Thursday, September 13, 2007

The secret joys of running a lab: helium.

In my lab, and in many condensed matter physics labs around the world, we use liquid helium to run many of our experiments. At low temperatures, many complicating effects in condensed matter systems are "frozen out", and it becomes easier to understand the effects that remain. Often we are interested in the ground state of some system and want to reduce thermal excitations. Quantum effects are usually more apparent at low temperatures because the inelastic processes that lead to decoherence are suppressed as T approaches zero. For example, the quantum coherence length (the distance scale over which the phase of an electron's wavefunction is well defined before it gets messed up due to inelastic effects of the environment) of an electron in a metal like silver at room temperature is on the order of 1 nm, while that length can be thousands of times longer at 4.2 K, the boiling point of liquid helium at atmospheric pressure. Those kinds of temperatures are also necessary for running good superconducting magnet systems.

The downside of liquid helium is that it's damned expensive, and getting more so by the minute. Running at full capacity I could blow through several thousand liters in a year, and at several dollars a liter minimum plus overhead, that's real money. As a bonus, lately our supplier of helium has become incredibly unreliable, missing orders and generally flaking out, while simultaneously raising prices because of actual production shortages. I just had to read the sales guy the riot act, and if service doesn't improve darn fast, we'll take our business elsewhere, as will the other users on campus. (Helium comes from the radioactive decay of uranium and other alpha emitters deep in the earth, and comes out of natural gas wells.) The long-term solutions are (a) set up as many cryogen-free systems as possible, and (b) get a helium liquifier to recycle the helium that we do use. Unfortunately, (a) requires an upfront cost comparable to about 8 years of a system's helium consumption per system, and (b) also necessitates big capital expenses as well as an ongoing maintenance issue. Of course none of these kinds of costs are the sort of thing that it's easy to convince a funding agency to support. Too boring and pedestrian.

Fortunately, when you work at really nanometer scales, interesting physics often happens at higher temperatures. I've been lucky that two major things going on in my lab right now don't require helium at all. Still, it's bad enough worrying about paying students without the added fun of helium concerns.

UPDATE: See here.

12 comments:

Abhay said...

Hi Doug,

Our lab is facing the helium crunch too - its making life miserable not knowing whether your experiment that's been running for a few weeks will make it though the next few days.

Cryofree has to be the way ahead. I'm betting that with the helium scarcity (I hear we are draining worldwide reserves at something like 10% a year!) and all the new security gizmos coming up that use low temperature spectrometers, cryofree technology is going to really take off.

CarlBrannen said...

Rather than re liquify your helium, is it possible to store it as a gas under pressure and let someone else cool it back down? Or liquify it at some central point on campus?

The helium that is sold to fill party balloons isn't refrigerated, I believe, but instead is simply pressurized gas. I wonder how big of a bottle you'd need, at 200 bar say, to store a week's worth of your evaporated gas. Hmm. PV = nRT or something like that.

Doug Natelson said...

700 gas liters at STP = 1 liquid liter. You need lots of high pressure tanks to hold a decent amount of helium. If you're going to all the trouble of collecting the gas and compressing it, you might as well liquify. Bear in mind that getting any air contamination in your gas is bad news. Good-sized liquifiers can handle up to maybe 1% of N2 contamination.

Dan M said...

When you complete your transition to full-time optics jock, it will be much less of an issue for you. Life is much easier when your energy scale is 2eV instead of 0.002.

Aaron said...

Some more fun LHe news. Mike Lilly was saying one of the main suppliers in the NW has to shut down their operations for a month to fix equipment. We were told to assume only 50% of our orders will be filled in October. Since we get our LHe from the outfit in Dallas, I think this crunch is going to be bad all over the country. Keep in mind that national labs get priority after hospitals so I can imagine things being even worse at universities. We just had the "liquifier vs. cryogen free" discussion a couple days ago.

thm said...

Perhaps you can blame John Reppy for the shortage... after all, he was co-chair of the NAS committee that produced the report that concluded, in part, that "The price of helium will probably remain stable through at least 2010." Has there been any analysis of whether the selling of the federal helium reserve is responsible for the current shortage?

