The basics of particle physics, the development of the particle accelerator, and the installation of the world’s largest such unit at CERN were discussed in Part 1. Here, we will take a closer look at this super collider and the cutting-edge vacuum technologies required to keep it operational.
Located 175 m (574 ft.) below ground on the border of Switzerland and France, the Large Hadron Collider (LHC) accelerates subatomic particles to speeds approaching the speed of light to produce head on collisions between atoms and split them into their elemental parts, thus shedding light on the fundamental nature of matter.
The LHC uses multiple vacuum systems, each serving a different purpose. Combined, these systems comprise the largest operational vacuum system in the world1. The first of these systems is used to put the “beam pipes” under ultra-high vacuum. These beam pipes form an oval track within the LHC and contain the beams of particles that are accelerated via a magnetic field. To prevent the particles from colliding with gas molecules inside the accelerator, the interior of the beam pipes is put under ultra-high vacuum to minimize the number of gas molecules present.
The LHC has 54 km (33 miles) of beam pipes under vacuum, which is twice the circumference of the LHC. Inside the LHC are two separate rings of beam pipes that carry the two particle beams in opposing directions (Fig. 1) before they are deflected into each other to collide head-on. The vacuum pressure in these beam pipes approaches the vacuum found in space and is one hundred trillion times less than atmospheric pressure, in the range of 10-10 to 10-11 mbar.
One of the key features of the LHC is the storage ring concept. The particles are not only accelerated in the beam pipes but can be stored for several hours, circling the LHC ring at 11,000 times per second. After several hours of ring storage, the particles have traveled a distance sufficient to reach Pluto.
The beam pipes feature both a set of arced sections (curved to match the circumference of the LHC), and straight sections (which house the beam-control systems and the insertion regions for the experiments). Within the arcs, the ultra-high vacuum is maintained by cryogenically pumping in 9,000 m3 (317,832 ft3) of helium gas which then bathes the exterior of the beam pipes cooling the beam pipes to the extremely low temperature of 1.9K (-271°C or -458°F), just above the theoretically lowest possible temperature of absolute zero.
At this low temperature, any stray gas molecules left inside the pipes will condense and adhere to the walls of the beam pipe in a process known as adsorption. It takes almost two weeks of pumping to bring the pressure down below 1 × 10-10 mbar. The inside of the round beam pipes are slightly square copper-coated beam screens (Fig. 2), which provide a further barrier to heat loss. These beam screens feature slits to allow gases to escape to the interior of the beam pipes, where they are captured cryogenically.
The straight sections of beam pipes, which are kept at room temperature, incorporate several technologies to maintain an ultra-high vacuum. First, these sections utilize a non-evaporable getter coating, first developed for use in the LHC, which absorbs gas molecules when heated, dissolving its native oxide layer, then cooled back to room temperature (Fig. 3). The coating consists of a thin layer of titanium-zirconium-vanadium alloy on the interior of the beam pipes.
The coating not only absorbs gas molecules but also blocks the outgassing of the beam pipe walls. In this way, the straight sections of beam pipes serve as a vacuum pump. This technology removes all gases except residual methane and noble gases, which are removed by a group of 780 ion pumps. Ion pumps (Fig. 4) ionize the residual gas within the pipe and impart a strong electrical potential to draw the electrically charged gas molecules to a titanium cathode. The straight, room temperature sections are periodically “baked out”, a procedure whereby the beam pipes are heated from the outside to replenish the effectiveness of the getter coating. Bakeout needs to be performed at regular intervals.
Another important use of vacuum technology in the LHC is insulating both the cryogenically cooled magnets and the helium distribution lines. The magnets, which are superconductors, operate using the same principle as common electromagnets in that a magnetic field is produced by running an alternating current through a coil of wire. The difference is that the superconducting coils are cooled to near absolute zero, which is below their critical temperature. As such, the temperature the winding material (most commonly a niobium-titanium or niobium-tin alloy) changes from the normal resistive state to that of a superconductor. Superconductors have no electrical resistance. In this condition the windings can carry much larger electric currents than at higher temperatures, allowing the creation of very large magnetic fields. In a fascination phenomenon known as “persistent mode”, the electrical power to a cryogenically cooled superconducting magnet can be turned off and since there is no resistance to current flow, the persistent current will flow for months, preserving the magnetic field without the addition of electrical power. The LHC cryogenic system is the largest in the world, containing 109,000 kg (240,000 lb.) of helium, utilizing 40,000 pipe seals and consuming 40 MW of electricity, 10 times that of a locomotive.
