[Amps] Ion Pump

[Roger (K8RI)]

On 7/19/2014 4:29 PM, Chris Wilson wrote:
>
> Hello John,
>
> On Saturday, July 19, 2014,  you wrote:
>
>> Ion Pumps are worthy of a wikipedia check. They are used in high vacuum
>> systems, once the vacuum is established with a roughing pump such as a
>> Roots blower style or mechanical chug-chug recipocating. They help

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.

I would think for production tubes we use, a simple roughing pump/ 
Diffusion pump would supply sufficient vacuum at 1 X 10^-6 Torr.

They also have a quantity limitation per use. IE: They can only absorbe 
a small quantity of gas.  Even with valving that lets the diffusion pump 
stay hot, the process of reaching 1 X 10^-6 can take a half hour or 
more. With the addition of an ion pump, the complexities can make a 1 X 
10^-7 system a royal PITA to set up and degas.  It's a system using gold 
O-rings that does not lend itself to connecting and removing things.  
Pumping a number of small tubes to a high vacuum would seem to me to be 
a problem because of the intricate surfaces involved.

As we are normally under 6 KV X-Rays should not be a problem and I would 
think 1 X 10^-6 would be plenty.  At 1 X 10^-6 virtually all the gas 
molecules are clinging to the surfaces rather than being dispersed 
through the volume of the tube.
I have never seen a reciprocating roughing pump.  We had 50 or 60 and 
all were 6 or 12 liter/min rotary vane types.  The 6 liter pumps were 
pulling on 3" diffusion pumps while the 12 liter ones were pulling on 
10" diffusion pumps.  We just used chilled water in the cold traps, 
while the mass spec used liquid N2.

73,

Roger (K8RI)


>> maintain the vacuum absorbing small qtys of free molecules. However,
>> they are mounted via a tabulation or tubing off the main vessel, so they
>> have limitations. Big tubes might have a 2 liter/sec 'appendage' pump.
>> They don't actually have any moving parts, using a voltage of 3000-7000
>> volts DC through a current limited power supply. In particle
>> accelerators, plasma sources, ion beam, or ion implantation sources,
>> they might be much larger, like the size of a keg. These require power
>> sources of 50 mA or so, while the little ones run off of 100 uA or less.
>> In vacuum tube manufacture they are used along with another more modern
>> devices called a cryo-pump. Like was said earlier, big tubes of over 100
>> kW dissipation often have appendage ion pumps mounted underneath. You
>> can tell the quality of the vacuum from the power supply current, and
>> when the tube is heated in operation and outgasses, the spikes in
>> current will indicate the event, can be used to even interlock the HV or
>> bias so that the tube won't run with trashed vacuum. This helps prevent
>> internal arcing from the outgassing from damaging the grids and cathode.
>> 73
>> John
>> K5PRO
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>
>
> Out  of  curiosity, what sort of vacuum levels do ceramic tubes, say a
> 3CX3000,  have within them when new? Has there ever been any papers on
> how  the  vacuum  is  created  and  the tube sealed? It's one of those
> subjects  I  just  accepted.  "oh, it has a vacuum in it, or it should
> have".  I  have  never really seen anything showing HOW it's developed
> and the tube sealed. Thanks, interesting stuff!
>
>


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