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: Vacuum Implosion
Date: Oct 08, 12:37 pm
Poster: Richard Hull

On Oct 08, 12:37 pm, Richard Hull wrote:


Stephen Coley has brought up an interesting point in his reply to the message on "notes".

He is concerned about vacuum implosions from a safety and pump protection standpoint.

The pump is normally protected by a fine wire mesh screen in the intake port reservoir.

The issue of implosion is part of the crucial safety issues associated with the fusor and its construction. I have a Safety FAQ published in the achieve of the HV LIST serv and on the web site in Germany recommended in an earlier post.

In this safety blurb I noted all the key technologies crossed into by the aspiring fusorite. Each discipline has definite hazards and related safety precautions.

Implosion is the number one hazard in vacuum work. The amateur will often not be able to afford a proper bell Jar or chamber made specifically for vacuum work. This is normally a heavy walled borosilicate glass vessel (Pyrex, Kimax, etc.) with special curvatures made to withstand atmospheric pressures. The manufacturers of these devices still recommend a safety shield of some sort. Professional implosion shields are most often made up from heavy gauge stainless steel wire mesh and can cost 60% of the price of the bell jar itself!

To make matters worse, we will stress the evacuated chamber by blasting the glass with needle fine electron beams which will easily reach welding temperatures!!! (there is such a thing as electron beam welding in real life!). I have been agast to see the fine beam in my fusor hit the glass and when I touch the glass get a severe burn! In addition I have even seen the glass have moments of tiny incandescence!! WHOA!! This is really frightening. Fortunately I use a professional jar which Corning annealed very well and all goes on as usual. Still this mistreatment of the glass is risky.

Most amateurs cast about looking for anything that looks as if it will work and cost almost nothing. If you do this, you absolutely must have an implosion shield in place and keep it there! Nothing would ruin your day more than having a 4" long sliver of high velocity 1/4" thick glass fly into you juggler or through your eye socket into you brain. The only thing that would be worse would be if it happened to an observer of your experiments.

Metals and polycarbonate plastics are pretty immune to launching hi-vel debris if they implode. Other kinds of plastics and glass are not so forgiving.

Glass and metal are among the cleanest vacuum environmental chambers. Glass allows unubstructed vision (probably best for a first demo fusor). Glass however soon becomes covered with ion or vapor depositied residues from the grids and other sources in the chamber. Also, glass is a risky chamber for higher power or neutron work. (due to high power density and very intense and powerful ion/electron beams.)

Stainless steel is the ideal vacuum chamber as it is implosion proof and can easily hit wall temperatures of 1000 degrees F and keep on ticking. Huge power densities can be supported with ease. The draw backs are expensive seals and welding/machining operations needed to contain the vacuum and add ports and attachments. Also, the need for a viewport which can be very expensive, if purchased from a vacuum supply house. 304 SS is the ideal choice for a neutron producing fusor where lots of energy will be input to the system.

Remember, always gueard against implosion with glass and most plastics.

Richard Hull