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: Molecular Sieve Sorption Pump
Date: Feb 15, 08:33 am
Poster: Richard Hull
On Feb 15, 08:33 am, Richard Hull wrote:
The sorption pump is a nice idea as Ely silk noted along with the major objections on the LN2 which demands a dewar. LN2 is about the price of beer in lot sizes under 25 liters. The real winner is that sorption pumping is totally oiless in operation.
The better idea with a mechanical pump is just a simple molecular sieve at room temp! This will elimenate virtually all of the oil and moisture from your system which are the major contributors to low ultimate pressures due to back streaming of oil vapor and high vapor pressures of water.
You can make your own sieve container with a screen wire basket inside of a cylinder with vacuum in and out lines attached, or purchase one. I chose to purchase a micro-nmaze from Lesker. (see my older post regarding the micromaze) This is even better than a straight sieve and I am pumping to submicron levels with it and nothing else on my pump.
Finally, as you operate your fusor and keep it sealed against air, the natural glow dishcarge cleaning and ion pumping effect will bury a lot of high vapor pressure stuff. These artifices might make the best mechanically pumped systems actually capable of 10-4 to 10-5 vacuums without a diff pump.
The above is not a cure all for a worn out pump! Re-read my large post on pump oil. You may just have bad or fouled oil. I find that if a naked pump will give a 5-10 micron vacuum in a chamber, then it is a first class pump hooked to a first class and well sealed system of good overall conductance.
Forget ever hitting these levels, even with a brand new pump, if you are using over 12" of pump line, expecially if that line is only 1/2" inside diameter or less. You are killing your conductance. (see a Lesker catalog for complete tech data on conductance). I find 5/8" ID hose as the absolute minimum inside diameter for rapid pump down and lowest ultimate pressures. Big chambers demand even large hoses of shorter length. Pump down times grow to incredible lengths will low conductance forelines.