaced anyway.  When we closed up the tank and 
> checked the
> accelerator beam output, the same symptoms recurred.  The next time we 
> opened the tank
> we also monitored the alternator output for an extended period, thinking 
> that could be
> causing us trouble, and it seemed to be stable (under a limited load).  We 
> also
> monitored the pentode circuit output for several minutes and noticed no 
> reduction in
> current.  So, all the diagnostics we have been able to run have revealed 
> nothing, yet
> the problem persists.  And we know we are not losing beam in the steering 
> process.
>
> We are not sure where to turn since HVEC no longer exists and we are not 
> aware of any
> companies in the U.S. that can provide service for these machines.  Any 
> assistance or
> suggestions that could be offered would be really appreciated.
>
> Fred Bateman
>
> -- 
> Dr. Fred B. Bateman
> NIST Radiation Interactions and Dosimetry
> 100 Bureau Dr. Stop 8460
> Gaithersburg, MD  20899
> ph: 301-975-5580 fax: 301-869-7682
>
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