[Sneap] electron Van de Graaff source troubles

[Fred B. Bateman]

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