The calculation that cryogen-free = 8 years of helium purchase could change drastically.

Are there any small-scale recovery/ liquifier solutions that are feasible for labs with a couple of cryostats?

CarlBrannen said...

Say 2000 liters of helium per year, or about 40 liquid liters per week.

One liquid liter = 600 gas liters, so get 24,000 gas liters per week.

A 12" balloon = 15 gas liters, so this amounts to 1600 standard helium balloons per week.

An "L" size party helium bottle fills about 500 or 600 12" balloons, so your surplus helium will be about 3 L bottles per week. This seems manageable.

Of course there will be a high degree of contamination in your captured gaseous helium. That's why it's not convenient to just reliquify it. You have to purify and reliquify with some sort of distillation apparatus I suppose. Instead, it'd be a lot easier to just refill the standard party helium bottles, and then collect those at a single site that makes liquid helium the usual way.

To do this, all you need is a 200 bar pump. Shouldn't be that much of a problem to set this up. All you have to do is set up a recycling agreement with your helium liquifier.

Narasimhan said...

Dear Doug,

I read your blog regularly and really like the way you explain the recent results.

I worked for a year at Low Temperature Lab in IIT Kanpur Physics department India (before coming to Stony brook for graduate school) where we had our own liquid nitrogen and liquid helium facility. I even used to do the liquification and it is 40 years old. When I was there, I was amazed at the extensive collection pipes that ran criss cross the big lab and we used to store the evaporating gas in a big plastic can and then pressurize them in ~30 cylinders. Of course there were over head losses, but it was an amazing system.
At that time I was told that in US they let the helium to evaporate.

The facility still runs and they even sell the helium to nearby labs outside of kanpur.

With Regards
Narasimhan

Andrew said...

We're facing the helium crunch here at Yale too. While delivery is still fairly reliable, we were just hit with a 70% price increase! That's quite a jump... Cryo-free certainly seems to be the way to go, at least for people who are looking to buy fridges anyway.

adrian said...

Has someone some experience with cryofree cryostats for quantum transport in nanosystems?

I saw once such a system, and it is very noisy (a lot more than ordinary pumps). The salesman said that the system is made to suppress vibrations at the sample. However, I am afraid that the vibrations cannot be completely suppressed, and that this generates noise in the electric signal.

In Europe, it is very expensive to buy He from a company (between 5 and 10 dollars per liter). Most of the labs have a liquefier. The investment is important. It costs about 500 kilo dollars for a liquefier that produces about 800 liters/week. The space for the liquefier is also an issue. You need more than 50 m^2. The floor has to sustain the few tons of the liquefier system. The compressors generate important mechanical vibrations.

Helium has become an important issue for me. I have started in a new research center without liquefier. I plan to have from next year a bill of about 70 kilo dollars per year for my two cryostats. I have the money for the next two years. However, what will happen after, if I am not successful with grants?

Doug Natelson said...

My group at Stanford ran a helium liquifier when I was a grad student. Liquifiers cost about $500K for the hardware, 50 m^2 for all the associated equipment, and a significant investment of time and cost in preventative maintenance. They're complicated machines with lots of moving parts. Screw up the recovery system so that your helium gas supply gets contaminated with air beyond 1% or so, and you have to basically throw it out and start over. The best way to run a facility like this is to have a dedicated (half-time) technician. You also need to have all of your helium users in buildings sufficiently close together that you can run the recovery plumbing.

As a price point of reference, Quantum Design will sell you an "evercool" dewar for their measurement system that's basically a closed-cycle liquifier for around $130K. Normal helium usage on that system would be around 2000L/yr.

Cryogen free systems still have some issues. You can get a 1.5 W cooling power 2 K pulse tube refrigerator that's pretty quiet vibrationally, but it requires an 11kW water-cooled compressor to run. Not too great a coefficient of performance there.

Dan M said...

Frank Hegmann tells me that they're paying upwards of $13/liter for LHe in Canada. And that's real dollars because Can$1 = US$1 these days. I can only imagine that it is more expensive there because of import tariffs or something. Seems to me that it should be cheaper in Canada, seeing as it's already 77K outdoors most of the time.