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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