The efficient use of energy resources for scientific research is a challenging and timely topic, in particular for scientific fields that require particle accelerator facilities, such as particle or nuclear physics. Accelerator facilities capable of providing high electron beam currents with minimum emittance to ultra-relativistic energies are needed for the production of intense, MeV-ranged, quasi-monochromatic photon beams for photonuclear research and nuclear photonics applications. This contribution presents a design for such an electron accelerator which is capable of recycling most of the kinetic energy of the ultra-relativistic electrons by beam deceleration after a quasi destruction-free usage, thereby lowering significantly the electrical power consumption footprint of operating the accelerator without compromising beam delivery performance. We provide for the first time a technically quantitative design for a thrice-recirculating, superconducting energy-recovery LINAC (ERL) with individual beam transport focusing primarily on its use as a driver for a source of MeV-ranged photons from a laser-Compton backscattering (LCB) scheme. As a specific example, we choose an ERL design for maximum electron energy of 520 MeV at an electron current of up to 26 mA in continous wave mode at 1.3 GHz. This allows for 0.79 mA average current in a pulsed mode at 39.4 MHz, providing unprecedented spectral densities on the order of 105 γ /(eV s) on target for photon energies around 5 MeV with the aforementioned repetition rate. Our Design concept of an Individually recirculating Compact ERL (DICE) is discussed. Options for its scientific use are sketched.

Bulk niobium is the standard material for superconducting RF cavities but requires operation at 2 K due to its critical temperature (T-c) of 9.3 K, leading to high cooling costs. Nb3Sn, with a higher T-c (similar to 18.2 K), allows operation at 4.2 K, reducing cryogenic demands and improving efficiency. Although Nb3Sn theoretically offers a higher quality factor (Q), achieving this in practice remains difficult. Co-sputtering Nb3Sn films on copper cavities may improve Q, thermal stability, and delay quenching at high accelerating fields. In this study, Nb3Sn films were co-sputtered on different substrates to investigate the T-c and lower critical field (H-c(1)). The films were deposited in a controlled atmosphere and analyzed using X-ray diffraction, resistivity measurements and superconducting quantum interference device magnetometry. The results confirm the successful formation of the Nb3Sn phase with well-defined superconducting transition. The measured T-c value for films grown on r-cut sapphire reached 17.8 K, close to the bulk Nb3Sn value. On the technically relevant substrate copper, a T-c value of 15 K was obtained. Notably, the H-c(1) value was found to be not only higher than that for bulk Nb3Sn but also higher than that of pure Nb. A high value of H-c(1) is critical for operating high Q cavities at accelerating fields above 20 MV/m.

Laser-driven ion beams offer significant advantages over radio-frequency accelerated beams, making them particularly promising for biomedical applications. Key features include their ultrashort pulse duration, on the order of picoseconds, and high peak fluxes ranging from 10^{11} to 10^{13} particles per shot. These characteristics enable the delivery of ultrahigh dose rates, potentially reaching the FLASH therapy regime. The future implementation of laser-driven ion beams in clinical settings relies on the development of advanced focusing and beam transport systems capable of precisely controlling parameters such as energy range, beam focus, and dose distribution to meet stringent therapeutic requirements. However, the integration of ultrashort laser-driven ion beams into medical treatments presents challenges, particularly due to their broad energy spectrum and high angular divergence. We present 3D simulation-based studies investigating the focusing effects of different high-current solenoid configurations on laser-plasma-accelerated proton beams. To identify the optimal focusing solution, we designed, analyzed, and compared high-current solenoids of varying dimensions and shapes, both as stand-alone magnetic elements and as part of two-solenoid focusing systems. Our analysis focuses on magnetic fields between 6 and 9 T, generated by the high-current solenoids. The study considers a proton beam with an energy range from a few MeV to 20 MeV and an initial divergence of 21°. Results are evaluated in terms of collection efficiency, beam focusing position, and beam profile. The optimized beamline configuration was then used to assess the dose distribution in a cylindrical water target with a volume of 78 mm^{3} by calculating the absorbed dose and dose delivery rates, demonstrating the potential of laser-plasma-accelerated proton beams for radiotherapy applications.

