Pump question

[Steven Sesselmann]

Would someone share an opinion with me, I am concidering wheather to
get a good two stage rotary vane vacuum pump vs. a oil diffusion pump
together with a lesser quality rughing pump.

Are there any real benefits in using an oil diffusion pump?

Do you guys normally run the pump during operation of your Fusors, or do
you just pump down the chamber and let in a small charge of D2?

Any suggestions from experienced Fusers would be welcome, at this
stage of my project.

Steven

[Frank Sanns]

A two stage pump if working completely like new will pull down to just about where you need to be for fusion. A little water in the oil, a trivial vacuum leak, a loose reed valve or a vane and you can forget about doing much fusion. You will still get nice electron beams and pretty colors and maybe even a star but you will always be right at the edge of what is enough to do fusion. Others may say a two stage is enough but in the real world, just being at the very edge of capability is not where I like to operate. There just is not enough extra capacity with a mechanical pump.

A diffusion pump on the other hand can take you down to 10-15 micron (millitorr) with no problem at all unless you have a significant leak. The pump rate on a diffusion pump is very high but as the pressure gets down lower and lower then it is not a pressure differential that removes molecules but diffusion. Hence the diffusion pump should be part of the chamber if you want to get to 10-7 numbers or better. For us, a half meter of a larger diameter (1"-2") vacuum line is quite fine.

I ALWAYS run a flow of deuterium through my chamber when I am doing a fusion run. The chamber lines out faster, runs higher neutron numbers and is more stable at all currents and voltages when there is a contiuous flow. Myself, I set vacuum throttle valve on the diffusion pump to drop a micron or two a second at around 40 microns then I open the deuterium as the pressure gets to around 15-20 microns to counteract the drop. This is the minimum deuterium flow that I like to run. Without a diffusion pump, I don't know if a mechanical pump alone could remove this flow of deuterium since it is so close to its pumping limit.

Hopefully others will chime in with their thoughts.

Frank S.

[UG!]

i haven't done fusion yet, but am close to getting the vacuum system sorted after a lengthy downtime.

i have a single stage vane pump and a turbo pump. originally i was going to get a 2 stage vane pump, but now have decided instead to invest in a system that will let me use the turbo pump to better effect.

the single stage pump sucks down to about 60 microns which is enough for the turbo pump. with turbo pump i will be able to reach pressures unattainable with any vane pump, hopefully enough to run my RGA (i eagerly await the arrival of my Lesker order)

another problem with vane pumps is that they vary widely in specification. iirc a robinair reffidgeration service pump is suppose to suck to about 15 microns but an edwards will pull to about 5 microns. This is about the critical range for fusion, and it will probably be very helpfull to have fine controll about here.

so your probably better off with a lesser roughing pump and a diffusion pump in my opinion

Oliver

[Steven Sesselmann]

Thanks for the comments, if I could work something around a single stage
rotary vane pump together with a secondary pump of some description, I
could save myself some money. I actually have two pretty good single
stage pumps on hand.

I shall keep a lookout for a turbo pump as well.

Steven

[DaveC]

Two single stage pumps in tandem, should result in a considerably lower pressure, although not a good as a true two stage pump. Depends on each pumps general condition, including the seals. Try it. Put a gage on the input and get an idea.

Dave Cooper

[Todd Massure]

What level of vacuum is required from a roughing pump for the average diffusion pump / turbo pump?

[Richard Hull]

100 microns is low enough, but you really would like it lower, for if you can't go lower, then your ultimate pressure with the diff pump will suffer. A good mechanical forepump on a diff pumped system should certainly hit 25 microns or it isn't worth beans in the long run. Good forepumps have little trouble hitting 10 microns.

Richard Hull

[Todd Massure]

Thanks Richard,
I ask, because as I said before, my Welch pump is only hitting 30 microns, and if I'm going to get a diff or turbo pump anyway maybe I shouldn't even bother with any kind of overhaul attempts.

[DaveC]

Some number for comparison,
I am running a 4" turbo backed by a direct drive rotary (Varian 300) .... System pressure for the experiments running is in the low -6s torr, at which point the foreline vacuum is about 18 microns. The Varian direct drives, when new, can reach low -5s torr (0.01 micron) when blanked off.

We rarely see a real system get below 10-20 microns on roughing only, because of outgassing, desorption, leaks and etc.

My home lab system uses a venerable Welch beltdrive, backing a 4 inch diffusion pump. Roughing stabilizes at around 20 microns with the diffusion pump fired up and the system pumped down into the -6s torr.

So, use these numbers for comparison, your pressures may vary, but should fall in line, all other things being about equal.

Dave Cooper