Re: TC in a vacuum ... Re: TC vs Electron Gun

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>
> 
> Well.. more just for testing general breakdown over surfaces and the like
> for antennas.. We also do multipactor testing, etc. (as you say, it's a big
> issue, particularly as Solid State Power Amps are getting up there in power,
> and folks want to build cavity filters and diplexers that have fairly high
> Q).  For the moment, there's more interest in breakdowns around the Paschen
> minimum (which oddly, happens to be what the atmosphere on Mars is like).
> Yes folks, I'm going to get to run a semi-TC in a simulated Martian
> atmosphere (assuming my little project gets funded (hope!)).
> 
> Multipactoring these days is mostly handled by analysis (you don't want to
> find out it has the problem after you build it), and just verified
> afterwards in the vacuum chamber.

	The analytical approach works fine, ONCE YOU HAVE BEEN BITTEN and
realize you should do it!  It's usually easy to pick non-resonant
geometry.  In our case we didn't anticipate operation during launch and
thought the whole structure would have been well degassed by the time
the big transmitter was turned on.  It probably would have, but that
wouldn't have prevented the breakdown we had.  In the course of later
work I ran the thing in a a VacIon pumped ultra high vacuum chamber
(pressure supposedly <10^-12 mm, but one always wonders) for weeks. 
Even at the end of that time there was a visible blue glow between the
two pieces of the cavity, and enough electrons were floating around to
excite a spot of fluorescent ZnS on the wall of the resonators.  The
discharge at this low pressure didn't have enough power associated with
it to cause any problems, which were apparently due to heating releasing
the occluded gas, which resulted in a discharge followed by an arc, even
though the power level was only 10 watts.  I seem to remember that was
Richard Woo's conclusion too; he had fried 50 ohm type N
connectors!!!!!!!!  The final solution, as there really wasn't time for
a new design, was to pressurize the cavities with helium, encapsulate
them in epoxy, temperature cycle and then helium leak check. There was
never any problem with the transformers which were flown, but I was
always uneasy about the epoxy and afraid of cracks. 

	We'll be very interested in the results of your low-pressure (~1 mm?)
TC experiment and hope you get to run it.  I assume the primary gap will
be pressurized.

Ed