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: Re: Neutron counters and other parts.
Date: Apr 04, 9:36 pm
Poster: Dave Cooper

On Apr 04, 9:36 pm, Dave Cooper wrote:

> Also I was wondering about vacuum pumps. I have a two stage vacuum pump similar to what Richard has in his pictures, not the belt driven one the rotary one.
>So, are there any objections to this form of pump or should I consider another kind of pump or pumps.

Nathan - as regards mechanical pumps the chief concerns are pumping speed, ultimate pressure and backstreaming of oil vapors. The pump speed is "probably" a bit less important than ultimate pressure. If your roughing pump is one of the newer direct drive pumps and in good condition, getting to 1 micron or even below that on a tight clean system is not too difficult. The key words are tight and clean. Also a molecular sieve on the inlet side will help keep backstreaming of pump oil to a minimum.

You should expect some backstreaming of roughing pumps below 100 microns.

However, along the lines of the "Deceleration of ions.." discussions of a few days ago, you really want to minimize collisions between the ions you make and neutral gas molecules. this means the longest mean free path (MFP) possible.. which means the lowest pressure possible.. before D2 is put in the fusor. Since MFP is inversely proportional to absolute pressure... you need to go substantially below roughing pump vacuum to get the innards clean and gas free. Ion pumps are a bit pricey but very nice, turbos are great, but also pricey. Personally I like the good old diffusion pumps. They are relatively cheap...and with an LN2 cold trap, quite acceptable...Oil backstreaming from long term use can be an issue. But they are bulletproof and the price is right. The ideal diff. pump would be air cooled. Water cooling usually requires 1/2 to 1 gpm... which if you can avoid.. is one less expense.

Hope this is useful..

Dave C.