The physics programme and the design are described of a new collider for particle and nuclear physics, the Large Hadron Electron Collider (LHeC), in which a newly built electron beam of 60 GeV, to possibly 140 GeV, energy collides with the intense hadron beams of the LHC. Compared to the first ep collider, HERA, the kinematic range covered is extended by a factor of twenty in the negative four-momentum squared, Q 2 , and in the inverse Bjorken x, while with the design luminosity of 10 33 cm −2 s −1 the LHeC is projected to exceed the integrated HERA luminosity by two orders of magnitude. The physics programme is devoted to an exploration of the energy frontier, complementing the LHC and its discovery potential for physics beyond the Standard Model with high precision deep inelastic scattering measurements. These are designed to investigate a variety of fundamental questions in strong and electroweak interactions. The LHeC thus continues the path of deep inelastic scattering (DIS) into unknown areas of physics and kinematics. The physics programme also includes electron-deuteron and electron-ion scattering in a (Q 2 1/x) range extended by four orders of magnitude as compared to previous lepton-nucleus DIS experiments for novel investigations of neutron's and nuclear structure, the initial conditions of Quark-Gluon Plasma formation and further quantum chromodynamic phenomena. The LHeC may be realised either as a ring-ring or as a linac-ring collider. Optics and beam dynamics studies are presented for both versions, along with technical design considerations on the interaction region, magnets including new dipole prototypes, cryogenics, RF, and further components. A design study is also presented of a detector suitable to perform high precision DIS measurements in a wide range of acceptance using state-ofthe art detector technology, which is modular and of limited size enabling its fast installation. The detector includes tagging devices for electron, photon, proton and neutron detection near to the beam pipe. Civil engineering and installation studies are presented for the accelerator and the detector. The LHeC can be built within a decade and thus be operated while the LHC runs in its high-luminosity phase. It so represents a major opportunity for progress in particle physics exploiting the investment made in the LHC.

Accelerator physics issues and their influence on performance are presented for the Large Electron Positron storage ring (LEP) at CERN in Geneva, Switzerland. After several years of operation on the Z boson resonance at beam energies around 45 GeV, the beam energy was increased in steps to over 100 GeV. The major power loss to synchrotron radiation and its consequences on the maximum beam energy are discussed. The subjects of luminosity optimization, beam-beam effect, instabilities, detector backgrounds and beam lifetime are addressed. The precise beam energy calibration, which is of particular importance for the determination of standard model parameters is described.

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The Large Hadron Collider (LHC) is the world's largest and most energetic particle collider. It took many years to plan and build this large complex machine which promises exciting, new physics results for many years to come. We describe and review the machine design and parameters, with emphasis on subjects like luminosity and beam conditions which are relevant for the large community of physicists involved in the experiments at the LHC. First collisions in the LHC were achieved at the end of 2009 and followed by a period of a rapid performance increase. We discuss what has been learned so far and what can be expected for the future. AbstractThe Large Hadron Collider (LHC) is the world's largest and most energetic particle collider. It took many years to plan and build this large complex machine which promises exciting, new physics results for many years to come. We describe and review the machine design and parameters, with emphasis on subjects like luminosity and beam conditions which are relevant for the large community of physicists involved in the experiments at the LHC. First collisions in the LHC were achieved at the end of 2009 and followed by a period of a rapid performance increase. We discuss what has been learned so far and what can be expected for the future.

The Large Hadron Collider (LHC) was first suggested (in a documented way) in 1983 (S. Myers and W. Schnell, Preliminary performance estimates for a LEP proton collider, LEP Note 440, April 1983) as a possible future hadron collider to be installed in the 27 km "LEP" tunnel. More than 30 years later the collider has been operated successfully with beam for three years with spectacular performance and has discovered the longsought-after Higgs boson. The LHC is the world's largest and most energetic particle collider. It took many years to plan and build this large complex machine which promises exciting, new physics results for many years to come.I describe the LHC design objectives, review some of the more relevant beam effects, define the major accelerator components and parameters, and finally give an overview of the commissioning and operational performance since the initial turn on of the collider. The latter will include the major accident which took place in September 2008 and the subsequent repair and redesign of the faulty components.The first attempt to circulate beam in the LHC in September 2008 were initially very successful. However, after only nine days of preliminary beam commissioning, on 19 September 2008, disaster struck: the last octant was being ramped up in preparation for high energy operation when a magnet interconnect failed and the enormous energy stored in the superconducting magnets was released in an uncontrolled way and damaged around 600 m of the LHC installed equipment. The next 14 months were crucial for the machine. A crash programme for the repair of the damaged sector was initiated as well as in depth studies to understand the cause of the failure and make design changes which would ensure that such an accident could never reoccur in the future.In this paper, the story of the four years of the intensive activity since the accident is described starting with the repair of the damaged area and followed by the three very successful years of beam operation.