The seminar on energy recovery linacs (ERLs) is giving an overview of the field: How does an ERL work? What have been important milestones in ERL history? What are the reasons to use an ERL instead of a conventional accelerator? As examples of the landscape of machines, ranging from ancient ERLs up to future projects, this chapter will give results of the runs from CBETA (USA) and S-DALINAC (Germany). The two facilities bERLin-Pro/SEALab (Germany) and MESA (Germany) will belong to the next ERLs to be in operation and will be introduced briefly. The way to future ERLs will also be addressed.

Electron-induced fission reactions will be deployed and studied at the S-DALINAC. For these experiments, it is desirable to limit the transverse displacements of the electron beam due to drifts and distortions to below 200 mu m. Prior to the present work, this requirement was not met at the S-DALINAC. A total of three systems have been developed, implemented and interconnected to monitor and improve the transverse beam stability: (i) The beam position monitoring system based on high-speed cameras provides transverse beam parameters with micrometer resolution at a kilohertz rate. (ii) A newly designed compensator device mitigates longitudinal and transverse perturbations from the mains frequency on the electron beam. (iii) Finally, an active beam stabilization system ensures high beam stability at the intended interaction point of the electron beam and the fission target. The design and implementation of these systems as well as performance measurements will be presented in this paper.

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.

Regenerative multibunch beam breakup instabilities are a well-known phenomenon in recirculating linacs where particle bunches pass multiple times through the same superconducting rf cavities with extremely high quality factor. This is in particular true for energy recovery linacs. Parasitic electromagnetic modes excited in the cavities can affect bunch dynamics in such a way, that on its subsequent passes it excites the modes further and a positive feedback loop is formed. Direct bunch tracking and a stability analysis technique can be used to study the instability. Usually only dipole modes are considered. In the present work, similar approaches are applied to monopole and quadrupole modes and illustrated with simulation results for the S-DALINAC and MESA facilities. An approximated stability analysis technique with better performance for the case of multiple recirculations is proposed. Countermeasures including betatron phase advance adjustment and additional betatron coupling are considered and a universal criterion for assessment of their effectiveness is proposed. A simple model of a damped oscillator with feedback is proposed as a universal example illustrating the phenomenon in general.

Energy-recovery linacs (ERLs) have been emphasised by the recent (2020) update of the European Strategy for Particle Physics as one of the most promising technologies for the accelerator base of future high-energy physics. The current paper has been written as a base document to support and specify details of the recently published European roadmap for the development of energy-recovery linacs. The paper summarises the previous achievements on ERLs and the status of the field and its basic technology items. The main possible future contributions and applications of ERLs to particle and nuclear physics as well as industrial developments are presented. The paper includes a vision for the further future, beyond 2030, as well as a comparative data base for the main existing and forthcoming ERL facilities. A series of continuous innovations, such as on intense electron sources or high-quality superconducting cavity technology, will massively contribute to the development of accelerator physics at large. Industrial applications are potentially revolutionary and may carry the development of ERLs much further, establishing another shining example of the impact of particle physics on society and its technical foundation with a special view on sustaining nature.

A new emittance measurement system has been installed at the superconducting, recirculating electron accelerator S-DALINAC. It is based on the quadrupole-scan technique and beam profile measurements with optical transition radiation. A first demonstration of this setup has been conducted as part of the beam-based alignment of an SRF cavity. The corresponding emittance measurement is described here, and the following data evaluation is discussed.

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 Energy Recovery Linac (ERL) paradigm offers the promise to generate intense electron beams of superior quality with extremely small six-dimensional phase space for many applications in the physical sciences, materials science, chemistry, health, information technology and security. Helmholtz-Zentrum Berlin started in 2010 an intensive R&D programme to address the challenges related to the ERL as driver for future light sources by setting up the bERLinPro (Berlin ERL Project) ERL with 50 MeV beam energy and high average current. The project is close to reach its major milestone in 2020, acceleration and recovery of a high brightness electron beam. The goal of bERLinProCamp 2019 was to discuss scientific opportunities for bERLinPro 2020+. bERLinProCamp 2019 was held on Tue, 17.09.2019 at Helmholtz-Zentrum Berlin, Berlin, Germany. This paper summarizes the main themes and output of the workshop.