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SF6 - Sulfur Hexafluoride

Ves Fulp

SF6 was used as a gaseous dielectric medium in all of the high power waveguide runs between the klystron output windows and the four antenna feed horn windows (located in the anechoic chambers). SF6 is a very heavy gas and has a density about 6 times heavier than air, and its primary purpose was two fold, (1) to increase the breakdown voltage of the waveguide, and (2) to aid in cooling the waveguide.

With the extremely high power being transmitted through the very small waveguide, cooling presented a major (but manageable) challenge to the designers. In addition to the SF6 gas under heavy pressure, all high power waveguide runs had 4 individual half inch square copper tubes soldered to the waveguide external surface and running the full length of each section of waveguide with cool water flowing through each (in effect, an external heat sink). A high pressure stainless steel flex hose was used to jumper the water around each waveguide joint.

Sustained arcing inside a waveguide can be very destructive, especially in dealing with the power levels at which we were operating. A waveguide arc is in reality a short circuit across the waveguide. If unchecked, it tends to walk back down the waveguide from the point of arcing toward its RF source, which is the klystron. Arcing did occur quite frequently at times. The Raytheon design included numerous unique fault monitoring techniques to detect a waveguide arc and to immediately switch off the RF pulse at its source (the low power exciter).

When any section of waveguide was opened for any reason, special burn in was required to condition the waveguide to withstand full power without arcing. Without this special burn in process, operation at the power levels required would have been extremely difficult and costly. This conditioning procedure was designed to remove/vaporize all minute particulate matter suspended in the waveguide.

For whatever reason, once a section of high level waveguide was opened, the following had to take place before full power operation could be restored. This was known as controlled "burn-in" using an "early off" procedure.

  1. Replace/repair as necessary to eliminate the problem.
  2. Close waveguide using new oxygen free copper gaskets. The waveguide flanges had a knife edge embedded into a recessed slot that cut into the copper gasket when two flanges were mated. This formed a high pressure seal strong enough to contain SF6 under about 26 psi pressure.
  3. Pull vacuum down (can't remember value, but near absolute zero).
  4. Release vacuum and backfill with 100% pure SF6, cap off at 26 lbs pressure.
  5. Activate water and SF6 systems.
  6. Energize final power amplifier system.
  7. Burn in of waveguide: start off at low power with very short RF pulse width and progress up to full power maximum RF pulse width. This vaporizes any suspended particulate matter and may take an undetermined amount of time as the progression of burn-in is controlled by amount of arcing, not time.