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P. Jenni and T. S. Virdee
6.3.2 The Challenges of the LHC Accelerator
In this section we outline some of the features and the technological challenges of
the LHC [1].
Protons are accelerated by high electric fields generated in superconducting r.f.
cavities and are guided around the accelerator by powerful superconducting dipole
magnets. The dipole magnets are designed to operate at 8.3 Tesla, allowing the
proton beams to be accelerated to 7 TeV, with the current carrying conductor cooled
down to 1.9 K in a bath of superfluid helium. The beam pipe in which the protons
circulate is under a better vacuum, and at a lower temperature, than that found in
inter-planetary space.
The choices of two-in-one high-field superconducting dipole magnets operating
at a temperature of 1.9 K, cooled by super-fluid helium were critical to a competitive
and affordable design. The LHC could only be competitive with the Superconducting Super Collider (SSC), whose construction had started in the early 1990s in
Texas, U.S.A, if the instantaneous luminosity could be an order of magnitude higher
(at 10 34 cm −2 s −1 ). However, the SSC was later cancelled in October 1993.
The main challenges for the accelerator were to build more than one thousand
two hundred 15 m long superconducting dipoles able to reach the required
magnetic field, the large distributed cryogenic plant to cool the magnets and other
superconducting accelerator structures, and the control of the beams, whose stored
energy will reach, in design operation, a value of 350 MJ. This magnitude requires
extraordinary precautions for beam handling, since if, for any reason this beam is
lost in an uncontrolled way, it can do considerable damage to the machine elements,
which would result in months of down time.
The counter-rotating LHC beams are organized in 2808 bunches, each of ~10 11
protons per bunch separated by 25 ns, leading to a bunch crossing rate of ~40 MHz.
Proton beams were first circulated in the LHC in September 2008, and in the
days that followed, rapid progress was made in getting a beam to circulate with
very good lifetime. Soon after the start a technical incident occurred in the last of
the eight sectors to be tested as it was being ramped up to the pre-agreed startup energy of 5 TeV. The root cause was a failure of one of the 50,000 soldered
joints. Substantial damage was done to a large part of the sector involved. After
repairs lasting about a year, the LHC started operating again in November 2009.
Collisions took place at the injection energy (450 GeV per beam), followed in 2010
and 2011, by a very successful operation at a centre-of-mass energy of 7 TeV. In
2012 the centre-of-mass energy was increased to 8 TeV. The performance surpassed
expectations and an integrated luminosity of ~25 fb −1 , corresponding to 2 × 10 15
proton-proton interactions, was delivered. This is labeled Run 1.
During the period 2015–2018 the LHC operated at a proton-proton centreof-mass energy of 13 TeV and delivered a total of over 150 fb −1 of integrated
luminosity. The collider performed close to, or beyond, its design values in many
parameters, operating at 13 TeV and reaching peak luminosities of 2 × 10 34
P. Jenni and T. S. Virdee
6.3.2 The Challenges of the LHC Accelerator
In this section we outline some of the features and the technological challenges of
the LHC [1].
Protons are accelerated by high electric fields generated in superconducting r.f.
cavities and are guided around the accelerator by powerful superconducting dipole
magnets. The dipole magnets are designed to operate at 8.3 Tesla, allowing the
proton beams to be accelerated to 7 TeV, with the current carrying conductor cooled
down to 1.9 K in a bath of superfluid helium. The beam pipe in which the protons
circulate is under a better vacuum, and at a lower temperature, than that found in
inter-planetary space.
The choices of two-in-one high-field superconducting dipole magnets operating
at a temperature of 1.9 K, cooled by super-fluid helium were critical to a competitive
and affordable design. The LHC could only be competitive with the Superconducting Super Collider (SSC), whose construction had started in the early 1990s in
Texas, U.S.A, if the instantaneous luminosity could be an order of magnitude higher
(at 10 34 cm −2 s −1 ). However, the SSC was later cancelled in October 1993.
The main challenges for the accelerator were to build more than one thousand
two hundred 15 m long superconducting dipoles able to reach the required
magnetic field, the large distributed cryogenic plant to cool the magnets and other
superconducting accelerator structures, and the control of the beams, whose stored
energy will reach, in design operation, a value of 350 MJ. This magnitude requires
extraordinary precautions for beam handling, since if, for any reason this beam is
lost in an uncontrolled way, it can do considerable damage to the machine elements,
which would result in months of down time.
The counter-rotating LHC beams are organized in 2808 bunches, each of ~10 11
protons per bunch separated by 25 ns, leading to a bunch crossing rate of ~40 MHz.
Proton beams were first circulated in the LHC in September 2008, and in the
days that followed, rapid progress was made in getting a beam to circulate with
very good lifetime. Soon after the start a technical incident occurred in the last of
the eight sectors to be tested as it was being ramped up to the pre-agreed startup energy of 5 TeV. The root cause was a failure of one of the 50,000 soldered
joints. Substantial damage was done to a large part of the sector involved. After
repairs lasting about a year, the LHC started operating again in November 2009.
Collisions took place at the injection energy (450 GeV per beam), followed in 2010
and 2011, by a very successful operation at a centre-of-mass energy of 7 TeV. In
2012 the centre-of-mass energy was increased to 8 TeV. The performance surpassed
expectations and an integrated luminosity of ~25 fb −1 , corresponding to 2 × 10 15
proton-proton interactions, was delivered. This is labeled Run 1.
During the period 2015–2018 the LHC operated at a proton-proton centreof-mass energy of 13 TeV and delivered a total of over 150 fb −1 of integrated
luminosity. The collider performed close to, or beyond, its design values in many
parameters, operating at 13 TeV and reaching peak luminosities of 2 × 10 34