[Amps] Ion Pump

Roger (K8RI) k8ri at rogerhalstead.com
Sun Jul 20 18:33:44 EDT 2014

On 7/20/2014 3:12 PM, John Lyles wrote:
> Although this doesn't have much to do with the building of amplifiers, 
> it does shed insight into some of the complications of making good 
> tubes and how the technology there has impacted other industries.
> General Electric was one of the first to recognize that vacuum tubes 
> (their Pliotrons and Kenotrons) required better vacuum than the 
> Audion's had that were made by DeForest companies. So naturally, a lot 
> of development came from GE and also benefited their Xray tubes.
> K8RI's experience (below) is helpful here. I am in RF engineering and 
> only observe vacuum experts doing their stuff from the sidelines. We 
> work hand in hand, of course, in not applying RF power until vacuum is 
> sufficient. I know of several instances at particle accelerator 
> laboratories where oil contamination from diffusion pumps created a 
> costly mess. I didn't think we have them for pumping between 10^-1 and 
> -6. The big roughing pumps are indeed rotary. We use cryo pumps and 
> ion pumps, maybe the cryos are for in-between? I know they have to be 
> regenerated regularly.

I should add that the chilled water or liquid N2 "cold traps" are also 
called "Back gassing traps" whose purpose is to prevent the gasses in 
the pumps from getting into the chambers on which they are pumping.  The 
liquid N2 traps also aid the pumping as they remove many molecules of 
gas.  When you fill the N2 trap, the vacuum goes down almost 
immediately, but they are not filled until there is a vacuum on the pump 
or any moisture would freeze to the baffle and then slowly sublime, 
slowing pumping substantially.

The diffusion pump is a very simple device, but might take a bit of 
space to explain.


Roger (K8RI)

> Our accelerator cavities need to be in the low 10^-7 Torr or we get 
> many discharges from the extreme RF voltages there. Electron 
> accelerators need much better vacuum. I believe that big tubes must be 
> better than -6 to minimize damaging flashovers during conditioning.
> 73
> John
>> From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
>> Roughing pumps do not normally get anywhere near the vacuum needed to
>> start an Ion pump.   We used a roughing pump to get to 10^-1 Torr, then
>> a diffusion pump to make 10^-6, (-7.with a liquid N2 cold trap) At that
>> point we'd start the ion pump.  Once it was working, we'd valve off the
>> diffusion pump inlet as the ion pump would not work in parallel with the
>> diffusion pump. It would pull oil, or mercury fumes out of the diffusion
>> pump.
>> We used ion pumps on a mass spectrometer. It created a beam of ions
>> containing the material to be analyzed. This beam passed between two
>> curved and charged plates inside a powerful electromagnet. IIRC the
>> poles were about 10" in diameter with the magnet weighing around a ton.
>> However the last time I worked on that was "maybe in or around 1980.
>>    An ion pump is basically a big diode that collects ions on the
>> surface, or getter.  They have no exhaust.  Starting one at the typical
>> 10^-1 Torr of a roughing pump would quickly contaminate the ion pump.
>> Penning tubes will light up with plasma around 1 X 10^-1. The ionization
>> will go out around 1 X 10^-2.  The ion pump would need much better than
>> that to start.
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