high vac req'd for mean free path in colliders

[Todd Massure]

How do engineers working on colliders get a high enough vacuum to have the mean free path required for particle colliders? From everything I've read here it seems difficult to get down to 5 microns or so, which I'm sure isn't nearly enough for particle colliders and some of those are HUGE volumes, and so is the mean free path required. I'm even amazed that Lawrence was able to get the vacuum required for his cyclotron in 1938. Is there a trick with the magnetic or electrostatic fields in these devices that helps in some way?

[Richard Hester]

For cyclotrons, absolutely huge diffusion pumps were used. I don't know what was used to pull the vacuum on the first cyclotron, but they had to practically coat the thing in wax to keep it from leaking.
The actual volume in the beam line of a synchrotron or collider is relatively small. For an extremely clean vacuum, you could rough with a molecular sieve pump, followed by a turbo, with an ion pump used to maintain the vacuum once everthing is pumped down and sealed off. Of course, the whole system is metal sealed, probably with some provision for bakedown. At SLAC, each of the giant klystrons used to power the linac has a middling sized ion pump hanging off it to keep the insides of the klystron clean and fresh....

[3l]

Hi Todd:

I can tell you about my beam line vacuum..... 10^-6 torr.
Your diff pump has to have a pumping capacity a minimum of eight times the volume to be pumped. You can manage it with a diff pump followed by an ion pump. It takes about six hours of pumping with a Welch forepump..... then the diff pump pulls the vacuum down to around 10-5 torr....finally four hours with ion pump. I heat the beam line with a propane hand torch during pump down. When the ion pump is running the diff pump is valved off from the system. That prevents backstreaming of oil. It is strickly a batch process in small scale. The 10^-6 will only last until the ion pump fills up. Runs rarely last more than an hour. I'm working up to heat tape that would elminate hand heating but I'm a cheapo.

Happy Fusoring!
Larry Leins
Fusor Tech

[raneyt]

Todd,

Hope you are well. As in any vacuum system, size matters. Evacuating a huge accelerator is no different than evacuating a 6” diameter Fusor chamber. At least in principle. They just use bigger pumps with tremendous flow rates.

However, the important factor (all things being equal) is that these large systems are always dynamic; the pumps never shut down. Good luck with your experiments.

Warm Regards,

TIM RANEY

[Richard Hull]

In large scientific systems, especially giant accelerators, 10e-7 torr is considered a very poor and dirty vacuum. (This is 10,000 times deeper and cleaner than the vacuum in a fusor)

Jefferson National Laboratory here in VA strives for vacuum levels of 10e-10 torr or better and normally work their best stuff at 10e-11 torr. They have achieved vacuum that are truly incapable of being measured in small systems. (10e-13 torr is considered the technical limit of measurement)

It is not hard to get a vacuum 10 million times deeper and cleaner than in our fusors if you have hundreds of millions of dollars to spend on the vacuum system alone. The main pumps on such large systems run 24-7, for the most part. Breaking a beam line down in a large machine for repairs can easily be a 40 day job with most of it in getting the vacuum back on line.

There is a big difference between a chincy little $1500 amatuer vacuum system and a million dollar plus vacuum system found in even the smallest of scientific experimental accelerator stations.

Good vacuums are purely a matter of cash! Some might say there is skill invovled too. NO! That reverts back to cash inorder to pay for the finest vacuum engineers and techs to design, assemble and tend monster systems.

It is true that a super skilled, crafty and gifted vacuumist can take $2000 and get you a 10e-7 vacuum at the pump head with no problem, but that is a zero volume chamber.

Amateur Scientists, for the most part, struggle along achieving only that level of vacuum needed for experiment at the minimalist cost level.

Neopyhte vacuumists have at least 6 unevenly spaced, very rude awakenings as they stumble innocently into the world of vacuum technology.

1. Vacuum stuff is expensive
2. Vacuum pumps do not suck the air out of anything
3. You can't just attempt to seal a jar and take it to any vacuum level you wish.
4. Water on planet earth is everywhere and is the last molecule to leave the party.
5. All things exhail into vacuums for hours and weeks.
6. As the vacuum demanded goes to lower orders of magnitude, the difficulty in paying for and maintaining it goes UP in inverse proportion.

