Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Novice Questions
Date: Jan 27, 10:34 pm
Poster: Don & Steve Estes

On Jan 27, 10:34 pm, Don & Steve Estes wrote:

My son (Steve) is interested in building a fusor, and I've volunteered to help, with no small interest in the project myself. I did have some vacuum experience working in a molecular beam lab while an undergraduate, but it's been 30 years since I've been around a vacuum pump. So, we have a number of very basic questions for which we would appreciate any assistance.

We have a stainless steel cold trap from an old vacuum pump to use as a chamber, about 8 inches in diameter and about 18 inches high, with a 1 inch flanged inlet we can use for the vacuum pump and hydrogen bottle, and two 8 inch mountings we can use for viewports. Along with the chamber we were given 2 1-inch thick Lucite plates to use in lieu of buying glass.

By the way, the staff at H K Equipment in Haverill, Mass. have been very supportive of our project, donating the chamber and making us a very nice deal on a pump. If you love junkyards, this place is heaven. www.hkequip.com.

So, questions:

1. We've been reading about the electronic beam heating, and how it can even be a problem with steel vessels. Are we fooling ourselves with these Lucite viewports? Should we bite the bullet and buy glass?

2. We went to our friendly local welding shop to buy some hydrogen. We were told we couldn't buy it unless we had hydrogen rated fittings for the gas bottle. I haven't seen anyone mention hydrogen rated fittings on any postings (not that I've read all of these yet!), so I'm wondering if they were being straight with us. Comments? If we should have hydrogen fittings, where can we get them? Bear in mind we need to connect both the vacuum pump and the gas fittings to the same flange connector on the chamber. Having a non-conducting vacuum hose also seems a very sensible idea.

3. The feedthrough seems a major question, particularly for a low budget project. Contemplating an $800-$1000 professional feedthrough encouraged me to seek alternatives. A friend suggested a sparkplug, which seems like it could work, if I'm prepared to use epoxy to seal it into place and modify it appropriately to create an electrode . Any thoughts? Is this crazy, and I should just pay the money?

4. Mounting the feedthrough is not a trivial issue with stainless this thick. I'm certainly not going to try to tap it myself. We've hit on the idea of using the top viewport and either drilling the Lucite or getting a steel replacement. This has two additional advantage. If we have to replace the feedthrough after epoxying it into place, the plate is easily removable. If we mount from the top, then changing the inner grid becomes simpler and makes it less likely that we might impact the outer grid once it has become fragile.

5. What do you recommend for a stand for the outer grid? Before you build your first prototype, these small fabrication issues loom pretty large, particularly for those of us whose constructions have been mostly logical rather than physical.

6. Is a vacuum gauge really necessary? It seems that we can pump for a reasonable time, then fire up the power supply until we have a glow, then pump until the glow is almost gone. (And therefore a non-conducting vacuum hose.) If we do need a gauge, does it really need to be the $800 ones?

7. We were planning on buying a small spot welder for grid construction until we found there is no such animal. The posting on building a spot welder was very helpful. I've got several fair sized capacitors rated at 3000 ufd @ 50 VDC. It seems too simple to just get a 50 volt DC power supply and hook it up to the capacitor and the spot welding electrodes. It seems we could easily get a nasty burn from this setup if we're anything other than totally alert. What else are we not understanding here being novices?

Our plan is to start working with protium rather than deuterium, and then switch when the bugs are worked out. However, we've had several people express serious concern about even getting marginally detectable levels of neutrons, including a couple of physicists from local universities. Well, I'm not planning on taking a nap in full view of the fusor when running, but what is the real risk here? Directly and through secondary emission? Should we view the plasma only through a video camera and/or a mirror? Do we really need to filter the output of the vacuum pump and treat it as low level waste? This is supposed to be cool, I don't want my son getting leukemia in 20 years from it.

The more realistic danger I worry about is chemical: hydrogen explosion in the basement of our house. I expect to release and pump down such small amounts of hydrogen that as long as the vacuum pump is vented directly to the outside the amount of hydrogen that escapes into the basement atmosphere should be trivial, even considering the way the pesky little atoms migrate through just about anything. However, I am the cautious type, and I'm happy to pay for the hydrogen fittings to reduce this risk if it makes sense.