Ewen Hamish Maclean

LHC@Home: a BOINC-based volunteer computing infrastructure for physics studies at CERN

Abstract The LHC@Home BOINC project has provided computing capacity for numerical simulations to researchers at CERN since 2004, and has since 2011 been expanded with a wider range of applications. The traditional CERN accelerator physics simulation code SixTrack enjoys continuing volunteers support, and thanks to virtualisation a number of applications from the LHC experiment collaborations and particle theory groups have joined the consolidated LHC@Home BOINC project. This paper addresses the challenges related to traditional and virtualized applications in the BOINC environment, and how volunteer computing has been integrated into the overall computing strategy of the laboratory through the consolidated LHC@Home service. Thanks to the computing power provided by volunteers joining LHC@Home, numerous accelerator beam physics studies have been carried out, yielding an improved understanding of charged particle dynamics in the CERN Large Hadron Collider (LHC) and its future upgrades. The main results are highlighted in this paper.

IOP : Experimental validation of the Achromatic Telescopic Squeezing (ATS) scheme at the LHC

The Achromatic Telescopic Squeezing (ATS) [1] scheme offers new techniques to deliver unprecedentedly small beam spot size at the interaction points of the ATLAS and CMS experiments of the LHC, while perfectly controlling the chromatic properties of the corresponding optics (linear and non-linear chromaticities, off-momentum beta-beating, spurious dispersion induced by the crossing bumps). The first series of beam tests with ATS optics were achieved during the LHC Run I (2011/2012) for a first validation of the basics of the scheme at small intensity. In 2016, a new generation of more performing ATS optics was developed and more extensively tested in the machine, still with probe beams for optics measurement and correction at β∗ = 10 cm, but also with a few nominal bunches to establish first collisions at nominal β∗ (40 cm) and beyond (33 cm), and to analysis the robustness of these optics in terms of collimation and machine protection. The paper will highlight the most relevant and conclusive results which were obtained during this second series of ATS tests.

Non-Linear Errors in the Experimental Insertions of the LHC

Correction of nonlinear magnetic errors in low-β insertions can be of critical significance for the operation of a collider. This is expected to be of particular relevance to LHC Run II and the HL-LHC upgrade, as well as to future colliders such as the FCC. Current correction strategies for these accelerators have assumed it will be possible to calculate optimized local corrections through the insertions using a magnetic model of the errors. To test this assumption the nonlinear errors in the LHC experimental insertions have been examined via feed-down and amplitude detuning. It will be shown that while in some cases the magnetic measurements provide a sufficient description of the errors, in others large discrepancies exist which will require beambased correction techniques. INTRODUCTION As the LHC progresses to more challenging β∗ regimes nonlinear errors in the low-β insertion regions (IRs) will play an increasing role in limiting the performance of the accelerator. In particular a ∼ 5σ reduction in dynamic aperture is expected in the HL-LHC due to these errors [1]. For this reason dedicated nonlinear correctors are provided in the common-beam regions left and right of the experimental insertions. A schematic of the corrector layout is shown in Fig. 1. Figure 1: Corrector layout in LHC experimental IRs [2]. Two correction strategies have been considered for the LHC and HL-LHC. The first method compensates magnetic errors in IR elements via local minimization of selected resonance driving terms [2]. The second method is based upon a direct compensation of the transverse map coefficients left and right of the interaction point (IP) [3]. For these strategies to be valid however, an accurate magnetic model of the insertions is required. Magnetic measurements performed on the LHC magnets during construction provide a foundation for such a model, but must be verified and refined through beam-based measurements to ensure the validity of the IR correction scheme. Strategies for nonlinear correction based upon feed-down to tune have previously been employed around the whole ring in SIS18 and CERN-SPS [4, 5], and in the RHIC experimental insertions [6]. In the RHIC method linear coupling was held constant during the feed-down scan, with correction attempted through minimization of observed tune shifts. At the LHC study of nonlinear multipoles in the IRs has been performed through feed-down to both tune and linear coupling. The focus of the studies in the LHC was also upon testing the magnetic model, rather than any beam-based minimization of the observable symptoms of the nonlinear errors. Table 1 summarizes the feed-down of normal and skew nonlinear multipoles, due to horizontal or vertical displacement from the magnetic axis, generating shifts in tune (ΔQ) and linear coupling (Δ|C−|). In Run I such studies were performed in the LHC by varying crossing angle bumps in the IRs, which are intended for prevention of collisions at parasitic crossing points either side of the IP (studies were performed with non-colliding probe bunches). More details of Run I studies may be found in [7, 8]. In 2015 feed-down scans were also performed [9], however new theoretical developments [10] also allowed use of an AC-dipole for measurement of amplitude detuning at top energy, providing an additional measure of normal octupole errors. MODEL VS MEASUREMENT Results from beam-based studies were compared to predictions of MAD-X simulations incorporating the best available knowledge of the magnetic errors in the IRs. This allowed for the validation of several components of the LHC magnetic model. Figure 2 shows an excellent agreement between modelled and measured variation of linear coupling with vertical crossing angle in the ALICE IR (IR2), dominated by the b3 component of the separation dipoles.

The Magnetic Model of the LHC at 6.5 TeV

After two years of shutdown, the Large Hadron Collider (LHC) operated in 2015 at an energy of 6.5 TeV. In this paper, we give the first outlook of the behavior of the LHC magnets operating at this field level, corresponding to 8-T peak field in the main dipoles. The main magnetic features are reconstructed through the beam measurements, mainly the tune (quadrupolar components) and the chromaticity (sextupolar components). The decay and snapback at injection and the start of the ramp are expected to increase by 50% due to the higher operational current. The behavior at high field will be affected by the saturation components, which were barely visible in one magnet family in the 4-TeV runs and totally invisible in the rest. Although the initial plan aims at a rather conservative value of β*, during the squeeze, the triplet and matching section quadrupole will become important. Beam measurements will provide interesting information about the magnetic behavior of these quadrupoles; in particular, through beta beating, we will estimate the absolute precision of the knowledge of the quadrupole strength. The statistics of the beam measurements will be rather limited in the first stages of commissioning, but should already reveal the main features of the 6.5-TeV operation.

Observations of Resonance Driving Terms in the LHC during Runs I and II

Future operations of the LHC will require a good understanding of the nonlinear beam dynamics. In 2012, turn-byturn measurements of large diagonal betatron excitations in LHC Beam 2 were taken at injection energy. Spectral analysis of these measurements shows an anomalous octupolar spectral line at frequency −Qx − 2Qy in the horizontal motion. The presence of this spectral line, as well as other lines, was confirmed by measurements taken for LHC Beam 1 and Beam 2 during the commissioning in 2015. We take a close look at the various spectral lines appearing in the LHC transverse motion in order to improve the LHC nonlinear model.