accelerate the Main Beam. Up to 90% of the energy of the Drive Beam is extracted and efficiently transferred to the Main Beam. Main beam The electrons needed for the main beam are produced by illuminating a GaAs-type cathode with a Q-switched polarised laser, and are longitudinally polarised at the level of 80%. The positrons for the main beam are produced by sending a 5 GeV electron beam on a tungsten target. After an initial acceleration up to 2.86 GeV, both electrons and positrons enter damping rings for emittance reduction by radiation damping. Both beams are then further accelerated to 9 GeV in a common booster linac. Long transfer lines transport the two beams to the beginning of the main linacs where they are accelerated up to 1.5 TeV before going into the Beam Delivery System (BDS), which squeezes and brings the beams into collision. The two beams collide at the IP with 20 mrad crossing angle in the horizontal plane. Drive beam Each Drive Beam complex is composed of a 2.5 km-long linac, followed by a Drive Beam Recombination Complex: a system of delay lines and combiner rings where the incoming beam pulses are interleaved to ultimately form a 12 GHz sequence and a local beam current as high as 100A. Each 2.5 km-long Drive Beam linac is powered by 1 GHz klystrons. This produces a 148 μs-long beam (for the 1.5 TeV energy stage scenario) with a bunching frequency of 0.5 GHz. Every 244 ns the bunching phase is switched by 180 degrees, i.e. odd and even buckets at 1 GHz are filled alternately. This phase-coding allows the first factor two recombination: the odd bunches are delayed in a Delay Loop (DL), while the even bunches bypass it. The time of flight of the DL is about 244 ns and tuned at the picosecond level such that the two trains of bunches can merge, forming several 244 ns-long trains with bunching frequency at 1 GHz, separated by 244 ns of empty space. This new time-structure allows for further factor 3 and factor 4 recombination in the following combiner rings with a similar mechanism as in the DL. The final time structure of the beam is made of several (up to 25) 244 ns-long trains of bunches at 12 GHz, spaced by gaps of about 5.5 μs. The recombination is timed such that each combined train arrives in its