[Sneap] High radiation at low terminal voltage

[tjgray2m at peoplepc.com]

Pertti:
    High terminal radiation is usually associated with electron loading in 
the acceleration tubes. If you have a capacitance pickoff for monitoring 
voltage fluctiations of the terminal, does it show any spiking 
characteristics that one see when there are breakdown between tube sections? 
What are the vacuum conditions in the low and high energy tubes? Breakdown 
leading to sparks to the terminal along the columns or from other things 
sparking to the terminal will not produce radiation because of the average 
low energy of the electrons moving in the dense insulating gas. Such sparks 
could trigger breakdowns between tube sections, thus causing electron 
loading with its attendent radiation. How many hours do you have on your 
acceleration tubes? When you inspect the acceleration tubes, is there any 
tracking between adjacent tube electrodes or are there any bubbles in the 
glue fillet joint between any of the tube electrodes and the glass 
insulators? Do any of the tube insulators show damage? Have any of the 
optics used to condition the ion beam for injection into the tandem changed 
their characteristics? Does putting an ion beam into the machine make the 
problem worse or have no effect?
    With the inner insulation bars not present the belt could flap about, 
hitting random equipotential planes and thus dump charge on the planes. This 
could raise that plane's potential thus creating a local high voltage 
gradient that could cause the tube section(s) in the proximity of that plane 
to breakdown with the release of an electron shower which triggers an 
increase of terminal radiation.
    When you first started up after putting in the new charging belt and you 
had reasonable operations, the belt was new and the tension was some value 
that you set it to. After some running the belt may stretch and run in a 
sloppy manner that could lead to what is discussed above. Has the belt 
tension changed from the original setting?
    I am certain that something is causing tube breakdown in one or more of 
the acceleration tubes, thus causing the high radiation levels that you are 
experiencing.
----- Original Message ----- 
From: "Dr. Pertti O. Tikkanen" <ptikkane at acclab.helsinki.fi>
To: "Symposium of Northeastern Accelerator Personnel" <sneap at tunl.duke.edu>
Sent: Wednesday, January 16, 2008 1:50 PM
Subject: [Sneap] High radiation at low terminal voltage


> Hello fellow sneapers,
>
> In our vertical 5-MV-, N2/CO2-insulated tandem we are having severe
> problems in reaching any HV higher than about 2 MV. Good ideas for
> locating the problem are welcome!
> Here is a description of how the symptoms developed:
>
> -After a tank opening in September 2007 we conditioned the machine up to
> 4,3 MV and did run some experiments where the tandem performed perfectly.
> There wasn't any need to go to higher TV's.
> The just-installed green Siegling belt ran also well. This time we had
> left out the inner equipotential bars. A bad mistake, perhaps?
> -The problems started to appear after a few days AMS run at 3.2 MV. In
> that run, the charging current was relatively high, as we used about
> 50-microamps of corona probe current. It wasn't exceptionally high,
> however.
> -Subsequently, when starting the tandem for another experiment, much
> higher radiation levels, tens of micro-Sv/hour (compared to what we
> usually have, only a few uSv/h) were observed already at 3.5 MV. It was
> impossible to go any higher than about 3.8 MV because of high radiation
> levels and very unstable TV.
> -As the insulating gas was a bit more humid than usual, we dried it once
> more to reach a dew point of about -69 Celsius. However, this made the
> problem even worse; the radiation started to inrease heavily at voltages
> lower than 3 MV's.
> -There were almost no tank sparks. At least none were related to increased
> radiation level.
> -Before opening the tank, we monitored the radiation to find the location
> of its source. All that we found was that it was concentrated near the
> terminal.
> -The tank was opened, but we didn't find any apparent reason for the extra
> radiation. The only difference compared to normal was that there were a
> lot of sparking tracks at the edges of the terminal shield. That's the
> place where the electric field is the strongest, I suppose.
> -We thought that there are some extra particles in the insulating gas and
> installed new filter elements into our pumping lines. Measured also the
> particle concentration in the gas. It was very small.
> -The resistors were checked carefully, a few were found defective (in
> random places), all surfaces were cleaned carefully, the belt was found in
> good condition. Looked for extra items like loose wires etc. in the
> column, found none. So everything looked just OK and we closed the tank,
> only to find out that
> -The high radiation level was still present!
> -We got an idea that there was some chemical contaminant in the gas
> because we had recently installed new absorbant in our gas dryer.
> -Replaced the insulating gas with a new mixture of N2/CO2. Furthermore,
> the gas dryer was regenerated carefully at higher temperature.
> -After all this we are in a situation that a lot of radiation is generated
> at about 2.2 MV or even at lower TV's.
>
> What did we miss?
> We are anyway going to open the tank again and try to locate the source of
> the problem.
>
> Best wishes and best regards,
>
> Pertti
>
> % Pertti O. Tikkanen, PhD         email: pertti.tikkanen(at)helsinki.fi
> % Senior Laboratory Manager
>
> % Accelerator Laboratory                        TEL:   +358 9 191 50006
> % P.O. Box 43 (Pietari Kalmin katu 2)           FAX:   +358 9 191 50042
> % FIN-00014 UNIVERSITY OF HELSINKI, FINLAND     GSM:   +358 40 72 65386
>
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