In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.Subject: We work a weird vacuum.
Date: Dec 10, 4:04 pm
Poster: Richard HullOn Dec 10, 4:04 pm, Richard Hull wrote:
The post just prior to this on glow cleaning sparked a thought in me which needs to be shared with vacuum newbees.
Over the entire range of vacuums for practical work, we work are at the most strange point. That point is the unit micron level hovering around 10e-3 torr. We are not low vacuum guys like the neon signage or deposition folks. We are not even in the medium vacuum range, but stradle the fence in betweeen medium and high vacuum.
So, as a class, we appear too snooty from the few torr people and too crude to mix with the very high vacuum folks who look down on us for using the "M" word (micron)and for not having diff or turbopumps. We work in unclean but decent vacuums. We are the vacuum "middle class".
As if this weren't bad enough, we are in a magical realm in between heavy ionic discharges of high current and brilliance and total extinction. The bulk of this action occurs over only a single decade span. OUR DECADE! This makes for subtle visible yet vast physical manifestations in our systems.
If we aren't paranoid enough already, nobody makes a good inexpensive electronic gauge for our triad of decades (10e-2 - 10e-4 torr). The theromcouple gauge peters out below 10 microns on most meters. (Though the 1518 tube can, with very careful calibration, serve to almost submicron levels - I use it alot).
Working an ionization gauge at these levels will crud it up in no time and only expensive iridium filaments survive. Likewise, the litle used cold cathode discharge gauge will get dirty quick at a few microns. Only a heated capacitive manometer in the 10 millitorr range will give superb results over the 10-2-10-5 range. These are about $1500.00 with controller.
Those who have really "fiddled" around in this region know exactly where I am coming from. It is a region of seemingly magical and counter intuitive goings on. Electron and ion beams can visually and instantly denote microsisms in pressure and or gas content. The best lessons are learned with a demo fusor with lights out and about 6 hours of visual study of the beams, quills, bugle jets, magnetic anomalies, etc. No one can appreciate this until it is experienced and seen.
Mean free paths in our decade go from almost worthless-(ten millimeters), to just beyond the dimensions of most vacuum vessels, or very useful-(tens of centimeters).
Whats more, this is a vacuum region where intense X-rays can be developed with no hot cathode. I had always heard that turn of the century cold cathode x-ray technicians were artisans practicing a black art. Using the cadmium getters and palladium gassers in the arms of the old medium vacuum x-ray tubes, they could tune the vacuum to such a fine degree over "OUR DECADE" that x-ray output could be set to any level desired.
This is the bizzarre world of vacuum we haunt, mostly because it is cheap, gives superb fusion density, accepable mean free path, and a visual display. It works for most of us in a marginal way.
Good luck and enjoy.
Richard Hull