Uses the big hadron collider superconductor

Magnets in the LHC - indispensable and sensitive

In September 2008, protons circled in the world's most powerful particle accelerator for the first time, but a short time later a technical accident occurred and paralyzed the LHC for many months. The cause: the energy stored in the magnets was suddenly discharged. Technicians and scientists have learned from the incident.

The LHC is a particle accelerator almost 27 kilometers long that brings protons and charged atomic nuclei to high energies and then collides. The most important components of the accelerator are the superconducting magnets: a total of 1232 dipole magnets - i.e. magnets with a north and a south pole - keep the particles on a closed path within the tunnel. The magnets, each 15 meters long, enclose the beam tubes in which the protons rotate and create a dipole field that is adapted to the particle energy: A beam energy of seven teraelectron volts, for example, requires a magnetic field of just over eight tesla - that is almost a hundred times stronger than a commercially available one Horseshoe magnet.

Such magnetic field strengths are generated at the LHC by electromagnets, i.e. with the help of an electric current that flows through a coil. Accuracy is required: the field may only deviate by around 0.01 percent from that of an ideal dipole over the entire length of the magnets. Otherwise the protons could not be kept in the storage ring long enough to carry out the desired experiments with them. The course and quality of the magnetic field lines are also determined by the arrangement of the coil windings. The requirements for the magnetic field quality in the LHC demand stability and accuracy in the order of a few micrometers.

Hardly any leeway

In order for the coils to achieve the required magnetic field strength, currents of more than ten thousand amperes are necessary - such a high current can no longer be conducted through ordinary copper cables. This is because, on the one hand, they would heat up so much due to ohmic losses that the coils melt during operation, and on the other hand they would mean unacceptable energy consumption for the operation of the accelerator. This is why superconducting cables are used in the LHC, in which the current flows without loss.

Insight into the storage ring

The superconductors consist of a niobium-titanium alloy, similar to other accelerators such as the Tevatron in the USA. However, the magnetic field in the LHC is almost twice as large as that in previous systems. The LHC thus fully utilizes the capabilities of the niobium-titanium cables. The operating conditions required for superconducting cables pose a particular challenge: With a magnetic field of eight Tesla and a corresponding current density of around one to two thousand amperes per square millimeter, niobium-titanium conductors have an operating temperature of less than minus 271 degrees Celsius, that is only a good two degrees above absolute zero. If the temperature exceeds this value when the magnets are in operation, the cables become normally conductive and, as in normal copper cables, ohmic heat losses occur, which causes the material to heat up.

Even a small, local heating of the cable can therefore lead to an inexorable loss of superconductivity. In such a case, the electromagnetic energy stored in the coils must be dissipated as quickly as possible - before the resulting heat damages the magnet. Physicists refer to the sudden transition from a superconducting to a normally conductive state as a "quench". Since a quench cannot technically be ruled out, every magnet must be designed and built in such a way that it can survive such an incident undamaged during operation.

Loss of superconductivity

During the LHC operation, a protection system continuously measures whether there is a voltage difference between the electrical input and output of the solenoid coil. Because with an ideal superconductor and constant current, this should not be the case. If the measured voltage exceeds a specified limit value, the so-called quench protection system initiates the following steps: The protons are diverted from the accelerator in order to avoid further secondary damage from the energy stored in the beams. In operation with 7 tera electron volts, corresponding to a collision energy of 14 tera electron volts, an energy of around 370 megajoules would be stored in each of the two proton beams - enough to suddenly bring a thousand liters of water to the boil. In addition, the power supply to the magnets is switched off and the affected magnets are uniformly warmed up by integrated heating cables in order to distribute the energy losses evenly over the coils. At the same time, the electricity stored in the magnets is dissipated via special diodes.

Consequences of the accident in September 2008

Despite these measures, the magnets cannot be operated at maximum power immediately. Rather, the scientists slowly feel their way to higher field strengths, with some of the magnets once again going through one or even several so-called training quenches. When the first injection tests began in both rings in August 2008 and the first proton beams circled in the accelerator a month later, not all magnets had yet been trained to operate at seven teraelectron volts. This was done between the injection tests - without any stored proton beams.

During such magnet training, a serious accident occurred on September 19, 2008: A quench occurred in a connection between two neighboring LHC magnets and was not registered by the quench protection system. The original early warning system tracked the voltage across each individual magnet, but not a possible voltage drop in the connecting pieces - and so the event remained undetected. As a result, the energy stored in the magnets was not dissipated via the diodes provided for this purpose, but instead discharged in the form of an arc over the affected connection point.

Discovery of the Higgs boson

The surrounding vacuum tubes and helium lines burst and caused a pressure wave that spread over several hundred meters. A number of magnets shifted as a result, in some cases they were even thrown from their supporting feet. The repair work, during which the affected magnets were replaced and the quench protection system fundamentally revised, took a year. In November 2009, the LHC resumed operations. It turned out that other magnetic connections in the LHC also have weak points that could cause damage similar to that in September 2008 if they are operated with the nominal current.

Location of the ring accelerator LHC

In order not to delay the start of the LHC any further, it was decided at the time to limit the beam energy to 3.5 teraelectron volts. In this way, the magnets can be operated with less current and the risk of damage to the machine due to similar accidents is reduced. The first collisions of the proton beams finally took place in March 2010. In the following twelve months, only a relatively small amount of data was generated. Because you first had to thoroughly check all components and ensure that all protective mechanisms were working correctly.

At the end of 2011, the LHC had already delivered 150 times more data than in the first year of operation. In order to enable the discovery of the Higgs boson before the first long break in operation of the LHC from 2013 to 2015, attempts were made to further increase the amount of data. After a thorough analysis of all magnetic connectors, the beam energy was then increased from 3.5 to 4 teraelectron volts in 2012. With this step, the LHC generated more data at the end of its first phase of operation than all previous hadron colliders combined. Thanks to this flood of data, scientists from the CMS and ATLAS experiments were finally able to announce the discovery of the Higgs boson in July 2012.