What is the world's lowest vacuum pressure achieved?

[dreens]

I'm trying to track down the best (lowest) vacuum pressure ever achieved. I'm talking about absolute pressures, not relative, so I wouldn't call the environment in a submarine the winner even though it achieves a very large magnitude negative pressure differential relative to the high ambient pressures deep in the ocean.

I've heard some talk about CERN having the best vacuum. They certainly have the largest, but I don't know about best. I'm sure they would, if they could keep the whole thing cryogenic, but my understanding is that the experimental hall portions of the beam line are kept at room temperature, and pumped with ion pumps for noble gases and getter coatings for everything else. In this piece, they talk about reaching 10^-10 torr in the beamline.

I've heard other people claim that the Wake vacuum measurement performed by dragging a shield from a space station is the record, but this is certainly not so. They are only claiming 10^-11 torr or so: http://www.sciencedirect.com/science/article/pii/S0042207X01003839. Their original expectation was 10^-14, but this was not reached, possibly just due to bakeout issues.

I've done better myself with an ion pumped vacuum chamber, as is common in AMO physics. I know of several labs that use ion pumps in combination with Titanium sublimation pumps, which deposit Titanium films on the walls to adsorb molecules, to reach pressures below 10^-12 torr.

My suspicion however is that experiments which maintain a completely cryogenic environment should do even better than this, because at those temperatures any metallic chamber wall behaves as a getter or cryopump without the need for titanium sublimated coatings, not to mention that whatever gas isn't adsorbed is traveling at lower speeds and thus exerting a lower pressure force. I believe I've heard people toss around numbers like 10^-16 torr in conference talks, but I haven't found any direct claims.

Can anyone substantiate this hypothesis or point me to an experiment operating in these extreme vacuum regimes? Does anyone know of a lab directly claiming to have the world's lowest vacuum pressure? I know that there aren't gauges at these pressures, but any trapping experiment would have a good guess at their pressure from the lifetime of the species of interest.

Thanks!

 

[Bystander]

http://www.sciencedirect.com/science/article/pii/S221181281400399X

 

[Cutter Ketch]

It's been a while, but long ago I used to be a UHV guy. With care we could open our chambers for maintenance and have them back in the 10s the next day (high E-10 Torr, I mean). We fairly often had chambers in the high 11s. We once worked hard over a long time (months) and had a chamber in the high 14s which is as low (or to be honest, probably lower with any reliability) as our gauges could read.

The lowest vacuums ever achieved are about
1E-12 pascal which is 7E-15 Torr. People have been able to do that for 40 years and it's been achieved many many times. This seems to be largely an instrumentation problem, and theoretically cold trapping below liquid helium temperature should make much lower pressure achievable. However I feel vaguely like I heard of techniques for measuring and claims of reaching the 17s. However I can't find anything to support that now.

Even at the time we laughed at the wake shield facility. We called it stupid from the first day we heard of it long before it flew. In the lab the transport of molecules through the metal seals can be reduced to a level that it never is the limiting factor. This was routinely demonstrated because we looked for leaks by bagging a chamber, filling the bag with Helium and looking for Helium with our high end quadrupole mass analyzer. (We once found a leak in the middle of a wall because the stainless steel casting was porous at this level) When we had all the leaks fixed the QMA read nothing. If I can bag a chamber with an atmosphere of Helium and not see Helium inside at 1E-14 Torr partial pressure sensitivity, then air from the outside is not the limiting factor. What limits the vacuum is outgassing from the walls of the chamber. On the wake shield they thought eliminating the atmosphere would result in miracle low pressure. However the shield itself is just another vacuum chamber wall. There was no reason to think that fighting the outgassing from half a chamber would be significantly easier than fighting the outgassing from a whole chamber, and yet with a limited time on orbit they are in a much worse position to do anything about it. Wrong headed.

 

[dreens]

Hey Cutter, thanks for your reply. That's amazing that you managed to find a porosity leak in steel casting! I've heard of leaks along weld joints but not that.

