Piotr Skowroński

APS : Stabilization of the arrival time of a relativistic electron beam to the 50 fs level

We report the results of a low-latency beam phase feed-forward system built to stabilise the arrival time of a relativistic electron beam. The system was operated at the Compact Linear Collider (CLIC) Test Facility (CTF3) at CERN where the beam arrival time was stabilised to approximately 50~fs. The system latency was \(350\)~ns and the correction bandwidth \(>23\)~MHz. The system meets the requirements for CLIC.

Effect of hard processes on momentum correlations in pp and pp collisions

The HBT radii extracted in p and pp collisions at SPS and Tevatron show a clear correlation with the charged particle rapidity density. We propose to explain the correlation using a simple model where the distance from the initial hard parton–parton scattering to the hadronization point depends on the energy of the partons emitted. Since the particle multiplicity is correlated with the mean energy of the partons produced, we can explain the experimental observations without invoking scenarios that assume a thermal fireball. The model has been applied with success to the existing experimental data both in the magnitude and the intensity of the correlation. Also, the model has been extended to pp collisions at the LHC energy of 14 TeV. The possibilities of a better insight into the string spatial development using 3D HBT analysis are discussed.

Lessons from CTF3

The CLIC Test Facility (CTF3) was built to demonstrate the feasibility of the CLIC two beam acceleration scheme. The main issues to be verified were the high current drive beam generation using a fully loaded highly efficient linac and a beam combination scheme, based on transverse RF deflectors, to increase beam current and bunch repetition frequency. The drive beam has been used for GW level RF power production and two beam acceleration experiments. CTF3 was also a test ground for development of many accelerator technologies.Its operation was concluded in 2016 and in this contribution the results relevant for the CLIC design as well as for the whole accelerator physics community will be presented.

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.

Recent Improvements in Drive Beam Stability in CTF3

The proposed Compact Linear Collider (CLIC) uses a high intensity, low energy drive beam producing the RF power to accelerate the low intensity main beam with 100 MeV/m gradient. This scheme puts stringent requirements on drive beam stability in terms of phase, energy and current. Finding and understanding the sources of jitter plays a key role in their mitigation. In this paper, we report on the recent studies in the CLIC Test Facility (CTF3). New jitter and drift sources were identified and adequate beam-based feed-backs were implemented and commissioned. Finally, we present the resulting improvement of drive beam stability.

First Experimental Results with the CLIC Drive Beam Phase Feedforward Prototype at the CLIC Test Facility CTF3

The two-beam acceleration scheme envisaged for CLIC will require a high degree of phase stability between two beams at the drive beam decelerator sections, to allow efficient acceleration of the main beam. There will be up to 48 such decelerator sections for the full 3 TeV design, and each decelerator section will be instrumented with a feed-forward system to correct the drive beam phase to a precision of 0.2 degrees at 12 GHz relative to the main beam, using a kicker system around a four-bend chicane. A prototype system has been developed and tested at the CLIC Test Facility (CTF3) complex, where the beam phase is measured upstream of the combiner ring and corrected with two kickers in a dog-leg chicane just upstream of the CLEX facility, where the resulting phase change is measured. This prototype is designed to demonstrate correction of a portion of the CTF3 bunch train to the level required for CLIC, with a bandwidth of greater than 30 MHz, and within a latency constraint of 380 ns as set by the beam time-of-flight through the combiner ring complex. A description of the hardware will be given and initial results from the first phase of the experiment will be presented.