Accelerator Configuration

The LHeC could be realised either as a ring-ring (with a new lepton ring in the LHC tunnel) or as a linac-ring (based on a superconducting electron linac, configured as a recirculator) collider. In the CDR 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. After careful consideration of installation issues and parameters of the electron beam, the linac-ring option has been chosen for the next phase of design as this is rather independent of the LHC.

The accelerator complex is of racetrack shape. A 500-MeV electron bunch coming from the injector is accelerated in each of the two 10-GeV SC linacs during three revolutions, after which it has obtained an energy of 60 GeV. The 60-GeV beam is focused and collided with the proton beam. It is then bent by 180° in the highest-energy arc beam line before it is sent back through the first linac, at a decelerating RF phase. After three revolutions with deceleration, re-converting the energy stored in the beam to RF energy, the beam energy is back at its original value of 500 MeV, and the beam is now disposed in a low-power 3.2-MW beam dump. A second, smaller (tune-up) dump could be installed behind the first linac.
Strictly speaking, with an injection energy into the rst linac of 0.5 GeV, the energy gain in the two accelerating linacs need not be 10 GeV each, but about 9.92 GeV, in order to reach 60 GeV after three passages through each linac. Considering a rough value of 10 GeV means that we overestimate the electrical power required by about 1%. Each arc contains three separate beam lines at energies of 10, 30 and 50 GeV on one side, and 20, 40 and 60 GeV on the other. Except for the highest energy level of 60 GeV, at which there is only one beam, in each of the other arc beam lines there always co-exist a decelerating and an accelerating beam. The effective arc radius of curvature is 1 km, with a dipole bending radius of 764 m. The ERL configuration is depicted in the figure below.

The shape, arc radius and number of passes have been optimised with respect to construction cost and with respect to synchrotron radiation effects. The two straight sections accommodate the 1-km long SC accelerating linacs. In addition to the 1km linac section, there is an additional space of 290 m in each straight section of the racetrack. In one straight of the racetrack 260 m of this additional length is allocated for the electron nal focus (plus matching and splitting), the residual 30 m on the other side of the same straight allows for combining the beam and matching the optics into the arc. In the second straight section of the racetrack the additional length of the straight sections houses the additional linacs for compensating the 1.88 GeV energy loss in the return arcs The total circumference of the ERL racetrack is chosen as 8.9 km, equal to one third of the LHC circumference. This choice has the advantage that one could introduce ion-clearing gaps in the electron beam which would match each other on successive revolutions. The length of individual components is as follows. The exact length of the 10-GeV linac is 1008 m. The individual cavity length is taken to be 1 m. The optics consists of 56-m long FODO cells with 32 cavities. The number of cavities per linac is 576. The linac cavity filling factor is 57.1%. The effective arc bending radius is set to be 1000 m. The bending radius of the dipole magnets is 764 m, corresponding to a dipole lling factor of 76.4% in the arcs. The longest SR compensation linac has a length of 84 m (replacing the energy lost by SR at 60 GeV). Combiners and splitters between straights and arcs require about 20-30 m space each. The electron final focus may have a length of 200-230 m.

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