Foreline traps?

[daniel_pflug]

I am drawing up my vacuum system set up now and am wondering how necessary it is to have a foreline trap. I am going for neutrons, so it has to be a prety good vacuum, however i dont have that much income b/c I am a high schooler. It would be nice to not have to purchase a trap and the 2 extra valves that would need to accompany it.
Also, i am considering raising a few extra bucks to get a manometer instead of tc and ionization gauges. if I do this, is it better to position it between the throttling valve and chamber, or on the chamber itself?

Thanks for the help!

[Tidbit77]

Foreline traps are certainly nice, but if you are just going for neutrons, I doubt it would be of much help. It seems (by rough estimation) 75% or so of people on fusor.net do not use foreline traps. Personally, I do not use one.

Capacitive manometers are certainly nice, and would allow for accurate pressure measurements, however, I am unsure of how fragile they are. TC gauges often fail in fusor environments, and I imagine CM gauges would not fare well either. All in all, if you have the cash for a CM gauge, I would give it a shot, however, if you feel another part of your system (gas valves, HV supply, neutron detection) is lacking, personally, I would focus the money there.

As to the mounting location of the gauge, I would recommend mounting it on the chamber itself. Mounting a gauge near a gas outlet or pump inlet could result in an inaccurate measurement of the chamber pressure. I would also recommend not mounting the gauge in a way that the head of it is in a line of sight of the plasma, it seems that this reduces greatly the chance of the gauge failing.

Just my $.02

-Will Jack

[SteveHansen]

If you are going to be running your system at base pressure for long periods a foreline trap is a good idea to keep mechanical pump oil from backstreaming or creeping into the DP or turbo pump. However, you will probably find that you pump to base and then proceed quickly to the operating pressure with some gas flow.

I use a copper wool trap in my DP foreline. It's pretty effective and doesn't require the maintenance that molecular sieve traps need. My hybrid turbo needs no trap as gas is bled into the foreline (held at about 0.1 Torr) and this acts as an effective ballast.

Plasmas and Pirani/TC gauges often do not mix well as the plasma can sometimes reach the filament and then the electronics. Capacitance gauges only present the inconel diaphragm to the system environment - the electrodes and electronics are shielded. If you can find a good CDG for your fusor, use it.

[Chris Bradley]

No harm in including one if you find one for small money. I have one in my system and seem to recall I got a couple from the same guy I got my turbo pump from for, essentially, next to nothing.

I'd not have gone out of my way to get one, but it serves its purpose and protects the turbo and experiments from any backstreaming.

One further useful function has not been mentioned yet: You can close off from the chamber and bake out the trap. This gives you a nice clean trap and if you isolate it then next time you come to crank everything up, the trap will quickly soak up all that troublesome water vapour out of the chamber. It is rather anecdotal, but if I do this - a bakedown of the trap, then leave the system for a week, then come back to it - I reckon I see a pump down to base pressure that is around 3 to 4 times as quick. And that can mean the difference between deciding to get on with some tests after a few weeks 'off' time, cranking it up in an evening after work and getting to base pressure ready for experiments, or not getting to base pressure nor any experiments done that day and waiting for tomorrow! It's a minor advantage, but serves the purpose.

[daniel_pflug]

Thanks for the help. I probably wont worry about a trap till a future fusor effort then.