The Large Hadron–Electron Collider (LHeC) is designed to move the field of deep inelastic scattering (DIS) to the energy and intensity frontier of particle physics. Exploiting energy-recovery technology, it collides a novel, intense electron beam with a proton or ion beam from the High-Luminosity Large Hadron Collider (HL-LHC). The accelerator and interaction region are designed for concurrent electron–proton and proton–proton operations. This report represents an update to the LHeC’s conceptual design report (CDR), published in 2012. It comprises new results on the parton structure of the proton and heavier nuclei, QCD dynamics, and electroweak and top-quark physics. It is shown how the LHeC will open a new chapter of nuclear particle physics by extending the accessible kinematic range of lepton–nucleus scattering by several orders of magnitude. Due to its enhanced luminosity and large energy and the cleanliness of the final hadronic states, the LHeC has a strong Higgs physics programme and its own discovery potential for new physics. Building on the 2012 CDR, this report contains a detailed updated design for the energy-recovery electron linac (ERL), including a new lattice, magnet and superconducting radio-frequency technology, and further components. Challenges of energy recovery are described, and the lower-energy, high-current, three-turn ERL facility, PERLE at Orsay, is presented, which uses the LHeC characteristics serving as a development facility for the design and operation of the LHeC. An updated detector design is presented corresponding to the acceptance, resolution, and calibration goals that arise from the Higgs and parton-density-function physics programmes. This paper also presents novel results for the Future Circular Collider in electron–hadron (FCC-eh) mode, which utilises the same ERL technology to further extend the reach of DIS to even higher centre-of-mass energies.

Active plasma lensing is a compact technology for strong focusing of charged particle beams, which has gained considerable interest for use in novel accelerator schemes. While providing kT/m focusing gradients, active plasma lenses can have aberrations caused by a radially nonuniform plasma temperature profile, leading to degradation of the beam quality. We present the first direct measurement of this aberration, consistent with theory, and show that it can be fully suppressed by changing from a light gas species (helium) to a heavier gas species (argon). Based on this result, we demonstrate emittance preservation for an electron beam focused by an argon-filled active plasma lens.

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Measurements of the beam emittance during bunch compression in the CLIC Test Facility (CTF-II) are described. The measurements were made with different beam charges and different energy correlations versus the bunch compressor settings which were varied from no compression through the point of full compression and to overcompression. Significant increases in the beam emittance were observed with the maximum emittance occurring near the point of full (maximal) compression. Finally, evaluation of possible emittance dilution mechanisms indicates that coherent synchrotron radiation was the most likely cause. PACS numbers: 29.27.Bd, 41.60.Ap, 41.75.Lx Magnetic bunch compressors have been and will be utilized in many high-energy electron accelerators to increase the longitudinal density of a particle beam. In particular, they are important in linear colliders, where short bunches are needed because of the short depth of focus at the interaction point, and in short wavelength FELs, where high peak current is needed to reduce the optical gain length.The compressors operate by first creating an energy variation along the length of the bunch and then passing the bunch through a series of bending magnets in which the path length is energy dependent. By appropriately choosing the energy correlation and magnet strengths, the bunch can be compressed as desired and, because the forces involved are conservative, the longitudinal and transverse phase-space densities should be conserved.The conservation of the transverse phase-space density, referred to as the transverse beam emittance, is usually of extreme importance-this is especially true in linear colliders and short wavelength FELs whose performance is very sensitive to the transverse emittances. There are a number of sources of emittance dilution which can increase the emittances, decreasing the phase-space density. The most obvious are chromatic effects that are important because of the large energy spread (typically a few percent) that is needed to compress the bunch length; thus the compressor optics are usually achromatic to second or higher order. Other standard sources of dilution include the longitudinal wakefields and the "classical" longitudinal space charge force which is inversely proportional to the square of the beam energy 1͞g 2 ; these break the achromaticity of the optics by generating small energy changes within the compressor.Recently, as designs have started requiring shorter bunches and smaller emittances, the more subtle issue of what happens as the beam is deflected in the bending magnets has been discussed. This includes the transformation of the longitudinal space charge force in the curved geometry [1] as well as the coherent synchrotron radiation [2-6]. The resulting emittance dilution, which is independent of the beam energy, appears to place very stringent limits on many of the future bunch compressor designs. Coherent synchrotron radiation itself has been observed in special experiments [7,8], but, at this time, the effect on the beam has no...