# How much power does it take to maintain the high vacuums in advanced physics experiments?

The LIGO Gravitational-Wave Observatory and CERNs Large Hadron Collider both have some impressive ultra-high vacuum systems. For my project proposal I need to demonstrate some understanding of how much power it takes (in practice) to maintain an ultra-high vacuum. This could be Watts as a function of vacuum level and volume (or surface area) of the evacuated system.

• CERN has a nice presentation on vacuum available: indico.cern.ch/event/173359/contributions/276007/attachments/… Commented Jan 11 at 17:27
• Consider that an off-the-shelf vacuum flask (e.g. Thermos/Dewar) is vacuumed once, sealed, and then requires no maintenance or upkeep. It's not an extreme vacuum, but it's evidence that it's not the maintenance of the vacuum that requires the energy; it's mostly keeping it cold, and keeping it cold is an easy way to make your vacuum slightly better. Commented Jan 12 at 5:44

As someone who has worked on an experiment with an ultra-high vacuum, I can tell you an order of magnitude. However, the real answer is that asking how much power it takes isn't really the right way to think about how a vacuum is maintained.

To my knowledge, the "strongest vacuum" (the experiment which has measured the lowest upper bound on how many molecules are in their vacuum) is BASE's. They phrase this as a limit on the lifetime of the antiproton, but assuming that the antiproton lasts forever, this is a really strong bound on how much ordinary matter is in their vacuum system.

Improved limit on the directly measured antiproton lifetime

BASE doesn't have any vacuum pumps pumping air out of their experiment. They "bake" their experiment (heat it to a reasonably high temperature) with pumps running about once a year. Then they pinch off a copper tube, sealing the experiment closed for the rest of the year. Then they immerse the experiment in liquid helium, and most of the lingering air is stuck to the walls of the experiment. However, it takes a lot of energy to produce liquid helium. IIRC from working in that building that BASE uses a $$500\,\mathrm{l}$$ tank of liquid helium once every two weeks, which probably takes about $$10,000\,\mathrm{kWh}$$ to produce. However however, BASE doesn't expend liquid helium because somehow the maintenance of the vacuum evaporates it, their loss of liquid helium just comes from thermal conduction from the outside world. It wouldn't be fair to call this "energy used to maintain a vacuum."

The experiment I work for, ALPHA, also uses cold pumping. We use a lot more helium, about $$2000\,\mathrm{l}$$/week. But we also run pumps because the experiment isn't closed off like Base's is. This is... idk... a thousand watts total. Maybe ten total pieces of equipment (ion pumps, turbomolecular pumps, roughing pumps), 30ish if you include things like controllers and vacuum gauges, all which can run on ordinary mains power. But it's not like the energy used to run those pumps comes from some kind of thermodynamic rule about how much energy it takes to remove gas from a vacuum. Indeed it does fundamentally require some energy to move gas from a low pressure system to a high pressure one, but mechanical resistance and the simple operation of electronics is really what requires power on those pumps. Once the vacuum is reasonably good their power draw just to remove the gas produced by off-gassing metals in the vacuum is extremely minimal. It only takes a few minutes of the pumps "straining" before they are no longer using significant power. Worth noting also that a lot of our experiment works on cryocoolers operating at $$4-6\,\mathrm{K}$$ instead of liquid helium, and when this is specifically for vacuum it's called a cryopump. This is essentially a fridge that uses high pressure helium as the working fluid. There's a thermodynamic limit on how much work it takes to take a certain amount of heat out of the low temperature system and put it into the rest of the room at $$300\,\mathrm{K}$$ (given by the Carnot efficiency). But again this is entirely limited by heat flux from the lab to the cold region; nothing to do with maintaining the vacuum, even if that's the goal of the cryopump.

The LHC is more like ALPHA, but at a much bigger scale and everything is done much better cern website on the LHC's vacuum. They bake at 300 celsius (which requires energy to keep something that hot for a few weeks); most of their beamline is immersed in liquid helium at $$1.7\,\mathrm{K}$$. They also have non-evaporable getter coating (which is just a coating on a wall; no energy needed). The LHC interior parts are also all vacuum fired (heated to many hundreds of degrees celsius in a specialized vacuum chamber), which definitely takes A LOT of energy. Again, lots of equipment that uses plenty of energy. But none of that energy usage is really fundamentally necessary. It's just how the technology works at the moment.

And in all three cases the cryogenics serve more purposes than maintaining the vacuum. ALPHA and the LHC also use the liquid helium to operate superconducting magnets. ALPHA and BASE also rely on the helium to maintain low particle temperatures.

• In order to get from the (very interesting -- thanks for the insider account!) engineering back to physics: As far as thermodynamics go, the bulk of the work is done when "most of the air" is pumped out, after a few minutes; everything else is, so to speak, upkeep of infrastructure not directly related to the underlying thermodynamic physics (i.e., to get the remaining few molecules out requires only negligible work as such). Commented Jan 11 at 16:24
• Yes, I really appreciated gaining that perspective. It is obvious in hindsight, but somehow I got it in my head that "more vacuum" meant "more power" and that there would be some kind of unfavorable power-law relationship between the two. Commented Jan 11 at 23:03
• @Peter-ReinstateMonica This site has an large majority of theorists and people who have only been exposed to physics in a classroom (ex. undergrad students) and as such it has a theory bias. The vast majority of physics departments in the world include experimental physics under the umbrella of physics. I think vacuums, cryogens, lasers, beams, magnets, etc (the main tools of experimental physics) are physics. They are typically built, characterized, studied, and optimized by people with the job title "physicist" not "engineer." My physics undergrad didnt teach me much about them though. Commented Jan 24 at 21:54