Yeah I'm with you on the wake shield. Seems like more of a pub stunt to try and get in on semiconductor industry high risk research capital or something. Although maybe they might have something to brag about as far as effective pumping speed is concerned? Not that you'd want to "pump" semiconductor etchants into the atmosphere.

 

[Cutter Ketch]

Hey Cutter, thanks for your reply. That's amazing that you managed to find a porosity leak in steel casting! I've heard of leaks along weld joints but not that.

Yeah I'm with you on the wake shield. Seems like more of a pub stunt to try and get in on semiconductor industry high risk research capital or something. Although maybe they might have something to brag about as far as effective pumping speed is concerned? Not that you'd want to "pump" semiconductor etchants into the atmosphere.

Ha! Yes, well giant rockets should give you a great pumping speed! Actually, as it turns out the pumping speed isn't really that exciting. In a typical UHV chamber molecules which go into the pump effectively never come back. Ion pumps are very good at ionizing everything and burying it deep in titanium (even Argon, but Helium doesn't stay put. That's the death of ion pumps.). We had titanium filaments that would lay down a fresh layer of titanium on the walls of the pump to getter Oxygen and Nitrogen, so those in particular wound up very well bound. Cold traps have essentially no vapor pressure for the adsorbed atoms so anything that lands stays. So the pumping speed is generally determined by how big the port into the pump is. Now the wake shield had a pump that covered more than 2 pi steradians so that's going to be faster than anything I ever used. However, some of those pumps had big throats and covered a fair bit of solid angle in the chamber. Our pump down rates probably were a healthy fraction of the wake shield. Of course the wake shield was also much larger than anything I ever used, so in terms of torr-l/s it was phenomenally fast.

 

[RonL]

I have always had a thought that the clear space behind the space shuttle and between it's vapor trail (during reentry) would be a perfect vacuum. I have never looked at NASA information for any possibility they took measurements in that zone, but have been curious, having watched a few returns moving across Texas.

 

[Cutter Ketch]

I have always had a thought that the clear space behind the space shuttle and between it's vapor trail (during reentry) would be a perfect vacuum. I have never looked at NASA information for any possibility they took measurements in that zone, but have been curious, having watched a few returns moving across Texas.

Well, even more so on orbit. The wake of the space shuttle would be a perfect vacuum if it weren't for all of the junk coming off of the space shuttle itself. That was the idea of the wake shield facility. They towed a giant stainless steel disk on a tether behind the space shuttle. The idea was that it would push aside the remaining atmosphere and in the wake of the shield there would be a perfect vacuum. The shield is necessary because even at the orbit of the space shuttle there is still a fair bit of atmosphere. As it happens it is almost all atomic oxygen (O, not O2) so it is very reactive. At the speed the space shuttle is moving the "stationary" atomic oxygen has a relative kinetic energy of 5eV, so there is plenty of energy to overcome any activation barriers. As a result the space shuttle environment was very oxidizing. In the first shuttle missions some materials in the cargo bay like polystyrene and kapton eroded away to almost nothing. Anyhow, the idea of the wake shield was to eliminate the atmosphere and make a perfect vacuum, but it didn't work. In vacuum chambers on Earth we have no difficulty sealing out the atmosphere. The problem is the walls of the chamber itself are constantly outgassing. That's what happened with the wake shield. It outgassed.

My question is how did they convince NASA to haul a 9 ft. disk of stainless steel into orbit. It must have weighed a ton.

 

[dreens]

I spent a while searching the literature for an actual claim on the best vacuum pressure. I ended up contacting a friend I know on this cryogenic electron beam ion trap (EBIT) experiment. Still didn't find anyone saying "this is the best" but he pointed me to these guys who must be close:

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.106.253001

They claim < 10^-14 Pa ~ 7.5*10^-17 on page 2 top left, and their proton can stay trapped for months.