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The Compact Linear Collider requires micron-level alignment of the key accelerator components within the two 21 km main LINACs to achieve the desired luminosity. Several Super Accelerating Structures (SAS) will be pre-aligned on a single common support structure to within 14μm. This common support structure will then be actively aligned relative to a Machine Reference Network, and in operation beam-based alignment will be used to achieve the final 1 μm alignment required. A design for the SAS pre-alignment system has been created, prototyped, and tested, and shown to match the design performance within 2.3 μm/turn, and to meet the specification requirements. A design for the active girder alignment system has also been created, prototyped, and initial testing has begun. This testing indicates that the system works as designed. Finally, the testing required to confirm this, and to fully validate the CLIC alignment procedure, is discussed.

Coherent synchrotron radiation (CSR) is a collective effect that mainly occurs when the trajectory of an electron beam is bent in a dipole magnet. It affects the electron beam by distorting the phase space along its slice distribution, which leads to emittance growth. Therefore, CSR should be suppressed to transport electron beams without further degradation of the emittance. In linear optics, CSR-induced emittance can be suppressed by controlling the Twiss parameters along the electron-beam transfer line. However, owing to some physical constraints, transfer-line optics may be governed by higher-order terms in the transfer map, and the use of a sextupole magnet to suppress these terms would be very challenging for low-energy-spread and low-emittance beams. Therefore, without using a sextupole magnet, we estimate the region of the Twiss parameters where the first-order terms are dominant along the transfer line by introducing chromatic amplitude. In this region, we can apply the suppression condition that is valid in a linear matrix system. This minimization of the emittance growth becomes even more important when the electron-beam transfer line is used for external injection into a plasma wakefield because mismatched beam conditions could induce an additional increase in the emittance during the acceleration. In this paper, we discuss a method of emittance-growth minimization driven by the CSR effect along the transfer line, which is particularly used for electron-beam injection into plasma wakefields. In addition, using the particle-in-cell simulation, we investigate the evolution of electron beam parameters during the acceleration through plasma wakefields in the presence of the CSR effect on the electron beam. We confirm that the beam emittance growth is minimized when the CSR effect is properly controlled. Otherwise, it is found that 11%–32% emittance growths by the CSR effect along the transfer line lead to additional 20%–40% increase of the maximum slice emittance.

Following the successful Run 1 experiment, the Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) Run2 experiment requires the design and implementation of a compact electron source. The “high-quality Electron Accelerator driven by a Reliable Laser wakefield for Industrial uses” (EARLI) project aims to design a stand-alone high-quality electron injector based on a laser wakefield accelerator (LWFA) as an alternative proposal to AWAKE’s baseline design of an X-band electron gun. This project is currently in the design phase, including simulations and experimental tests. Exhaustive beam physics studies for conventional accelerators are applied to LWFA physics.

An active plasma lens (APL) focuses the beam in both the horizontal and vertical planes simultaneously using a magnetic field generated by a discharge current through the plasma. A beam size of5–10μmcan be achieved within a short distance using a focusing gradient on the order of100T/m. The APL is therefore an attractive element for plasma wakefield acceleration, because an ultrasmall size of the witness electron beam is required for injection into the plasma wakefield to minimize emittance growth and to enhance the capturing efficiency. When the drive beam and witness electron beam copropagate through the APL, interactions between the drive and witness beams, and the plasma must be considered. In this paper, through particle-in-cell simulations, we discuss the possibility of using an APL for the final focusing of the electron beam for the AWAKE RUN 2 experiments. It is confirmed that the amplitude of the plasma wakefield excited by proton bunches remains the same even after propagation through the APL. The emittance of the witness electron beam increases rapidly in the plasma density ramp regions of the lens. Nevertheless, when the witness electron beam has a charge of 100 pC, emittance of 10 mm mrad, and bunch length of60μm, its emittance growth is not significant along the active plasma lens. For small emittance, such as 2 mm mrad, the emittance growth is found to be strongly dependent on the rms beam size, plasma density, and multiple Coulomb scattering.

For the Compact Linear Collider project at CERN, the power for the main linacs is extracted from a drive beam generated from a high current electron source. The design of the electron source and its subsequent focusing channel has a great impact on the beam dynamic considerations of the drive beam. We report the design of a thermionic electron source and the subsequent focusing channels with the goal of production of a high quality beam with a very small emittance.

