Role of surface microgeometries on electron escape probability and secondary electron yield of metal surfaces

The influence of microgeometries on the Secondary Electron Yield (SEY) of surfaces is investigated. Laser written structures of different aspect ratio (height to width) on a copper surface tuned the SEY of the surface and reduced its value to less than unity. The aspect ratio of microstructures was methodically controlled by varying the laser parameters. The results obtained corroborate a recent theoretical model of SEY reduction as a function of the aspect ratio of microstructures. Nanostructures - which are formed inside the microstructures during the interaction with the laser beam - provided further reduction in SEY comparable to that obtained in the simulation of structures which were coated with an absorptive layer suppressing secondary electron emission

First beam test of Laser Engineered Surface Structures (LESS) at cryogenic temperature in CERN SPS accelerator

Electron cloud mitigation is an essential requirement for accelerators of positive particles with high intensity beams to guarantee beam stability and limited heat load in cryogenic systems. Laser Engineered Surface Structures (LESS) are being considered, within the High Luminosity upgrade of the LHC collider at CERN (HL-LHC), as an option to reduce the Secondary Electron Yield (SEY) of the surfaces facing the beam, thus suppressing the elec-tron cloud phenomenon. As part of this study, a 2.2 m long Beam Screen (BS) with LESS has been tested at cryogenic temperature in the COLD bore EXperiment (COLDEX) facility in the SPS accelerator at CERN. In this paper, we describe the manufacturing procedure of the beam screen, the employed laser treatment technique and discuss our first observations in COLDEX confirming electron cloud suppression.Electron cloud mitigation is an essential requirement for accelerators of positive particles with high intensity beams to guarantee beam stability and limited heat load in cryogenic systems. Laser Engineered Surface Structures (LESS) are being considered, within the High Luminosity upgrade of the LHC collider at CERN (HL-LHC), as an option to reduce the Secondary Electron Yield (SEY) of the surfaces facing the beam, thus suppressing the electron cloud phenomenon. As part of this study, a 2.2 m long Beam Screen (BS) with LESS has been tested at cryogenic temperature in the COLD bore EXperiment (COLDEX) facility in the SPS accelerator at CERN. In this paper, we describe the manufacturing procedure of the beam screen, the employed laser treatment technique and discuss our first observations in COLDEX confirming electron cloud suppression

Optics model

The LHC optics quality after the 2015 commissioning is reviewed. Optics corrections were limited due to a variety of problems. This includes disturbed dispersion measurements due to orbit drifts and an unexpected shift of the IP β-function (β *) waist. The resulting issues will become more critical at the smaller β * of 40 cm foreseen for 2016. Therefore , an improved optics correction strategy with desired actions for the 2016 commissioning is proposed. The optics quality during combined ramp and squeeze is presented and the benefits of ballistic optics measurements are discussed. The optics stability over time and the correction of interaction region (IR) non-linearities are also reviewed

LHC optics commissioning: A journey towards 1% optics control

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*=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* 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

Nonlinear optics commissioning in the LHC

So far, the LHC has operated without any dedicated commissioning of the nonlinear optics at top energy. As is reduced however, the impact of nonlinear errors in experimental insertions may become sizable. Below , and in particular with possible LHC con gurations approaching , an operational impact from uncompen-sated nonlinear errors in the IRs is to be expected. Notably the contribution of normal octupole errors in IR1 and IR5 to the tune footprint becomes comparable to that created by the Landau octupoles, with implications for the performance of instrumentation and Landau damping of in-stabilities. In the HL-LHC compensation of IR-nonlinear sources may become a critical issue, with feed-down from IR-sextupole errors having the potential to generate substantial linear optics perturbations. This effect will require an evolution of the linear optics commissioning strategy in the LHC and HL-LHC. Current understanding of the impact of these errors and our ability to correct them will be reported. Nonlinear optics commissioning activities under-taken elsewhere in the machine cycle will also be introduced

New LHC optics correction approaches in 2017

The optics commissioning strategy for low-$\beta ^{*}$ operation of the LHC underwent significant revision in 2017 [1]. The beam-based correction strategy transitioned from being concerned exclusively with linear optics errors, to a combined linear and nonlinear optics commissioning, with the traditional optics corrections at flat-orbit being both preceded and followed by corrections for the effect of nonlinear errors in the ATLAS and CMS insertions

Optics control in 2016

In 2016 the fi-functions at the interaction points of ATLAS and CMS have been squeezed down to 0.4 m. This is below the design fi" = 0.55 m at 7 TeV and has been instrumental to surpass the design luminosity. Even though the fl-beating for the virgin machine was above 100% the corrections reduced it to an rms B-beating below 1% at the two main experiments and below 2% rms around the ring. These results are presented together with the fl-beating deriving from the crossing angles in combination with the sextupolar errors in the IRs. A way to correct the errors using sextupo-lar correctors is referenced and how this could be integrated in the commissioning is outlined. Furthermore. the progress towards an automatic coupling correction is described

New approach to LHC optics commissioning for the nonlinear era

In 2017, optics commissioning strategy for low-β* operation of the CERN Large Hadron Collider (LHC) underwent a major revision. This was prompted by a need to extend the scope of beam-based commissioning at high energy, beyond the exclusively linear realm considered previously, and into the nonlinear regime. It also stemmed from a recognition that, due to operation with crossing angles in the experimental insertions, the linear and nonlinear optics quality were intrinsically linked through potentially significant feed-down at these locations. Following the usual linear optics commissioning therefore, corrections for (normal and skew) sextupole and (normal and skew) octupole errors in the high-luminosity insertions were implemented. For the first time, the LHC now operates at top energy with beam-based corrections for nonlinear dynamics, and for the effect of the crossing scheme on beta-beating and dispersion. The new commissioning procedure has improved the control of various linear and nonlinear characteristics of the LHC, yielding clear operational benefits