Plans for an ERL Test Facility at CERN

In order to make this facility work at the usual European frequencies 704.42 MHz (ESS, SPL,…), 801.59 MHz (LHC harmonic, SPS, LHeC, FCC,…) and 1,300 MHz (XFEL, ILC, …), one could use an injector with a photocathode pulsed at

𝑓𝑓 = 12.1474 ± 0.0022 MHz.

12.142 12.144 12.146 12.148 12.150

0.2

0.1

0.1

0.2

Operation at multiple frequencies

107 𝑓𝑓 − 1,300 MHz

58 𝑓𝑓 − 704.42 MHz 66 𝑓𝑓 − 801.59 MHz 𝛿𝛿𝑓𝑓 MHz⁄

𝑓𝑓 MHz⁄

The ERL test facility is designed to be constructed in stages. A first phase with recirculation would only use two 4-cavity cryomodules and single recirculation – it could reach 150 MeV. A second phase could feature multi-pass operation to reach 300 MeV (2 passes) or 450 MeV (3 passes). Adding two more cryomodules could boost the top energy to 900 MeV. In its full energy version, the ERL test facility will consist of two anti-parallel linacs with two 4-cavity cryomodules in each. Vertical spreaders/combiners separate the beams into up to 3 vertically separated arcs, each of which is optimized for its nominal energy. The highest energy arc is adjusted in length to assure arrival in the decelerating phase when entering the linac again.

Layout/staged design

Strong synergy exists with the MESA project at Mainz University; help with the design and construction of the 802 MHz cavities and cryomodules will result from collaboration with JLAB, who have already contributed significantly to the lattice and with their relevant experience operating CEBAF and the FEL in ERL mode, Cornell University has studied an injector with similar characteristics. CERN invites further collaborations, e.g. for the injector, magnets and other subsystems.

International Collaboration This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under Grant Agreement no 289485. The authors wish to thank H. Schopper for his encouragement and support.

Acknowledgements

The baseline electron accelerator for LHeC and one option for FCC-he is an Energy Recovery Linac. To prepare and study the necessary key technologies, CERN has started – in collaboration with JLAB and Mainz University – the conceptual design of an ERL Test Facility (ERL-TF). Staged construction will allow the study under different conditions with up to 3 passes, beam energies of up to about 1 GeV and currents of up to 50 mA. The design and development of superconducting cavity modules, including coupler and HOM damper designs, are also of central importance for other existing and future accelerators and their tests are at the heart of the current ERL-TF goals. However, the ERL-TF could also provide a unique infrastructure for several applications that go beyond developing and testing the ERL technology at CERN. In addition to experimental studies of beam dynamics, operational and reliability issues in an ERL, it could equally serve for quench tests of superconducting magnets, as physics experimental facility on its own right or as test stand for detector developments. This contribution will describe the goals and the concept of the facility and the status of the R&D.

Abstract

Recirculation arcs: Appropriate recirculation optics are of fundamental concern in a multi-pass machine to preserve beam quality. The design comprises three different regions, the linac optics, the recirculation optics and the merger optics. Our present design involves an FMC cell based lattice. Specifications require isochronicity, path length controllability, large energy acceptance, small higher-order aberrations and tunability. An example layout is shown here for the 150 MeV, 450 MeV and 755 MeV return arcs (left to right). Shown are: red: 𝛽𝛽𝑥𝑥, green: 𝛽𝛽𝑦𝑦, blue: 𝐷𝐷𝑥𝑥, black: 𝐷𝐷𝑦𝑦. Multi-pass linac optics: Here a concise representation of the multi-pass ERL for Linac 1 (left) and Linac 2 (right). Shown are 𝛽𝛽𝑦𝑦(green) and 𝐷𝐷𝑥𝑥 (blue). Arrows below indicate the passages of acceleration and deceleration through both linacs.

Optics

Linac 1

Linac 2

Initial cavity design for 801.59 MHz The initial cavity design is based on existing SPL, JLAB and BNL experience; the considered 802 MHz cavity shapes is sketched below (left: 150 mm ∅ , right: 160mm ∅ );

it is derived from an earlier 704 MHz design. The cavities should be optimized for 𝑄𝑄0 while the accelerating gradient is kept at about 15 MV/m. Scans of the parameter space around this starting point (varying iris aperture, iris width and cavity wall angle) revealed that this first guess was acceptable in terms of 𝑅𝑅 𝑄𝑄⁄ and peak surface fields. Longitudinal and transverse impedance spectra are shown below for the above 2 cavity versions (blue and red). Cryomodule design JLAB had designed an 805 MHz cryomodule for SNS, which is the concept for the 802 MHz baseline design (left). CERN is following for SPL (704 MHz ) an alternative path using SS helium vessels and support by the power couplers (right).

Cavity/Cryomodule design

E. Jensen, O. Brüning, R. Calaga, K. Schirm, R. Torres-Sanchez, A. Valloni – CERN, Geneva, Switzerland A. Bogacz, A. Hutton – Jefferson Lab, Newport News, Virginia, USA K. Aulenbacher – Johannes Gutenberg Universität, Mainz, Germany M. Klein – University Liverpool, UK & CERN

• The European Strategy for Particle Physics, Update 2013, http://council.web.cern.ch/council/en/EuropeanStrategy/esc-e-106.pdf • LHeC Study Group: A Large Hadron Electron Collider at CERN, JPhysG 39(2012) • Future Circular Collider Study – Kickoff Meeting, Geveva, Switzerland 2014, http://indico.cern.ch/event/fcc-kickoff • E. Jensen et al., “Design Study of an ERL Test Facility at CERN”, Proceedings of IPAC 2014, Dresden • A. Valloni et al., “Strawman Optics Design for the LHeC ERL Test Facility”, IPAC2013, Shanghai • A. Hutton, “JLAB ERL and a 802 MHz Cavity Design”, LHeC Workshop 2014, Chavannes-de-Bogis • K. Aulenbacher and F. Maas, “MESA – Mainz Energy-Recovering Superconducting Accelerator”.

Some references