The Higgs Story-Part 17: Other design challenges of the LHC


Magnetism is weird but in a fun way. Who as a child has not played with magnets and wondered how they worked? And for many a scientist it was what first attracted them to their field. Magnets are our first introduction to the idea of invisible forces that seem to permeate all space and can act to move objects without being in contact with them. Gravity is also such a force but it is too ubiquitous and outside our control for us to notice its peculiarity. We grow up so used to the idea that released objects fall to the ground that we do not give a second thought as to why they behave that way. (For previous posts in this series, click on the Higgs folder just below the blog post title.)

But magnets are different. We can play with them, see how they attract some things and not others, how two magnets can either attract or repel, and how the force varies with distance. The only other force that comes close to displaying acting at a distance in everyday life is static electricity but that is not nearly as commonplace or as much fun.

One of the most important features about magnetism is that a moving electric charge in a magnetic field experiences a force that is at right angles to its direction of motion. This results in the charged particle being deflected from a straight line and moving in a circle. The direction of the deflection (whether the particle is pulled to the left or the right) depends of the sign of the charge, while the amount by which the path is deflected (the radius of the resulting circular path) depends upon the speed of the particle, its mass, the size of the magnetic field, and the amount of electrical charge.

This property has proved to be invaluable is physics and gives us one of the most important tools for identifying particles. By sending particles through detectors (like cloud and bubble chambers) that produce tracks (like airplane contrails), we can observe the curvature of their tracks due to magnetic fields and from them we can deduce the charge, energy, and mass and from that we can infer what particle we are seeing.

This feature is also used in designing the LHC. The accelerator has two proton beams moving in the same circular tunnel but in opposite directions. The circular motion is produced by magnetic fields that act along the entire length of the beam path. The relationship between the size of the magnetic field and these other quantities can be derived quite simply for the relativistic case where particles are moving close to the speed of light and can be done by anyone who has done a college-level introductory physics course. The strength of the magnetic field B that is required is given by B=E/qcR, where E is the energy of the charged particle, q is its charge, c is the speed of light, and R is the radius of the circle around which the particles move.

The easiest way to produce magnetic fields on this scale is to use electrical currents, because electric currents have been known for well over a century to produce magnetic fields. Producing large magnetic fields is not easy, since it requires large currents that not only cost a lot but also result in the wires carrying them becoming hot. A design challenge for the LHC was the fact that, according to the above equation, as the energy of a charged particle increases, it becomes harder to make it go around in a circle because the magnetic field required to do so becomes larger. With the LHC, we were going to energies that had not been reached before.

The size of the required magnetic field can be lowered by increasing the radius of the circle, but in order to save construction costs for the tunnel, it was decided to shut down the LEP (Large Electron-Positron) accelerator that was already operational at CERN and use that same tunnel for proton-proton collisions. But just as there is no free lunch, there is no free tunnel either. For LEP, the maximum energy of the electron-positron collisions was about 200 GeV, or 100 GeV for each particle. For the LHC, the target energy is 14 TeV or 7 TeV for each particle, which means that the magnetic field needed in order to keep the beams moving in a circle of the same radius had to be 70 times larger than what had been necessary for the LEP.

The LHC needed magnetic fields of about 8.5 Tesla. (The SSC would have reached roughly three times the energy of the LHC but that tunnel was designed to have three times the radius, thus requiring roughly the same size magnetic field as the LHC.)

8.5 T is a huge magnetic field. To get some idea of its size, it is about one hundred thousand times the strength of the Earth’s magnetic field. The world record for the largest magnetic field ever produced is 91.4 T but this was produced for a short time in a small region of space. The LHC magnetic fields are about the size of the fields found in modern MRI machines but in the case of the LHC, these large fields have to be kept uniform and precise and stable over the length of the accelerator of over 26 kilometers. Producing such a field over such a distance requires miles and miles of wiring carrying large electrical currents that, in order to prevent overheating and meltdowns, require them to be cooled down to liquid helium temperatures of minus 456 degrees Fahrenheit so that they become superconductors and thus have zero resistance. This is what made the LHC a massive engineering project requiring some of the best scientists and engineers to pull it off.

The slightest glitch in the cooling system can cause a runaway destructive process to occur in which the wires cease to be superconductors and become ordinary resistors which results in greater heat being produced, which results in yet greater resistance, and so on. This happened just nine days after the LHC was turned on September 10, 2008, when a small electrical arc created a hole in the cooling system that released the liquid helium in an explosion, destroying some of the magnets in that area. It took more than a year, until November 20, 2009, for the repairs to be done and the beams turned on again. There have been no problems since then.

Next: What else is the Higgs good for?

Comments

  1. Doug Little says

    The only other force that comes close to displaying acting at a distance in everyday life is static electricity but that is not nearly as commonplace or as much fun.

    Surely you jest, static electricity not fun? You never rubbed your feet on the carpet and then bought your finger close to a friends ear? Never rubbed a balloon on your sweater and then made it stick to someones head or make their hair stand on end? Not fun, are you crazy. What’s fun about watching iron filings align with a magnetic field?

  2. Doug Little says

    Well yeah it’s hard to compete with nature, on the static electricity side we have lightning!

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