A list of vacuum level requirements for various devices covered here might be appropriate for the newbies and as a one stop summary of what to expect and plan for in fusor construction and operation.
The terms for pressures related to vacuums are many and varied. All of us must get used to the term "torr" it is simply one millimeter of pressure. One atmosphere is considered a sea level pressure of about 760 torr. So a 1 torr vacuum is roughly a thousandths part of one atmosphere. Simple fusors operate in glow mode between one one hundred thousandths of an atmosphere and 1 millionths of one atmosphere (.01 to .001 torr). There is a term for one millionths of an atmosphere (.001 torr) called one micron.
Fusorites and many medium vacuum users use microns a lot as their pressures are all easily stated in microns. All mechanical pumps, from the worst, water laden, old worn out pump to the finest, shiney new 6,000 dollar units, work in easily stated micron ranges.
In general we have little need for discussion outside of the 100 to .1 micron ranges. In any discussions with professionals or involving vacuum levels outside of this fusion arena are undertaken, we MUST speak in torr and scientific notation torr, was well! One micron or .001 torr becomes 10e-3 torr. Vacuum tubes work in the 10e-5 to 10e-6 torr ranges. For the sake of the fusorite locked into our group, and to avoid straining the brains of the non-scientist and non-engineer types, this post will be in microns. Scientific and engineer types, like mysself and many others on this list will have no trouble farting with the exponents if they can't think, or don't want to think in the micron venu.
Electrical "glow modes" in gases (plasmas) can start at many thousands of microns pressure and extend to what is called "extinction" at, or just below, 1 micron.
Most simple, amateur, and even some professional fusion operations are carried out in the 20 to 0.1 micron range.
The modern dual stage mechanical pumps, working alone, can easily pull a vacuum of between 100 and 20 microns, even if in poor condition, providing that the system under evacuation is sealed perfectly and is clean and dry.
New or immacualtely clean rebuilt pumps, working into ultraclean systems of good design, can easily pump to between 10 and 1 micron vacuums. Simple, passive traps added in the foreline can reduce this to the 0.1 micron range.
To go below this and start speaking the exponented "torr speak" demands a new "high vacuum" pump be added, such as a diffusion, cryo, or turbo pump. We will not elaborate on them here.
Measuring fusor vacuums can be accomplished with a simple thermocouple gauge which even new, will cost under $300.00 with tube and gauge metering. (Duniway) Used, such instruments can be found in the under $100.00 price range.
Finally, How much vacuum is enough?
1. Simple demo fusors (good starting point) 50-10 microns 2. Simple neutron producing fusors 10-.1 microns 3. Advanced ion gunned or ionizer grid units <.01 micrcons
In #1 above, the neophyte will get interesting patterns at about 150 microns, but the quills, bugles, and smaller plasma regions aren't seen until 50 microns or a little lower. Star mode operation in air demo fusors occurs in the 15 micron and below range. (by demo fusor I mean a non-neutron producing, non-fusing fusor usually with just remenant air or inert gas filling.)
#2. This is the fusor where the first fusion is seen. It should be a stainless steel vessel which is perfectly sealed and has a low outgassing figure. Pressures above 20 microns are just too high! Your base pressure should be at least 1 micron to begin fusion. Deuterium is backfilled to operating levels between 5 and 15 microns for normal operation.
The levels mentioned in #3 are nebulous and depend on the system size and the scope of the fusion work to be done. In this scenario, base pressures of 10e-7 torr are not uncommon with run pressures in the 10e-4 to 10e-5 torr range. The star modes so brilliantly seen in the simple fusors may disappear of become faint. Special ion gun or ionizer grid schemes are required to create the deuterons.
Vacuum newbies will discover it is not so much the pump, but the sealing of the system and choice of materials contained within it that makes and maintains the vacuum.
Richard Hull
Created on Friday, January 19, 2001 11:22 AM EDT by Richard Hull