10 The Largest Accelerators and Colliders of Their Time
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10.6.3 Performance
The LHC was first operated with beam for short periods in 2008 and 2009. In 2010
a first experience with the machine was gained at a beam energy of 3.5 TeV, and
moderate beam intensity of up to around 200 bunches of 1.1 × 10 11 protons per
bunch (ppb). The reduced energy was chosen such as to minimize risks associated to
the quality issue of the soldering of the busbar cables in the magnet interconnections.
In 2011 the beam intensity was pushed to around 1400 bunches of 1.4 × 10 11 ppb
while 2012 was dedicated to luminosity production with higher bunch intensities
1.6 × 10 11 ppb and a beam energy of 4 TeV. A bunch spacing of 50 ns was used in
2011 and 2012 to minimize the complication of electron clouds. The first operation
period is commonly referred to as run 1. In early 2013 beam operation was stopped
for a 2-year long shutdown (LS1) to consolidate the magnet interconnections in view
of reaching the design beam energy. Many details on LHC operation during Run 1
may be found in [190].
Beam operation resumed in 2015 at 6.5 TeV following a dipole training
campaign of 169 quenches at the end of Long Shutdown 1 (LS1). The LHC
experiments expressed a strong preference for beams with 25 ns bunch spacing,
as opposed to the 50 ns spacing used in 2011–2012, as this would result in a too
high number of inelastic collisions per crossing (pile-up). On the machine side
25 ns beams pose additional challenges, e.g. the formation of electron clouds (eclouds) in the vacuum chamber and a higher number of fast loss events, named
Unidentified Falling Objects (UFOs). Given the number of new territories had to be
explored, 2015 was considered a re-commissioning and a learning year, dedicated
to preparing the machine for full luminosity production in 2016–2018. In 2016 the
machine performance was pushed for the first time above the design luminosity of
10 34 cm −2 s −1 , and by the end of that year the design had been exceeded by 40%.
This excellent performance was possible thanks to beams of much lower emittance
produced by the LHC injector chain, coupled to a reduced β ∗ of 40 cm as compared
to the design value of 55 cm. In 2017 the performance could be pushed further
with more than twice the design luminosity with a further reduction of the beam
emittances and of β ∗ to 30 cm. The performance in 2017 was so exceptional that the
luminosities of ATLAS and CMS had to levelled down to limit the event pile-up to
60. Figures 10.26 and 10.27 present the evolution of the peak and of the integrated
luminosity between 2011 and 2017. A summary of the main parameters is presented
in Table 10.16. Details on operation in LHC Run 2 up to and including 2016 can be
found in [191].
The remarkable performance of LHC during run 2 between 2015 and 2017
was achieved despite unexpected limitations. In 2016 the SPS beam dump was
damaged when high intensity LHC beams were dumped onto the block at 450 GeV,
developing a vacuum leak. Because it was not practical to exchange the dump during
653
10.6.3 Performance
The LHC was first operated with beam for short periods in 2008 and 2009. In 2010
a first experience with the machine was gained at a beam energy of 3.5 TeV, and
moderate beam intensity of up to around 200 bunches of 1.1 × 10 11 protons per
bunch (ppb). The reduced energy was chosen such as to minimize risks associated to
the quality issue of the soldering of the busbar cables in the magnet interconnections.
In 2011 the beam intensity was pushed to around 1400 bunches of 1.4 × 10 11 ppb
while 2012 was dedicated to luminosity production with higher bunch intensities
1.6 × 10 11 ppb and a beam energy of 4 TeV. A bunch spacing of 50 ns was used in
2011 and 2012 to minimize the complication of electron clouds. The first operation
period is commonly referred to as run 1. In early 2013 beam operation was stopped
for a 2-year long shutdown (LS1) to consolidate the magnet interconnections in view
of reaching the design beam energy. Many details on LHC operation during Run 1
may be found in [190].
Beam operation resumed in 2015 at 6.5 TeV following a dipole training
campaign of 169 quenches at the end of Long Shutdown 1 (LS1). The LHC
experiments expressed a strong preference for beams with 25 ns bunch spacing,
as opposed to the 50 ns spacing used in 2011–2012, as this would result in a too
high number of inelastic collisions per crossing (pile-up). On the machine side
25 ns beams pose additional challenges, e.g. the formation of electron clouds (eclouds) in the vacuum chamber and a higher number of fast loss events, named
Unidentified Falling Objects (UFOs). Given the number of new territories had to be
explored, 2015 was considered a re-commissioning and a learning year, dedicated
to preparing the machine for full luminosity production in 2016–2018. In 2016 the
machine performance was pushed for the first time above the design luminosity of
10 34 cm −2 s −1 , and by the end of that year the design had been exceeded by 40%.
This excellent performance was possible thanks to beams of much lower emittance
produced by the LHC injector chain, coupled to a reduced β ∗ of 40 cm as compared
to the design value of 55 cm. In 2017 the performance could be pushed further
with more than twice the design luminosity with a further reduction of the beam
emittances and of β ∗ to 30 cm. The performance in 2017 was so exceptional that the
luminosities of ATLAS and CMS had to levelled down to limit the event pile-up to
60. Figures 10.26 and 10.27 present the evolution of the peak and of the integrated
luminosity between 2011 and 2017. A summary of the main parameters is presented
in Table 10.16. Details on operation in LHC Run 2 up to and including 2016 can be
found in [191].
The remarkable performance of LHC during run 2 between 2015 and 2017
was achieved despite unexpected limitations. In 2016 the SPS beam dump was
damaged when high intensity LHC beams were dumped onto the block at 450 GeV,
developing a vacuum leak. Because it was not practical to exchange the dump during