Conventional electron linear accelerators are essential research tools but limited in providing high beam currents. Energy recovery technology enables high beam currents with reasonable and sustainable power supply requirements by recycling the electrons’ kinetic energy. Independently, higher beam energies can be achieved if electrons are accelerated multiple times in a linear accelerator. The combination of both techniques results in a multi-turn energy recovery linear accelerator, which is capable of providing high beam power. Here we report the demonstration of efficient energy recycling in multi-turn operation where we saved up to 87% of the consumed beam power in the main linear accelerator of the superconducting Darmstadt electron linear accelerator (S-DALINAC). In this setting, the cumulative phase slippage effect, caused by the different speeds of the electrons per main linear accelerator pass and the resulting different interactions with the alternating electric field, cannot be neglected and was compensated. Our proof-of-principle demonstration shows how multi-turn energy recovery linear accelerators can outperform conventional machines due to the potential for considerable power saving while providing higher beam power.

The superconducting Darmstadt linear electron accelerator (S-DALINAC) has been operated as an energy recovery linac (ERL) for the first time. The S-DALINAC is a recirculating superconducting radio-frequency (SRF) accelerator and had been upgraded with an additional recirculation beamline. It features a path length adjustment system that provides a freedom of choice of 360° for the rf phase difference between the electron bunches recirculated through the new beamline and the phase of the accelerating TM_{010} mode of the oscillating electromagnetic field in the SRF cavities of the accelerator. A choice of around 180° for this phase difference results in a deceleration of the recirculated beam and a corresponding transfer of the particles’ kinetic energy back to the energy of the electromagnetic field in the cavities. The main components relevant for ERL operation are described and data of the first operation as an SRF-ERL are presented.

The S-DALINAC* is a thrice-recirculating linear electron accelerator operating in cw-mode at a frequency of 3 GHz. A path-length adjustment system in the second recirculation beam line allows to shift the beam phase by 360° and thus to operate in ERL mode. For the multi-turn ERL operation, the beam will be accelerated twice and subsequently decelerated twice again (not demonstrated yet). For this mode, it is necessary to develop a nondestructive beam diagnostics system in order to measure the beam position, phase and beam current of both, the accelerated and the decelerated beam, simultaneously in the same beamline. A particular challenge will be the operation at low beam currents of 100 nA, which corresponds to bunch charges of about 30 aC. The conceptional study of a 6 GHz resonant cavity beam position monitor will be presented together with alternative solutions.

The S-DALINAC is a thrice-recirculating electron accelerator, capable of being operated in energy-recovery linac (ERL) mode. The change between conventional, accelerating operation and ERL mode is done by changing the distance traveled by the bunches in the second recirculation beam line. This distance is increased or decreased by moving dedicated magnets on a rail system. These so called pathlength adjustment systems exist in all recirculation beam lines but only the stroke in the second recirculation beam line enables a phase shift of up to 360° of the RF phase. After having proven the functionality of the new systems the once-recirculating ERL operation was demonstrated successfully in August 2017. This contribution will introduce the path-length adjustment systems and present the commissioning of them. Results of the operation of the first ERL in Germany will be discussed.