For the LHeC, electrons are accelerated to 60 GeV by means of an Energy Recovery Linac (ERL). This modern concept allows to provide a large number of electrons (109 every 25 ns or 6.4 mA, corresponding to a virtual power of 384 MW) without actually using this power. The trick uses the fact that only a few electrons get lost in the interaction with the proton beam, so they can be fed through the linac again, but in the decelerating phase, which allows them to return their kinetic energy back to the electromagnetic energy stored in the cavities, thus reducing significantly the overall energy consumption.

Superconducting cavities with extremely large quality factors (>1010) can make this process extremely efficient in spite of the power needed to cool the cavities to 1.8 K. A cavity like the one shown in the Figure (courtesy JLAB) accelerates electrons by approximately 18 MV, so about 3’300 would be needed. Since however the ERL is conceived for 3 subsequent passes, this number is reduced to 1’104 such cavities grouped together in 128 cryomodules containing 8 cavities each.

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