In the Accelerator R&D Roadmap for particle physics a comprehensive R&D program is developed with the objective to demonstrate the high-power multi-turn ERL technique. In the realm of particle physics, the main R&D objectives for ERL are geared towards high-energy and high-intensity accelerators, including synergies with industry. The various aspects in the program are formulated in the Roadmap document, elaborated on in a dedicated publication, and summarized in the figure below.
Several critical aspects for next-generation high-energy accelerator-based facilities for particle physics were identified. They include:
- Generation of, high-brightness CW electron beams for high-luminosity operation, i.e., continuous reliable injection of high-current CW electron beams into a LINAC with high-charge, low-emittance bunches. Here SRF photoinjectors, with their ability to operate CW at high-voltage and high electric field to minimize disruptive non-linear space charge effects while handling high currents have the greatest potential to satisfy the requirements. Future systems must go more than an order of magnitude beyond the state-of-the-art.
- Very low levels of beam loss and a high degree of beam control will be essential for energy efficiency, radiation protection and machine protection. Here novel beam diagnostics are essential, with a dynamic range many orders of magnitude for beam commissioning or to discern “wanted” beam from “unwanted” beam (such as beam halo).
- Ability for LINACs to accelerate a very high average beam current without compromising the beam quality. The accelerator will require special cavity and higher-order mode absorber designs that are able to extract up to kWs of HOM power efficiently to minimize the beam disruption due to wakefields. Systems such as waveguide dampers attached directly to the cavity cell should be investigated.
- An improvement of the operating efficiency of the LINAC systems by more than a factor of two is essential for future high-energy facilities to be environmentally and economically feasible. This includes RF source efficiency (e.g., that of solid-state amplifiers or magnetrons), efficient cavity field control by eliminating noise sources such as microphonics with fast reactive tuners and, critically, cryogenic efficiency by moving beyond state-of-the-art niobium to operate SRF systems at 4.2 K and above. Nb3Sn is presently the superconductor that holds the greatest promise.
- Efficient recovery of the beam power after the experiment: At up to 10 GW and more of beam power, it is mandatory that essentially all power is recovered before the beam is dumped. Here the ERL principle comes into play. While existing facilities have demonstrated the principle, they fall short in terms of beam current, power and energy by up to three orders of magnitude. All the R&D aspects above will play a critical role, and special attention must be paid to beam sources, beam dynamics and cavity design, including new dual-axis cavities. Multi-turn ERL operation presents additional challenges. It is insufficient to simply demonstrate successful operation of subsystems: it is essential to validate full operation in an ERL environment.
The research programs at the emerging PERLE and bERLinPro R&D facilities embrace in a comprehensive way most of these ERL R&D objectives with a view to greatly enhance the capability and sustainability of future particle physics colliders.
