LHC optics commissioning: A journey towards 1% optics control

Abstract

Since 2015 the LHC has been operating at 6.5 TeV. In 2016 the β-functions at the interaction points of ATLAS and CMS were squeezed to 0.4 m. This is below the design β Ã ¼ 0.55 m at 7 TeV, and has been instrumental to surpass the design luminosity of 10 34 cm −2 s −1. Achieving a lower than nominal β Ã has been possible thanks to the extraordinary performance of the LHC, in which the control of the optics has played a fundamental role. Even though the β-beating for the virgin machine was above 100%, corrections reduced the rms β-beating below 1% at the two main experiments and below 2% rms around the ring. This guarantees a safe operation as well as providing equal amount of luminosity for the two experiments. In this article we describe the recent improvements to the measurement, correction algorithms and technical equipment which allowed this unprecedented control of the optics for a high-energy hadron collider.

Measuring β* in SuperKEKB with K modulation

“…Note that a 11 ¼ −a 22 such that a 11 þ a 22 ¼ 0 in Eq. (9). We know that the trace of T new is also equal to 2 cosð2πðQ þ ΔQÞÞ.…”

“…K modulation directly measures the average β function at a modulated quadrupole by measuring the corresponding tune shift. As a result K modulation is slow for global measurements, but can be used for measuring β Ã and has been used to do so in other machines with uncertainty below 1% [8,9]. However, it has been shown that the uncertainty of β Ã measurements using K modulation grows significantly for small values of β Ã [10], raising the question whether K modulation will give accurate measurements in SuperKEKB.…”

Luminosity scans for beam diagnostics

“…For the crossing plane, it is assumed that the longitudinal bunch shape is taken into account up to a residual error of 5 % as described in the previous section. 5% 5% β * (optics uncertainty) [12] 3% 3% head-on beam-beam kick [19,20] 2% 2% dynamic β * (beam-beam) [20] 2% 2% transverse distribution [15] 5% 5% crossing angle -5% longitudinal distribution -5% combined error 9.6 % 11.9%…”

New local optics measurements and correction techniques for the LHC and its luminosity upgrade

“…β-beating in the machine to the 2% level and the r.m.s. β Ã -beating down to the 1% level [1][2][3]. However, the continuous effort to reduce the beam sizes at the interaction points is starting to bring up the limitations of current optics measurements and correction methods.…”