[Sneap] High radiation at low terminal voltage (cont.)

[Dr. Pertti O. Tikkanen]

Hello,

First of all, many thanks for the good ideas!
Here is the story:
After opening our tank, we didn't find anything exceptional. There were 
some belt dust and even some traces of corona on some equiv. bars, 
especially near mid-section and on the tube side of the belt. It might 
have been possible that some charge-exchange between the belt and the 
bars have taken place resulting in local increase of field between some 
electrodes in the tube---producing electrons that accelerated towards 
the terminal in the vacuum. As the inner bars were missing, there was no 
balancing field. So we decided to put the inner bars back.
It also seems that the effects of the belt (transversal) vibrations are 
worst half-way between the pulleys, which is understandable because we do 
not have any extra belt guides in our machine. I can think of four 
different modes of vibration: 1) spring-like transversal movement, 2) 
flapping of the belt edges, 3) torsional waves, and 4) any combination of 
these. Furthermore, the belt joint will also have a non-negligible effect 
which interferes with the other modes. After studying some of the 
literature on belt vibrations, I'm convinced that in some unfavourable 
circumstances the belt-pulley system could switch to bifurcation. OK, 
let's not speculate further on this.
We thus did the usual final checks and closed the tank hoping to see 
improvement. But the radiation was still there, beginning at 2 MV!
Being now more brave (we were equipped with your good advices!), we let 
some Argon gas that is used for stripping, up to 3 x 10^-5 mbar, into the 
tubes. That really helped, and we could finally go up to 3.5 MV after a 
few hours. Although the radiation level was first about 1 mSv/h, it 
decreased down to a few micro-Sv/h. Everything seems to be normal, more or 
less, and we are going to pressurize the tank to the final 14 bar. Now we 
only had 7 bars.
No tank sparks were observed, but the CPU-signal looked very funny: It was 
almost a square-wave in the beginning (maybe saturated), but went back to 
more "random" towards the end of the day.

Best regards,

Pertti


% Pertti O. Tikkanen, PhD         email: pertti.tikkanen(at)helsinki.fi
% Senior Laboratory Manager

% Accelerator Laboratory                        TEL:   +358 9 191 50006
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