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Cheap First-Time Pump

Posted: Wed Mar 04, 2009 12:21 am
by Tyler Christensen
I've been looking around at pump information for a while and seem like I'm getting nowhere. There are so many types and brands and the use of multiple stages etc. What is the cheapest I can go to get fusion going (used is ok)? What type/model should I look into?

Re: Cheap First-Time Pump

Posted: Wed Mar 04, 2009 3:57 pm
by Richard Hull
Probably, for a demo fusor, a used refrigeration servicing pump or a an old welch 1400 would be a starting point. Note* If you ever plan on doing full fusion, get a pump that is worth having or your first pump above will probably not be satisfactory and you will be out hunting for a better pump at some point.

Richard Hull

Re: Cheap First-Time Pump

Posted: Wed Mar 04, 2009 10:40 pm
by Tyler Christensen
I want to do actual fusion, not demo. This means I will need more than a 1400 welch?

Re: Cheap First-Time Pump

Posted: Thu Mar 05, 2009 2:50 pm
by Richard Hull
A 1400 welch in really good condition might do, but a diffusion pump added to this would do even better.

Richard Hull

Re: Cheap First-Time Pump

Posted: Fri Mar 06, 2009 1:24 am
by Tyler Christensen
I was under the impression that fusion only requires a vacuum of 5-10 microns, and a 1400 welch should hit 1 micron max (unless my math was flawed). Wouldn't it be more than enough for this? Or is my 5-10 requirement not correct

Re: Cheap First-Time Pump

Posted: Fri Mar 06, 2009 4:20 pm
by Richard Hull
No 1400 will ever hit 1 micron, especially a used one. The math is good but deceiving.

Let us say that you have a very good 1400 and it hits 5 microns. The required pressure for fusion is 10 microns. You have 50% of your chamber filled with non-fusible gas. Eventually, you will have higher percentages of deuterium in the chamber, but you will waste a lot of valuable deuterium gas replacing that non-pumped intial 50% garbage by l"uck of the draw" diffusion of fresh D2.

I operated in just this manner for the first year or two of my true fusing fusor. Wasted about $200.00 wourth of D2 running this way, too.

It is your choice, of course.

Remember, there is no real zero micron vacuum. 1 micron leads to a deeper .1 micron vacuum ten times deeper and then a .01 micron level 100 times better, etc.
All of us folks in the vacuum biz revert back to torr when you go below 1 micron.

We only speak in microns as that is the normal operating pressure range for a fusor and the range where most glow discharges function and are extinguished. We get to speak in whole numbers and not scientific notation or millitorrs.

Richard Hull

Re: Cheap First-Time Pump

Posted: Fri Mar 06, 2009 4:58 pm
by Carl Willis
Hi Tyler,

To add briefly to Richard's fine response, the pump's quoted "ultimate vacuum" is the pressure at which the pump (in ideal condition) has effectively zero speed with the port blanked off.

Comparing the vacuum achieved in a real chamber to that in a blanked-off pump inlet requires accounting for the increased load from outgassing, permeation of seals, vapor pressures of materials, and so forth in the real chamber. This is why, in practice, that quoted "ultimate vacuum" is beyond reach. Since the pump speed curve is so steep at pressures on the order of the ultimate vacuum, small loads (those mentioned, plus flowing deuterium gas if that is in use) result in working pressures far higher than the "ultimate vacuum."

To have some useful pumping speed at under 1 micron, even if the pressures needed are above the "ultimate vacuum" for the mechanical pump, a high vacuum pump (turbo or diffusion pump) is called for.

-Carl

Re: Cheap First-Time Pump

Posted: Sat Mar 07, 2009 5:59 am
by Tyler Christensen
So if I got something like this (http://cgi.ebay.com/10-inch-diffusion-p ... 7C294%3A50), I would hook that onto the base of the vacuum chamber then the welch 1400 onto a different chamber port? First I would run the 1400 down to its minimum pressure then enable the diffusion pump. At this point, would I keep the 1400 running, or turn it off before running the diffusion?

Re: Cheap First-Time Pump

Posted: Sat Mar 07, 2009 6:33 am
by Richard Hester
A 10-inch diffusion pump is way too big. Hold out for something like a two inch pump. The larger pump will bankrupt you with its oil requirements, and suck more power for its heater than all the rest of your gear combined. Smaller pumps aren't that hard to come by. CVC and Veeco made several appropriate smaller models - so did Edwards. An air-cooled pump will be easier to deal with than a water cooled model, since all you will need is a fan.