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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Harry wrote ...
The Scientific American Book of Projects for the Amateur Scientist (Simon and Schuster, 1960) p. 350 has an "atom smasher" linear accelerator with an end window of 1/4" aluminium plate, in which is cut a haxagon of 37 3/16"* holes to support aluminium foil of 0.001"thickness.
The target for bombardment with beta particles, or protons (when the tube is filled with H2) is held just outside the tube window.
*It is unclear to me, from the .pdf copy I have, whether this is 3/10th" or 3/16" inch.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
plazmatron wrote ...
Harry wrote ...
The Scientific American Book of Projects for the Amateur Scientist (Simon and Schuster, 1960) p. 350 has an "atom smasher" linear accelerator with an end window of 1/4" aluminium plate, in which is cut a haxagon of 37 3/16"* holes to support aluminium foil of 0.001"thickness.
The target for bombardment with beta particles, or protons (when the tube is filled with H2) is held just outside the tube window.
*It is unclear to me, from the .pdf copy I have, whether this is 3/10th" or 3/16" inch.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Hello Plazmatron,
One reason I'm interested in pulsed accelerators is because they apparently operate quite well with a very short anode-cathode gap of a few mm or so (AFAIK they generally use "explosive" emission cathodes and the acceleration gap is adjusted to give the necessary high electric field and low impedance).
According to my calculations, the mean free path (MFP) of an electron in air at 10 millitorr pressure, would be about 2.9 cm (and it would be about 29 cm at 1 millitorr).
I could be wrong, but it seems to me therefore that for acceleration gaps of several mm or so, a two stage rotary vane pump followed by a molecular sieve trap would provide a sufficient vacuum.
BTW, here's a paper describing a nanoscale field emitter structure whose dimensions are so small that it operates in air at atmospheric pressure:
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
jpsmith123 wrote ...
Hello Plazmatron,
One reason I'm interested in pulsed accelerators is because they apparently operate quite well with a very short anode-cathode gap of a few mm or so (AFAIK they generally use "explosive" emission cathodes and the acceleration gap is adjusted to give the necessary high electric field and low impedance).
According to my calculations, the mean free path (MFP) of an electron in air at 10 millitorr pressure, would be about 2.9 cm (and it would be about 29 cm at 1 millitorr).
I could be wrong, but it seems to me therefore that for acceleration gaps of several mm or so, a two stage rotary vane pump followed by a molecular sieve trap would provide a sufficient vacuum.
BTW, here's a paper describing a nanoscale field emitter structure whose dimensions are so small that it operates in air at atmospheric pressure:
Hi, yes, you are correct in saying that one could build a miniature pulsed accelerator, at high pressure. Indeed if you look at rapp-intruments site, he at one point, used a xenon lamp as pulsed x-ray source on his marx generator, which I thought was very cool! This would obviate the need for a short pulse generator, but certainly looks do-able on a modest budget.
I have experimented with molecular sieve traps, but have found they are no too efficient with the Javac rotary vane pumps that show up on evilBay, since the vapour pressure of the oil used in these is quite high (10micron at 20C), but these were only designed for filling air con units after all!. If you can get a decent pump, like an Edwards or Varian, with suitable oil there is no reason a relatively high vacuum cant be pulled (as described in "the bell jar" article.
I finally went with diffusion pumps, since they are dead simple in principle, and show up very cheaply, the only caveat being I have to do a little work to get it up and running. The lure of having a 3ft Linac in the lab, offsets the work I will have to do on the pump though !
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
In my vacuum system, I have a shreadded copper (i.e. "Chore Boy") trap between the pump and the molecular sieve. This way, the first trap blocks the backstreaming pump oil and the 13x mol. sieve adsorbs the H2O vapor, CO2, etc.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
jpsmith123 wrote ...
In my vacuum system, I have a shreadded copper (i.e. "Chore Boy") trap between the pump and the molecular sieve. This way, the first trap blocks the backstreaming pump oil and the 13x mol. sieve adsorbs the H2O vapor, CO2, etc.
Thats an interesting way of doing it, I might give that a shot. I have just gotten half way through building an experimental peltier cooled trap, so I will let everyone know how I get on with that once it is assembled. Part of the problem with the javac pumps is that they run hot, raising the vapour pressure of the oil.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Yes I read that article, but I have no source of silicone adhesive, that will withstand the bakeout temp. I was using microbore pipe filled with 13X for the trap, and whilst there was some improvement to the vacuum pulled, it wasn't truly marvelous. I'm pretty sure the flowrate through the microbore had a lot to do with that, but I figure since I`m serious about vacuum, I might as well have a pump that can pull as hard as I like, hence the investment in the Diff pump.
Registered Member #531
Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125
Have you considered using a "cold finger" (i.e. small cold trap)? Although I have not tried it personally, it seems to me that a Peltier junction cooled system might be applicable here as a (inferior but inexpensive) substitute to LN2 cooling .
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
I'm very interested in the possibility of using Peltier devices in vacuum cold traps. To get the lowest temperatures you need to stack 3 or more Peltiers with the coldest being relatively small and each subsequent Peltier being larger to carry away all the heat from the Peltiers above. To keep the power supply simple it would be great if all the Peltiers in the stack ran at the same current so they can be connected in series. You need to take steps to prevent heat from lower in the stack leaking back up the stack and a good start would be to have the stack held in a high vacuum. Once the cold plate has reached its base temperature its Peltier will only have to pump heat that leaks to it as only very small amounts of material will be condensed out of the vacuum, so a clever design here will really help get the lowest possible temperature. I don't think you could possibly reach liquid nitrogen temperatures but it'd be a lot more convenient! There used to be some software at one of the manufacturers sites to design efficient stacks but I can't find it any more.
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