The Hollow Electron Lens (HEL) will provide, by means of a hollow electron beam overlapping over about 3 m length with the HL-LHC hadron beam, an active way to enhance particle diffusion and controlled depletion of the hadron beams tails. It is planned to be built partly as an in-kind contribution from Russia and the UK, and partly by CERN. A conceptual design of the HEL, developed in collaboration with the Budker Institute of Nuclear Physics (BINP, Novosibirsk) studying the electron beam dynamics and the magnetic system, and Cockcroft Institute (CI, Daresbury) developing the Beam Gas Curtain (BGC) monitor, was instrumental to integrate the system into the HL-LHC baseline (following the 4th C&S review, November 2019). This work allowed demonstrating the HEL feasibility and its integration in the LHC tunnel and infrastructure. In addition, the study clarified the requirements for magnet protection and powering, including the development of the electron gun (in collaboration with Fermilab – Batavia), collector and instrumentation.
In April 2021, a “Kick-Off” meeting, chaired by Adriana Rossi as technical coordinator, gathered all parties involved at CERN, with the goal of validating the HEL functional specifications and a HEL reference design (as a starting point to organize the work), defining the work break-down structure and responsibilities, clarifying what has to be done at CERN (and with what additional in-kind) and what can be outsourced including validation measurements, assembly, and testing. The outcome provides a solid base for the detailed technical agreement with BINP.
After feedback from the meeting, the final version of the functional specifications (CERN EDMS doc. No. 2514085) is now being circulated for approval at CERN. This document presents the HEL design requirements and its operation at LHC, and includes parameters established for the magnetic and powering systems, and for the cryogenic integration.
The components to be built, tested, and installed entirely by CERN are the cryogenic services and their connections to the HEL (for which a first study has been completed), quench detection and energy extraction system (already dimensioned according to the estimated magnet stored energy), power converters (for which the circuits parameters have been defined and integration is almost completed), controls (yet to be specified), cabling, and instrumentation read-out electronics. Integration studies (ancillaries included) are also part of the work at CERN, and they are very well advanced, allowing the drafting of an installation plan, which includes a full modification of the cryogenic line at the HEL location.
The functional design of the magnet system has reached a good level of maturity. Further studies are being performed, also in collaboration with BINP experts, to improve the electron trajectory and minimize the residual field in the center of the hollow electron beam, which would otherwise cause perturbations to the LHC beam, and to better integrate the BGC. The functional specification documents and drawings for the magnet system are due to be completed by November 2021 and reviewed internally at CERN in December 2021, before handing over to BINP for review and implementation. The strategy of where to assemble, align and test the magnets has been analyzed at the kick-off meeting, revealing the need to clarify the purpose and location of a cryo-test facility for the full assembly, in addition to the already planned capability to test the individual cold masses at BINP.
The program for the design and prototyping of the electron gun, collector and Beam Position Monitor (BPM) has progressed, in spite of COVID-19. The full engineering specification and design should be completed in the second half of 2023. These components are then to be built by BINP and validated at the CERN Electron Beam Test Stand (with normal conductive magnets).