Plasma wakefield dynamics over timescales up to 800 ps, approximately 100 plasma periods, are studied experimentally at the Advanced Wakefield Experiment (AWAKE). The development of the longitudinal wakefield amplitude driven by a self-modulated proton bunch is measured using the external injection of witness electrons that sample the fields. In simulation, resonant excitation of the wakefield causes plasma electron trajectory crossing, resulting in the development of a potential outside the plasma boundary as electrons are transversely ejected. Trends consistent with the presence of this potential are experimentally measured and their dependence on wakefield amplitude are studied via seed laser timing scans and electron injection delay scans.

High-energy particle accelerators have been crucial in providing a deeper understanding of fundamental particles and the forces that govern their interactions. To increase the energy of the particles or to reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma wakefield acceleration 1 – 5 , in which the electrons in a plasma are excited, leading to strong electric fields (so called ‘wakefields’), is one such promising acceleration technique. Experiments have shown that an intense laser pulse 6 – 9 or electron bunch 10 , 11 traversing a plasma can drive electric fields of tens of gigavolts per metre and above—well beyond those achieved in conventional radio-frequency accelerators (about 0.1 gigavolt per metre). However, the low stored energy of laser pulses and electron bunches means that multiple acceleration stages are needed to reach very high particle energies 5 , 12 . The use of proton bunches is compelling because they have the potential to drive wakefields and to accelerate electrons to high energy in a single acceleration stage 13 . Long, thin proton bunches can be used because they undergo a process called self-modulation 14 – 16 , a particle–plasma interaction that splits the bunch longitudinally into a series of high-density microbunches, which then act resonantly to create large wakefields. The Advanced Wakefield (AWAKE) experiment at CERN 17 – 19 uses high-intensity proton bunches—in which each proton has an energy of 400 gigaelectronvolts, resulting in a total bunch energy of 19 kilojoules—to drive a wakefield in a ten-metre-long plasma. Electron bunches are then injected into this wakefield. Here we present measurements of electrons accelerated up to two gigaelectronvolts at the AWAKE experiment, in a demonstration of proton-driven plasma wakefield acceleration. Measurements were conducted under various plasma conditions and the acceleration was found to be consistent and reliable. The potential for this scheme to produce very high-energy electron bunches in a single accelerating stage 20 means that our results are an important step towards the development of future high-energy particle accelerators 21 , 22 . Electron acceleration to very high energies is achieved in a single step by injecting electrons into a ‘wake’ of charge created in a 10-metre-long plasma by speeding long proton bunches.

A long, narrow, relativistic charged particle bunch propagating in plasma is subject to the self -modulation (SM) instability. We show that SM of a proton bunch can be seeded by the wakefields driven by a preceding electron bunch. SM timing reproducibility and control are at the level of a small fraction of the modulation period. With this seeding method, we independently control the amplitude of the seed wakefields with the charge of the electron bunch and the growth rate of SM with the charge of the proton bunch. Seeding leads to larger growth of the wakefields than in the instability case.

The Advanced Wakefield (AWAKE) Experiment is a proof-of-principle experiment demonstrating the acceleration of electron beams via proton-driven plasma wakefield acceleration. AWAKE Run 2 aims to build on the results of Run 1 by achieving higher energies with an improved beam quality. As part of the upgrade to Run 2, the existing proton and electron beamlines will be adapted and a second plasma cell and new 150-MeV electron beamline will be added. The specification for this new 150-MeV beamline will be challenging as it will be required to inject electron bunches with micron-level beam size and stability into the second plasma cell while being subject to tight spatial constraints. In this paper, we describe the techniques used (e.g., numerical optimizers and genetic algorithms) to produce the design of this electron line. We present a comparison of the methods used in this paper with other optimization algorithms commonly used within accelerator physics. Operational techniques are also studied including steering and alignment methods utilizing numerical optimizers and beam measurement techniques employing neural networks. We compare the performance of algorithms for online optimization and beam-based alignment in terms of their efficiency and effectiveness.

We study experimentally the longitudinal and transverse wakefields driven by a highly relativistic proton bunch during self-modulation in plasma. We show that the wakefields’ growth and amplitude increase with increasing seed amplitude as well as with the proton bunch charge in the plasma. We study transverse wakefields using the maximum radius of the proton bunch distribution measured on a screen downstream from the plasma. We study longitudinal wakefields by externally injecting electrons and measuring their final energy. Measurements agree with trends predicted by theory and numerical simulations and validate our understanding of the development of self-modulation. Experiments were performed in the context of the Advanced Wakefield Experiment (AWAKE).