When in the Course of Particle Events....

People often ask me "What is ATLAS?" -- which is a difficult question to answer, as it's a highly complex and massive device. In short, it's a general purpose particle detector, which means it's designed to help study a range of phenomena from the Standard Model to SUSY to things beyond our ken. The ATLAS detector has four main subsystems: the Inner Detector, the Calorimter, the Muon Spectrometer, and the Magnet System. My own work involves the Muon Spectrometer -- specifically the End Cap, but that's matter for another post -- but some of more exciting developments of late have occurred in the Magnet System.

The Magnet System is what bends the charged particles produced in each event allowing us to track them and deduce the charge and momenta of these products. As the particles are going nearly the speed of light, the magnets need to be extremely powerful: the Barrel and End Cap toroids generate 3-8 Tesla (T), while the Central Solenoid generates 2 T. How powerful is this? Well, the Earth's magnetic field ranges from 30 mT to 60 mT (the largest values being, unsurprisingly, at the magnetic poles where the magnetic flux lines converge). So the magnetic fields we'll be generating at ATLAS are up to x100,000 the Earth's magnetic field. For this reason, it's strongly recommended for people with pacemakers not to do calisthenics in the Pit when the detector is functioning.

At present, the folks over on Team Magnet are ramping up their tests in the Pit (where the detector does its business). They recently ran a successful test at half the final operational current of 20,000 Amps (A). Yes, you heard me: 20 kA. Forget going up to 11, this sucker is electrical. For reference, heart fibrillation can occur at low voltages (110-200 V) from 60 to 300 mA (depending on AC or DC and path taken to the heart) while a bolt of lightening usually has 30 to 300 kA.

While the test at 10 kA was successful, reaching the halfway mark current-wise doesn't spell smooth sailing from here out. Why not? The current induces a magnetic field (which is the whole point of the current), which then creates a force on the superstructure of the system: the magnetic force essentially tries to bend the system (which is an oval) into a minimal energy state (a circle). So how does the force depend on the current? Let's go back to E&M and first recall Gauss' Law and the Lorentz Force Law....

Guass' Law states that (in a properly chosen geometry) the magnetic field is proportional to the enclosed current. The Lorentz Force Law tells us that the force created by a magnetic field is q v x B (I'll try and upload some nice equations later, but I can't right now). With a little algebraic trickery, we can restate this as I l x B. As B ~ I, we get that the force is proportional to the square of the current.

So the 1/2 full current test is really just 1/4 of the force the system must withstand. Fortunately for us, there will be a full test this weekend to see how the whole thing holds up. Here's hoping....