Dr. Bortrum
10/09/2008
Helium Spill
Next to hydrogen, helium is the most abundant element in the universe, comprising nearly a quarter of all ordinary matter. Most of of the helium in the universe was formed shortly after the Big Bang. Another source is hydrogen nuclei getting together (fusing) to make helium in our sun and other stars. Here on Earth, helium is not so abundant, almost all of it formed when uranium or radium decay giving off so-called alpha particles, which are simply helium atoms without their electrons. These alpha particles quickly find some electrons to make themselves into helium atoms. Fortunately, significant amounts of helium are found in natural gas deposits, the source of most of the helium produced in the United States.
An article by Marc Reisch in the October 8, 2007 Chemical & Engineering News deals with a worldwide shortage of helium. I won’t go into the reasons for this shortage here but you might be surprised to find that Exxon Mobil is a big supplier of helium. Reisch noted that Exxon Mobil’s Shute Creek plant in Wyoming supplies some 20 percent of the world’s supply of helium. Liquid helium, which boils only a few degrees above Absolute Zero, is used to cool superconducting magnets in such things as MRI machines used in medicine. I have an attachment to helium dating back to my days as a graduate student at the University of Pittsburgh.
My first publication was as a co-author of a note in the Review of Scientific Instruments on a vacuum drybox we constructed to handle reactive materials such as molten soduim-potassium alloys. We had purchased one of the first commercially available machines to make liquid helium and used helium as the inert atmosphere in our drybox. Of course, we played with the helium, inhaling it in order to get that squeaky high-pitched voice.
Helium was in the news recently. Until the past couple of weeks and the $700 billion bailout, I thought $8 billion was a lot of money. That’s one figure quoted for the cost of the Large Hadron Collider (LHC) in Europe. The LHC is the humongous machine that will send proton beams hurtling towards each other in the hopes that the collisions will create conditions not seen since the Big Bang. If you saw "60 Minutes" a couple weeks ago, you have some idea of the enormity of the LHC. The LHC is also the machine that some hopefully ill-advised individuals think will create tiny black holes that will gobble up us and the Earth. Last month, the LHC was turned on and, on September 10, the first proton beams were sent racing around the 17-mile underground tunnel. Researchers were ecstatic.
However, only nine days later came a crushing blow that puts off at least until spring any hopes of collecting data on colliding protons. According to a news item by Adrian Cho in the September 26 issue of Science, the incident that shut down the LHC was a "quench". To me in my past life as a sometimes metallurgist, quenching meant taking a hot chunk of metal or alloy and plunging it into water in order to preserve a high-temperature alloy structure. Another way to quench an alloy was to "splat" a molten blob onto a cold copper or other metal block, instantly freezing the sample, sometimes in a glassy state unobtainable by other means. In either case, the rapid cooling freezes in the higher temperature structure by preventing the atoms from rearranging themselves to form the normal low temperature structure.
But I digress. The LHC quench involved not a cooling but a disastrous heating. To contain the bunches of protons on their proper paths around the tunnel requires a very strong magnetic field generated by huge superconducting magnets weighing 30 tons or so. The superconducting magnets are essentially coils of superconducting wire through which high currents are passed to generate the magnetic field. The more turns in the coils of the magnets and the higher the current, the higher the magnetic field. Ordinary magnets would not do the job because of the high currents. The more turns in the coils of a magnet the higher the electrical resistance. When you pass current through a wire it heats up; think of your toaster. The higher the current, the higher the temperature.
In a superconductor the electrical resistance goes to zero below a certain critical temperature. No resistance, no heating (at least up to a certain critical current). Hence the use of superconducting magnets; with no electrical resistance, very high currents can travel through the coils without generating heat that could melt or deform ordinary coils. The coils are cooled with liquid helium to temperatures only two degrees above Absolute Zero, lower than the temperature in outer space. According to an article by Gabrielle Walker in a Discover magazine (I believe it was the August 2007 issue), 185,000 gallons of liquid helium are used to chill those coils in more than 1,200 superconducting magnets in the LHC.
On September 19, the CERN researchers were increasing the current in a chain of magnets when something happened. A superconducting connection between two of the magnets heated up. When this happens, the superconductor is no longer superconducting but has electrical resistance. Now you have a toaster. The connection melted and a ruptured helium line vented a bunch of helium into the tunnel. It was a big bunch of helium, about a tonne (slightly more than our ton) according to some accounts. An AP item in Tuesday’s (October 7) Star-Ledger says the quench is believed to have been due to a faulty soldering job, just one soldering job among thousands of solders. It’s amazing how one simple defect can bring down a colossus.
Last summer, I was shocked when my electrical bill was over $300 for the month. When the various fees were added in I was paying about 19 cents per kilowatt-hour, not the 10 cents or less back in the not-too-distant past. Well, the LHC researchers must have the same problem. I don’t know the figure for the power consumption of the LHC but it must be tremendous. Even before the quench, plans were to shut down the collider for the winter months because the power rates in Europe are significantly higher in winter. As a result of the "quench", the LHC has to be warmed up and then cooled back down before any further operations can begin. Just the "simple" process of heating up the LHC in order to fix the problem and then cooling it back down takes about two months or more!
You don’t just pour in liquid nitrogen to cool the LHC down. It takes something like 10,000 tonnes of liquid nitrogen to first cool things down to about 80 degrees Kelvin. Then you start cooling down to 2 degrees Kelvin with liquid helium. As far as damage to the LHC is concerned, I saw in different articles speculation that the actual fixing of the machine will only take a few hours; there was also speculation that a $900,000 magnet might have to be replaced. I sure hope it’s the former!
I can imagine the disappointment of the researchers at the LHC at having to put off colliding those protons. In addition, the financial meltdown in Europe, as well as in the U.S., must have further sobering effects. Perhaps, to lighten the tension at the LHC, the workers can take whiffs of helium and tell some of those awful jokes that circulate on the Web in the resulting high squeaky voices. Or, maybe not.
Allen F. Bortrum
