Ian, I always learn something from your longer
postings. Often it's a summary of bits and pieces I
already knew, more often it's new data. Thanks for
this.
I would concur with the notion that the path length is
of very small concern when it comes to arcs. During
my "day job" I have occasion to be involved in the
design and fabrication of thick-film circuits -
precision attenuators, IC carriers and the like. In
our ceramics processing facility, we perform
sputtering, both DC-enabled and RF-enabled. It is not
unusual to have flash arcs occur in the sputtering
tank, whose walls are of translucent glass. We can
clearly witness the arcs occurring, and they seem
never to occur where the two conductors come closest
together. We also have never actually seen an
arc-mark afterwards. It must depend on how much
material is transferred during the arc. It is also
interesting to note that there is no gain in a
sputtering tank - we have high voltage, but no
amplification going on. So, this is purely an arc in
a high vacuum environment. Generally, we "train" the
tank each time we use it, allowing it to arc many
times before it settles down. Only after tank
"training" as we call it, do we apply heat to the
plating material, so it can vaporize and plate the
ceramics which we are processing.
Interesting bit of physics, all told!
73,
Dave W8NF
==== in reply to ===
There are two long-term sources of gas in tubes. One
is a leak to
atmosphere, and the other is "outgassing" of
structural materials.
Leaks can be large or small, but they are always a
one-way trip, with
only one end. Outgassing is more complex, and not
necessarily fatal.
Impurities in metals are the main source of
outgassing. When the metals
are refined from ores, they always have built-in
impurities. Materials
for use in vacuum tubes are extensively refined, by a
combination of
chemical processing and heating in a vacuum furnace...
but still some
impurities remain in the atomic lattice structures.
There are also gas molecules chemically bound onto the
exposed surfaces
of metals, ceramic and glass. These are largely
removed in the later
stages of vacuum pumping, by heating the whole tube
far above its normal
operating temperature while continuing to pump. The
better the
materials, and the longer the time for which the tube
is pumped, the
better the vacuum will be... but they can't pump and
bake forever, so
eventually the tube has to be sealed off.
At that point, the getter is activated. The getter is
a chemically
active material that has been placed inside the tube
to act as a kind of
'fly paper' for stray gas molecules that might appear
in the future. The
getter in receiving tubes (and small glass
transmitting tubes like the
807) is typically barium metal, which had been left
inside the tube in a
little tray. On the production line, an induction coil
heats up the
tray, evaporating the barium onto the glass as a
slivery-looking film
with a highly reactive surface. This will continue to
mop up stray gas
molecules for the life of the tube. (If that film has
turned white, it
means there has been a gross air leak - the barium
metal has turned to
oxide, and the tube is done for.)
Transmitting tubes are different because they run much
hotter and
operate at higher voltages, and a volatile metal like
barium would
evaporate from where it had been deposited, and then
condense in all the
wrong places. Instead, the getter materials are
typically zirconium or
tantalum, which are non-volatile but *need* to run hot
in order to
operate effectively. That is why the main getter in a
glass tube like a
3-500Z is located on the metal anode (the grey surface
finish is the
zirconium getter) and in a metal/ceramic tube it is
located on the
heater (the next hottest location). Most transmitting
tubes actually
have multiple getters to mop up the various kinds of
gas molecules,
using different materials in locations at different
temperatures.
Immediately after manufacture, the vacuum will
probably be about 10^-8
mmHg, which is really quite good for a routine
production-line process,
but no great shakes by the standards of a vacuum lab.
At this standard
of vacuum, a typical tube may contain anything between
a million and a
billion gas molecules! (PV=nRT... work it out)
Most of the time, a "vacuum" tube operates perfectly
well in spite of
sharing the space with very large numbers of gas
molecules. But
sometimes you need to remember that the tube is also a
reaction vessel
for some complex low-pressure chemistry.
Coming back to impurities... immediately after
manufacture, the vacuum
is probably pretty good because all the surface
impurities were flashed
off. However, impurities that were trapped deeper
inside the metal can
continue to diffuse to the surface over the lifetime
of the tube, and
can be released as gas into the "vacuum" space causing
a small increase
in pressure.
If the getter is active, it will mop up the impurities
within typically
a few seconds (determined by the time it takes for the
molecules to
bounce around until they strike the getter surface,
and by the
probability that a molecule will hit a chemically
active spot that can
form a strong enough bond to make it stick). But a few
seconds is far
too slow to prevent an arc, which can strike within
microseconds if all
the other conditions are right.
This explains why tubes can arc for no apparent
reason, but if you try
again a short time later, the tube goes back to normal
as if nothing had
happened. (Obviously this requires an amplifier with
good HV surge
limiting and fast shutdown protection. If the arc is
allowed to persist
at high current, it will damage both the tube and the
amp.)
It also explains why transmitting tubes generally need
to be pre-heated
after a long period out of use. The process of slow
diffusion to the
surface of the materials means that gas will probably
have accumulated,
and the getter needs some time at a high temperature
in order to do its
job.
--
73 from Ian GM3SEK
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