When discussing cryogenic temperatures, it is useful to take a moment to mention that Kelvin (K) is the preferred unit of measurement (not degrees Kelvin or ºK). The Kelvin temperature unit corresponds to the number of degrees Celsius above absolute zero, such that 10K is the same as 10°C above absolute zero, or -263° C (-441ºF).
Cooling the entire mass of the superconducting magnets takes several weeks and consists of three separate cooling stages. During the first stage, a refrigeration system using 9,090,000 kg (2,000,000 lb.) of nitrogen cools the helium to 80 Kelvin, whereupon it is injected into the magnets to start the cooling process. In stage two, turbines are used to cool the helium circulating through the magnets down to 4.5K. In the final stage, the magnets are cooled even further to achieve a temperature of 2.5K.
In order to sufficiently insulate the 50 km (31 miles) of piping used in the cryogenic systems, a vacuum insulation system is used (Fig. 5), which insulates against all three modes of heat transfer to the pipe carrying the liquid helium; convection, radiation, and conduction. In the vacuum insulation system, the helium piping is surrounded by a second, larger diameter pipe, and a vacuum system is used to remove the air from the space between the inner and outer pipes, creating a high-performance insulation. This vacuum virtually eliminates convection heat transfer from the outer pipe to the inner, because this mode of heat transfer requires the presence of gas molecules to transfer heat energy by bulk movement and there are few gas molecules remaining in the evacuated space. To reduce heat transfer by radiation, radiant heat shields are located in the evacuated space. They serve to reflect radiant heat away from the cryogenic piping.
Low conductivity spacers are used to minimize the heat transfer by conduction while holding the interior cryogenic pipe in place inside the outer pipe.
The vacuum insulation system operates using a two stage process. Turbomolecular pumps (Fig. 6) are used for initial drawdown. This type of pump is referred to as a kinetic pump. Kinetic pumps work on the principle of momentum transfer, directing gas through the pump mechanically. Turbomolecular pumps use impellers rotating at high speed to deflect the gas molecules from impellors (rotors) to stators during successive impacts. The stators consist of angled stationary blades fixed to the pump housing, located between the impellers, along the length of the pump from the inlet to the outlet (Fig. 7). The impeller rotation speed is such that the impeller surfaces travel at a speed similar to that of gas molecules themselves, roughly 460 m/s (1,500 ft./sec., or 1,000 miles/hr.). This is fast enough to direct the molecules through the pump by “bouncing” them off the rotors and stators without them escaping between the rotor-stator gaps, or moving in a reverse direction through the pump.
After the turbomolecular pumps draw the initial vacuum on the insulation system, cryogenic pumps are used to achieve high vacuum, roughly 10-6 mbar. Cryogenic pumps (aka “cryopumps”) belong to the entrapment category of pumps because they capture and hold the gas molecules, rather than transfer them through the pump. Cryopump interior surfaces are cooled to a temperature, typically -260° C (-436° F), low enough that the saturation pressure of the gas at that temperature (Fig. 8) is below the desired vacuum pressure in the chamber. The attainable pressure is determined by the gas saturation pressure at the temperature of the cold surfaces inside the pump. When the gas molecules make contact with the cold surfaces, they undergo phase transition from the gaseous to the solid phase, essentially freezing them to the pump surface.
In reviewing the gas saturation curves for various gases, a temperature of about 100 K would be sufficient to condense and freeze water and hydrocarbons; 20 K would be sufficient for nitrogen and oxygen; and 4 K would be sufficient to condense and freeze the hydrogen isotopes and neon.
Next Time: In our final installment on CERN, we take a closer look at the technologies used in the LHC, discuss some of the discoveries it led to, and review the past contributors to the field of vacuum technology whose innovations made the LHC possible.