The Large Hadron Collider (LHC) is scheduled to restart for physics early in 2015 after two years of maintenance and upgrading. The collision energy at restart will be 13 TeV, a significant increase over the initial three-year LHC run, which began with a collision energy of 7 TeV, rising to 8 TeV. But the LHC was designed to run at a maximum collision energy of 14 TeV, so why has CERN decided to start the second run at a lower energy?

The decision to begin the LHC’s second run at 13 TeV has been taken in order to optimise the delivery of particle collisions for physics research, and thereby speed the route to potential new physics. It is based on the properties of the 1232 superconducting dipole magnets that guide the beams around the LHC’s 27-kilometre ring. The higher the beam energy, the higher the magnetic field needed to maintain a constant orbit, and the higher the electric current flowing in the magnet’s superconducting coils.

At LHC beam energies, the electric currents are extremely high, up to 12,000 Amperes, and superconducting cables have to be used. Superconductivity is a low-temperature phenomenon, so the coils have to be kept very cold, just 1.9 degrees above absolute zero to be precise, or about -271°C. Even a tiny amount of energy released into the magnet for any reason can warm the coils up, stopping them from superconducting. When this happens, the current has to be safely extracted in a very short time. This is called a quench, and just one millijoule – the energy deposited by a 1-centime euro coin falling from 5 cm – is enough to provoke one. Magnet protection in case of quenches is a crucial part of the design of the LHC’s magnetic system.

When a new superconducting magnet is qualified for use, it needs to be trained. That involves steadily increasing the current until the magnet quenches, then starting again. At first, the quenches may occur at relatively low current, but over time, as the components of the magnet settle in, the current increases until the magnet can be operated routinely at its nominal current. If a new training cycle is started after an extended period during which the magnet is warm, the magnet usually restarts training at a value that is higher than first quench in the first training cycle but lower than the maximum previously reached. In other words, the magnet’s ‘memory’ is usually less than 100%.

Before the LHC started operation, all of its magnets were trained up to a current equivalent to a collision energy of over 14 TeV. Tests with individual magnets, along with hardware commissioning tests in 2008, have shown that for some dipole magnets the memory is slightly lower than expected, demanding a larger number of quenches to reach nominal field. However, retraining these magnets to 13 TeV should require only a short period of time, whereas retraining to 14 TeV would take longer, taking time away from physics research. That’s why the best way to get to new results quickly, at an energy considerably higher than ever achieved before, is to start operation at 13 TeV. A decision on when to go higher will be taken at a later date in the LHC’s second run.