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FAQ - Secondary vacuum pump notes for the Noobie

Posted: Wed Oct 21, 2020 7:48 pm
by Richard Hull
By the term "secondary pump" we typically mean a diffusion ("diff") pump or Turbo molecular ("turbo") pump. These pumps are recommended for all fusor vacuum systems to avoid the wasting of the precious and expensive deuterium gas needed to do fusion. The secondary pump is placed between the mechanical ("fore-pump") and the fusion reactor chamber. The fore-pump evacuates the "fore-line" and the body of the secondary pump. The secondary pump is turned on and further reduces the pressure (increases the vacuum level), within the fusor vessel.

The problem with all surplus diff pumps and all turbo pumps found on e-bay or such auction sites is that the seller thinks they are gold (Au) when many turbos or diffs do not rise to the standard of mangled lead (Pb). Also most all sellers have never in their lives pulled a vacuum beyond the draw on a cigarette! But then, many buyers are of the same caliber. (first timer, vacuum neophytes)

About diff pumps

Diff pumps are either air cooled or water cooled. For the rank amateur, an air cooled is more desirable. All Diff pumps have no moving parts and just demand a suitable wall outlet to work. Thus, the only electrical worry is, "does the boiler heater have continuity"? If this element is good, then the next bother is.... Does it have its guts intact? The guts consist of a single "stack" or "Christmas tree" occupying the diff pump's inner tube. If the stack is there and intact, the last thing would be....Is the stack bright and shiny and clean along with the interior hollow tube of the pump itself? If all these criteria are met, a Diff pump is at least worth $200. On smaller pumps, (2-3 inch throat pumps), expect to pay $50 for the required special pump oil. One of the worst parts of buying and using a diff pump is its adaptation to common, modern fittings. Diff pumps tend to never have a workable "common fitting" having been yanked from custom made and custom designed OEM systems of the 40s, 50's or 60's. A 1940's diff pump 300 years from now will still work! Not necessarily a turbo of current vintage. Be prepared for some custom machine work to adapt a diff pump to you fusion system.

Diff pumps require an intelligent vacuumist to operate them in a carefully controlled start up, but a very special shut down effort. Fail in the shut down of a diff pumped vacuum system and it can mean a complete tear down of the system and many hours of work attempting to clean a burned tar like mess off the pump and the stack assembly.

About turbo pumps

Turbo pumps have three critical components, the pump itself, an electronic controller, and a proper, multi-pinned, manufacturer's cable to connect the two. The turbo pump itself is 100% perfection of rotational mechanical motion. A flawlessly balanced rotor with hundreds of blades must spin up to 90,000 rpm on any number of different bearing surfaces based on its original design. A multipole motor in the base of the pump body must have every winding in good shape. The rather complex electronic controller box must be fully functional. The cable is a must have item. If all three are present, functional, and working, $1000 is not too much to pay for such a warranted system.

Normally, you tend to find just a pump.... untested. Regardless of what the seller says, assume it is dead, damaged or otherwise not worth over $100 max! Even then, you might receive a nice $100 door stop. Likewise, a lone controller, if found, that will match and run the pump, regardless of promises, might also be a $100 door stop. Cables are very rare if not part of a warranted functional system consisting of all three items as a functional set. (be prepared to manufacture your own cable, provided you can find and purchase the proper multi-pin plugs.)

Be advised that almost no living being buys a fully functional, surplus turbo pump system, complete, ready to rock and roll on the cheap, (like I did). Due to their complexity, Turbo pump systems are easy to destroy in the hands of a vacuum neophyte. Running turbo pumps can have material fall into them of the most inconsequential and trivial nature that would not impact, in the least, a working diff pump! If this happens, the turbo pump can self destruct in a second with some frightening violence. In high voltage systems, like the fusor, an arc or other RF or EM pulse can ruin the turbo pump's electronic controller. (believe me, I have blown up such a controller, however, my electronic knowledge allowed me to repair it over a two week period and almost $100 in components and shipping charges.)

I used an air cooled diff pump for 17 years on fusor IV and never had an issue. (Rigidly obeyed the operating procedures) I have used a Turbo now for 1 year and have blown up my controller once, (chalked up to a learning experience.) Like all turbo pump owners, I love the turbo pump due to its simplistic "button on-button off" operation when in a properly designed vacuum system. Make no mistake, however, If my turbo ever goes down for good, I will never buy another, but instead, un-pack my faithful old diff pump from fusor IV and use it the rest of my days on this earth.

Caveat emptor! Buyer beware! If you can't see it, test it or touch it, assume it is a piece of crap. Assume always that the seller is an idiot regarding what they are selling until they prove otherwise.

Note: Many sellers might give a 90 day money back warranty.....Buy a turbo pump by itself....Find a good matching controller to run it 5 months later, hand wire a cable, find out the pump is dead, your pump warranty has now expired.........To recover the cost of the controller, you must now purchase another pump specific to that controller which you just found and purchased!!

1. Buy only under a full money back warrantee from a seller who states that the pump, be it diff or turbo, is fully functional!
2. Be prepared to adapt any and all pumps to your system component fittings. (This may mean some custom machining)
3. Read up on how to operate and protect your pump. Be knowledgeable on the theory of its operation.

Richard Hull