Ion Gun basics

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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John Futter
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Ion Gun basics

Post by John Futter » Sat Aug 25, 2012 3:22 am

I have started a new thread so as not to muddy Richards excellent FAQ
This is primarily aimed at Preston
Ion guns are used so as to provide a charged particle that is looking for something of the opposite charge hence they tend to move towards this opposite charge. The rate at which they move depends on what potential they differ from the other electrode.
With ions having the same charge they naturally want to repel from each other and therefore the ion beam expands (defocuses).
The slower the ion beam the more time they have to repel from each other so lower potential beams defocus further than beams at high potential.
Another thing that defocuses beams is the ions hitting neutral gas atoms (mean free path )--so the better the vacuum the better the eventual spot.
Now on to Prestons inquiry
a 100mA beam is considered a big beam 10kV is considered a low accelerating potential so the chance of getting this number of ions into a 2mm spot is not possible. Also the vacuum of only a micron is such that the mean free path is only a few mm so there is yet another defocusing mechanism. Also a 100mA @ 10kV over a 2mm spot is a heat load of 33 kWatt per sq cm.

Check out what 16 watts per sqcm looks like with a target stage cooled to -179 degrees celsius
Note that in the following the vacuum in the beam line is 1x 10 to the minus 7 torr

http://www.youtube.com/watch?v=oALDWvav ... ults_video

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Mike Beauford
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Re: Ion Gun basics

Post by Mike Beauford » Sat Aug 25, 2012 3:20 pm

I love the floating head and also a very good intro into the subject.
Mike Beauford

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Carl Willis
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Re: Ion Gun basics

Post by Carl Willis » Sun Aug 26, 2012 2:24 am

I concur, from my years in the high-current linear accelerator industry, that 100 mA is a very high current. Not impossible to accommodate, but an unusual capability. For one thing, very few accelerator structures are capable of handling this current in the beam, so there is limited demand. A notable exception was the LEDA RFQ accelerator at Los Alamos, which used an ECR-type ion source whose close descendants probably fill all remaining demand in the proton/deuteron linac world for this kind of intensity. Such current causes strong space-charge effects that dictate the extraction geometry and have important influences on beam optics. Sometimes at high currents it is desirable not to have a pure ion beam, but to allow that beam to pass through enough background gas to create electrons that allow some space-charge neutralization. An extraction voltage in the 50-kV or higher ballpark is probably necessitated in order to overcome space charge limitations on apertures small enough to be effectively differentially pumped. A 100-mA / 10-kV ion source would be a custom-engineered device, easily with a $100k price tag for continuous duty and maybe $25k for <10% duty.

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