From: My Account \<me@leidecker.gsfc.nasa.gov\>

Date: Thu, 6 Jan 2000 19:27:22 -0500

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John.S.Canham.1@gsfc.nasa.gov

Subject: Mechanically agitating the fiber optic cable does produce HF

Gentles,

Dr John S Canham (a UNISYS employee who supports our Materials Branch)
has

attached a 10 foot long segment of ISS fiber optic cable (cut from the
link

I\'ve been working with, called P7) to a mass spectrometer.

The distant end of the segment is sealed with \"Apiezon Wax W\": this
stuff

has a vapor pressure of 0.6 microtorr at 20 degC, melts at 44 degC, and
is

applied at about 100 degC. The buffer tube of the near end of the cable
is

inserted into a stainless steel tube whose inside diameter is chosen to
be a

close fit to the outside diameter of the FEP buffer tube, and the joint
is

covered with Apiezon Wax W. Some external splices were added to
strengthen

this connection. John arranged the specimen as a coil of about ten
turns.

The stainless steel tube is a few inches long, and enters a chamber

containing a mass spectrometer. The chamber is 4 inches in diameter; I
do

not know how high it is. A port on the side of this chamber connects to
a

pressure gauge and turbomolecular pump, which is backed by a molecular
sieve

and then a roughing pump. (The sieve limits backstreaming from the
rougher

into the turbo.)

It takes about a day for this system to pump out all the gases normally

exhaled from the segment of cable after it has been soaked in air. After

pump-down, the total pressure approaches roughly 0.5 microtorr with the
cable

attached. The main gases seen by the mass spectrometer are from our

atmosphere: nitrogen, oxygen, neon, water; these gases leak through the

jacketing and then the FEP buffer tube, and finally get to the mass

spectrometer. (The total pressure is more two orders of magnitude lower
when

the chamber is sealed, without the cable attached.)

John twisted the coil vigorously for several seconds, and noted the
presence

of HF within several seconds after the start of twisting. The HF
persisted

for several minutes; the amount detected decreased substantially after
some

five minutes.

John stimulated the coil using a hand-held Tesla coil, moving it back
and

forth over several inches of one side of the coiled cable. John
estimates

that about as much as half the total length of the fiber optic cable was

stimulated by the Tesla coil. The duration of excitation was roughly two

seconds. HF became visible within a few seconds after the start of

excitation, and persisted over the next five minutes. The Tesla coil

produced roughly five times the amount of HF than twisting the coil did.

John will be attempting to estimate the number of moles (nanomoles is
likely

to be a more appropriate unit) of HF produced in these adventures. But,

while we do not know production rates yet, I believe we can conclude
that

John has established that HF is produced by motion of the cable (perhaps

because this motion creates sparks between the polyimide and the FEP?),
and

also by direct electrical action on this cable. Also, I believe we can

conclude that easily detectable amounts of HF are produced within
seconds of

a several second-long stimulus, and that this HF can be detected for
minutes.

We know that a few dozen nanomoles of HF is enough to produce a \"rocket

engine defect\".

We learn from this that we cannot simply irradiate the cable and then

transport it to John\'s mass spectrometer \-\-- atmospheric gases
quickly

dissolve into the cable to such an extent that the instrument is
saturated by

them for many hours, while the HF and associated products (i.e., COF2)
are

so active (reactive) that they \"disappear\" within (nominally) five
minutes.

So we must irradiate the cable while it is in a good vacuum, and then
\-\--

keeping it under vacuum \-\-- transport it to the mass spectrometer
within a

minute or two. We are presently studying how best to do this.

It seems clear that the source of the HF is the FEP. BICC has kindly
sent

us some FEP pellets (the same as the stock that feeds their extruder).
John

and I believe that we can learn something useful by exposing these
pellets to

x-rays from an industrial machine, and some others of these pellets to
the

gamma rays of cobalt-60. It can be much easier to arrange to keep these

pellets in a good vacuum during the irradiation, and then to measure the

amount of HF released, than arranging this for the cable.

Henning Leidecker