On 7/20/2014 8:07 PM, Hardy Landskov wrote:
This is well outside my realm of experience. The claim of 1 X 10^-12
would be extremely difficult to obtain.
https://en.wikipedia.org/wiki/Vacuum . So far, I've not found a
definitive answer to the actual vacuum required in normal production,
power tubes tat are use at amateur power levels, or several times higher.
73
Roger (K8RI)
Roger,
As an EE myself now retired, have any experiments or production units
of vacuum tubes been done while the shuttle program was ongoing? Maybe
there is the vacuum of outer space that might be better than what's
achieveable on earth but you might collect particles floating by as
well from space junk that would negate the advantages. Just curious or
maybe it's classified....or maybe just impractible.
73 Hardy N7RT
----- Original Message ----- From: "Roger (K8RI)"
<k8ri@rogerhalstead.com>
To: <amps@contesting.com>
Sent: Sunday, July 20, 2014 3:33 PM
Subject: Re: [Amps] Ion Pump
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.
73
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
K5PRO
From: "Roger (K8RI)" <k8ri@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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