Richard Hull

[Todd Massure]

Thanks for all the great info everyone.
As I am getting much closer to beginning an actual fusor project I'm finding myself taking more interest in all forums rather that the fusor theory forum as was my main forum of interest before.
I've been searching this forum but can anyone tell me some good threads to read about
1) What is the difference between different types of vacuum pumps (turbo, diffusion, ion) / brands available
2) What are those pumps cabable of
3)What kind of prices can I expect to see for these
Links to threads are great, but if anyone is willing to really tackle all these questions, in one place that would be really appreciated if so, please put a F-A-Q on it
thanks
-Todd

[Roberto Ferrari]

Richard,
I will make a comment on your panoramic view of amateur scientist high vacuum technology.
Fifteeen years ago, I optimized in my small lab a cheap technology to reach 10-9 torr, learned from....Electronics magazines from the 30 or 40s.
In those days, there were full page advertisements of vacuum pumps and a side point was the technology involved. If you go to your collection of Electronics (I am sure you have it!), you will learn that in those days, after a diffusion pump they used a "charcoal trap". It was a trap with a cage full of coconut charcoal, with a low voltage heater, with a feedtrough for the positive pole, ground used to close the circuit.
Rather weird to tame, the system was turned on in the following way:
1) mechanical pump on and charcoal heater on,
2) then you monitor low vacuum at the point between the diffusion pump and the mechanical pump
3) after a long time of evolving a lot of gas, the vacuum started to improve,
4) then you turn on the diffusion pump
5) finally, you have a good diffusion vacuum of around 10-6 torr...
6) then, you turn off heating to your charcoal trap...
7) and in half an hour your vacuum improved two orders of magnitude!

In my experiments, I replaced the charcoal by molecular sieve pellets, in a SS system, that every time the vacuum was broken ,was filled with dry N2.
Then, after several cycles, I finally reached 10-9 torr, several hours sustainable.
I employed this technique to rebuild power grid tubes and x-ray tubes.
Hope this will be helpful for "the poor man approach".
Roberto

[DaveC]

A tight, clean vacuum system, glass or metal pumped by a clean diffusion pump and a clean, tight roughing pump, should routinely reach 1.0E -7 Torr, without a Liquid nitrogen trap. With LN2 you can expect low 10-8torr pressures.

But there are a few caveats here. Tight and clean are immensely important. At 1E-7 torr, the residual pressure is 10 billion times lower than atmospheric pressure. What this means is that every cc of atmospheric pressure gas that leaks in, expands to 10 billion cc's.... about 10,000 cubic meters. That will take a very long time to pump out. So... gas in-leak is a very important issue...Hence a "TIGHT" system is a must.

Then there is "CLEAN". The oil from a single fingerprint will evaporate so much vapor that it will load up a good vacuum pump for hours or even days. So wise experimenters don the latex gloves during cleaning and assembly. What you gain in pumpdown speed, is well worth the inconvenience and expense of working with the gloves on. Just be sure they are the dust free type.

All surfaces adsorb gasses, water and contaminants. The gases desorb fairly easily, especially at higher temps.... so a good bakeout is a big help here. Water is a somewhat different material to remove. Being a polar molecule, it bonds a lot more strongly, and desorbs rather slowly at room temperature. So, again, a good bakeout will help remove water.... but.....you must keep the lines hot all the way to the low vacuum pump (out past the diffusion pump, the turbo-molecular pump, or the ion pump). If only some of the plumbing from the chamber to the roughing pump is heated, then water simply leaves the hot place and re-adsorbs at the next cold site.. Thus, you will never get rid of water, unless the entire route through the HiVac valve to the roughing pump is heated.

Then there is the outgassing of all surfaces... This gas, which can include water, comes from all depths of the material. Residual hydrocarbons, oils, solvents, hydrogen and etc. slooooooowly diffuse up to the surface and escape into the vacuum system. Stainless steel at room temperature will outgas for hundreds of hrs. A high temperature bakeout is needed to shorten this process. Typically, you need to be at about 800 C for a couple hours to get most of the gas baked out. (Obviously, this would rule out any non metallic O ring seals, and would kill the seals on virtually all of the HiVac valves we would commonly use.) The alternate is a much lower temperature bakeout...say 100C... and hold this for a week or two.

But ... most of this is unnecessary for the Fusor realm. A clean system is very nice, but... every time a fusor run is made, the chamber is filled to 1 - 20 micron pressures with deuterium or in the case of the demo mode.. plain ole atmosphere, replete with 1000's of ppm of water and other things.

So...while this quest for ultra-clean systems is a noble challenge, and would be needed for a permanently sealed system, it is really an expensive luxury for us penny pinchers..on limited budgets trying to do big research. Be content to practice good high vacuum technique... bakeout (100-150 C) dry gas flushing, use of gloves, and save your money for the istrumentation without which you cannot prove that fusion is taking place.

Dave Cooper