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: Re: Tour d'force
Date: Apr 29, 09:58 am
Poster: Richard HullOn Apr 29, 09:58 am, Richard Hull wrote:
No plastics allowed!!!!!
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>--Are you referring to the fusor region proper or the vacuum system in general? I would point out that no OBSERVABLE outgassing has seemed to occur from the Lucite fittings per se. The stuff is used extensively in Bickel's lab on many vacuum systems. Hell, there's a Helnholtz mirror made entirely of the stuff. I will repent of Lucite if it is a mortal sin to vacuum but I have not found much reference to problems with it outgassing.
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Kim Goins is our local group's resident physicist. Se works at the Naval Research Laboratory in DC as a research Physicist. She is a plastics nut and expert. She has an special fondness for plexiglass, and the polycarbonates. In the NRL NIKE laser group, they use plexiglass, lucite and a very few other plastics in vacuum systems involved with the huge terawatt laser.
She notes that all glastics outgas in vacuo although at 1 micron levels, you could effectively have very minor outgasing provided the plastic was taken to that vacuum and left there for a very long time. They process and vacuum cure there stuff over very long periods. They have a rack of old plastic like vintage wine which has sat around for years. She warns to never use new plastic. The stuff oozes polymerization distillables for years. Also she claims that not all plastics of the same type are equal. Sometimes you can get a "not quite done yet" batch of plastics where the mix was wrong or other chemical processes have failed. The plastic is quite usable as it was intended, but the stuff will never stop outgassing.
Kim notes that even tiny beam currents or stray charged particle bombardment will load up a chamber in seconds should the material be impacted. If it were not for its neat characteristics in other areas and extreme workability, they would never use the stuff.
Plastics in vacuo at the amateur level are a crap shoot. The key to success is to take the stuff down and leave it there forever. Never bring the thing back up to air! So, if you are in an experimental situation with lots of ups and downs be prepared for a lot of long pump cycles.
Remember on my tape how I was struggling to get 70 microns in that polycarbonate dessicator? AND that was specialing designed to be placed under moderate vacuum for drying! Ultimately, as noted on the tape, I hit 27 microns by pumping over days and valving the system off after use to keep the vacuum at some lower level when not in use. Plastics are not the best vacuum materials and below 1 micron require special vacuum aging and special selection batch to batch. RH
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>>Gerald's new "good" Sorenson supply should serve him well. Current limiting is something I gotta' work on here for my modified X-ray supply.
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>--Amen! I recommend CT's on the high side to monitor the current and be used to trip the contacts. You ARE using an HV contactor I hope?
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Not really! I use a variac to go from zero volts up to 30kv now. I do have a manual crowbar for servicing the fusor. I just leave the power unit perpetually connected to the chamber. I have a lot of nice Jennings 60KV vacuum contactors, but just don't use them in this particular application. RH
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>>Gerald's theoretical treatment, echo's Hirsch's original work near the end of the ITT/Farnsworth period.
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>--It is so based. Hirsch refers to the derivation actually being done by the team of Elmore, Tuck and Watson in his article. I put it in my presentation to allay the doubts about the issue of virtual electrodes. The math isn't that hard to follow.
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I am glad that you found their reference. Elmore, Tuck and Watson should be credited with their own special contribution to the IEC Saga, even though they never did fusion. RH
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>>At reduced pressures of 10e-5 torr and lower, we lose a lot of those atoms of deuterium, but nice deuteron recirculation becomes possible. Go too low, and a dispenser cathode arrangement becomes a must.
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>--Yep! I haven't used the diffusion pump since December. Also, my cheap video camcorder isn't doing my star modes much justice. They are much better defined to the naked eye.
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Use a black and white CCD camera hooked to a VTR. They have the IR filter mask removed, usually, and the beams can be see on video often better than with the eye! Try it! they can see IR leds like a flashlite beam. RH
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>--Don't leave out some brains Richard. One reason I sermonized on certain of my own atrocities was for the benefit of the unwary.
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>G. M.
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Agreed! I often harp on specific problems and tips to allow others an opportunity to avoid the potholes in the fusor road. Our mistakes and mis-steps can benefit others.
Oh, I forgot.... I have not used a stainless inner grid system since late 1997!!! I have been 100% pure tantalum based from the first pyrex belljar system onward. I figure that the stuff's 5,400 degrees F. melting point will give me a large breathing space. I couldn't very well investment cast tantalum, so I use Ta wire.
Richard Hull