Superconducting Magnets in Particle Accelerators

Human curiosity has guided mankind to its greatest achievements. The old challenges though of mapping new territories and building transcontinental transportation have been met. Focus has now shifted and leads deeper into the infinitesimally small building blocks of life. Using the brute force of particle accelerators we have been able to smash open these particles and write new chapters in our textbooks. For this exploration to continue though, we have to introduce new technologies in order to improve our methods. Superconductivity, one of the few perfections mother nature has given us, has found its place in particle accelerators. Through the use of superconducting magnets, we can now harness the power of these accelerators to their fullest extent. Without thinking of superconductivity an obvious question would be how magnets play a role in particle accelerators. We first must understand a beam is just a large amount of charge particles and during these accelerator experiments the goal is to get as energetic and dense of a beam as possible onto your target. In order to do this we must focus and guide our beam. Through the introduction of two simple concepts, the Lorentz force and the Biot-Savart law, we can understand how magnets can fill this role. The Lorentz force tells us that inside a uniform magnetic field moving charge particles are deflected at right angles to the field. This will create a circular path moving along the direction of the magnetic field. This can be thought of as beads on string. The string representing the magnetic field and the beads are the particles. The beads are always “frozen” to the magnetic field moving in its direction. Now we must create the magnetic field somehow. In order to do this we must introduce the Biot-Savart law. This simply states that a magnetic field will arise from a steady current. Think of a wire with a current going through it. A magnetic field will form around the wire. Both of these are positive effects allowing us to guide our beam in a desired direction, we do have a problem though. Looking at the the Lorentz force again we must realize that the intensity of this force is dependent on the intensity of the magnetic field. In particle accelerators we require a large field which then gives us a lot of force which creates major strain on the conductors being used to run the current through. Now that we have the ability to guide the beam we must now look at how focus it. When it comes to focusing we again can look to the Lorentz force. We can see that the magnetic force depends on velocity but the electric field does not. We can use perpendicular magnetic and electric field lines to select out particles of a particular velocity. It they are of the required velocity they will be unaffected. This will clean up our beam and not have it diverge into different directions. Another tool we have is to use a wedge shape magnet in which particles farther out will be bent back into the focus of the beam. This is called weak focusing. We can then use quadrupoles to focus our beam. In this instance we use a focusing magnet followed by a gap then a defocusing magnet then another gap and it repeats. This is referred to as strong focusing. This is the role magnets can play in out accelerators. Having built a foundation of the basics we can now appreciate the advantages and see the disadvantages of superconductors in accelerators. The most obvious advantage in superconductivity is the fact that we have zero resistance when running a current. This will save on power consumption significantly. The power cost will instead come from cooling the superconducting material to the required temperature. Another advantage is the ability to obtain a high current density. This will allow for stronger magnetic fields. The downsides include initial cost of the magnetic is very high. Dangers in the use of the magnet can be significant if it is not cooled properly. Looking more closely at the high current density we can see why superconducting magnets are so much appealing. The more powerful accelerators require a larger magnetic field to focus the beam. Standard Iron magnets are limited to 2T while other materials such as copper may reach 100T but for very short periods of time. Superconducting magnets reaching 10-20T with much better current densities are superior to any regular magnet. The most popular material used is Niobium Titanium for its good mechanical properties and durability. Other materials can provide better fields and current densities but are more brittle.