Subatomic traction

The trajectories of protons in the LHC are controlled by magnetic fields. An upward-pointing magnetic field (B) applies a force (F) to the right on protons flowing through the beam pipe (into the plane of the picture), and this steers them around the (imperceptible) curve of the ring. Image courtesy of Jim Pivarski

Manipulating small objects, such as the cogs of an old-fashioned watch, is difficult with bulky fingers, so special tools are needed to fit them together into a working system. The emerging field of nanotechnology is concerned with manipulating objects the size of atoms and making molecular machines. Protons, electrons and other subatomic particles are hundreds of thousands of times smaller than an atom, so even the techniques of nanotechnology cannot help. How do you grab and move a proton?

Perhaps surprisingly, all you need are simple electric and magnetic fields, such as what might be covered in a first-year physics class. There are two basic interactions: Electric fields accelerate positively charged particles in the direction that the electric field points, and magnetic fields accelerate them at right angles to the magnetic field and the particle’s original direction of motion.

The latter case is illustrated in the photo of an LHC section above. Positively charged protons travel through the beam pipe, and magnets around the beam produce an upward-pointing magnetic field. The direction that is perpendicular to both the protons’ trajectories and the upward-pointing field is to the right. By always turning the protons toward the center of the ring, they stay within the beam pipe as it curves around its 17-mile circumference.

The trick is building a strong enough magnet to keep 7-TeV protons within the ring — the LHC magnets need to produce 8.3 teslas of field strength, which is 130,000 times stronger than the Earth’s field. This is accomplished by making the coils of wire in the electromagnet out of superconducting wire. The bulk of an LHC magnet is for cryogenics to keep the wires at a low enough temperature to superconduct (hold a current with zero resistance).

Although a magnetic field can bend the path of a stream of protons, it cannot increase their speeds. This is because the magnetic force is always perpendicular to the direction of the protons’ motions. To accelerate protons up to 7 TeV, one needs a force pointing in the direction of their motion, which can only be accomplished by an electric field.

Strong, steady electric fields are hard to build, since they tend to discharge by emitting an electric spark. Strong oscillating fields — also known as radio waves — are easier, since they switch directions before a spark has a chance to develop. Unfortunately for an accelerator, this means that the electric field is pointing in the wrong direction half of the time. The solution is to replace a continuous beam with a staccato beam of short pulses known as bunches, and coordinate the bunches to enter the electric field only at those moments when it is pointing in the right direction. Needless to say, the timing is tricky.

The basic physics of a proton accelerator is straightforward enough to be within reach of a first-year physics student, but building an accelerator for energy or intensity frontier physics pushes the limits of modern technology. A minute to learn, a lifetime to master.

Jim Pivarski

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One proton beam at the LHC is propelled forward by electric fields and inward by magnetic fields, thus traveling in a clockwise direction. A second proton beam is similarly steered by electric and magnetic fields in the opposite direction, guided to collide with the first.