By nature, HL-LHC magnet circuits span many different technical domains and teams. Two dedicated international reviews – in 2016 and 2019 – and three internal CERN reviews – in 2017, 2020 and 2023 – have helped shape the HL-LHC magnet circuits, the associated instrumentation and the constituting systems being prepared at CERN and throughout the collaborations, and to be installed in the LHC tunnel during Long Shutdown 3.
Figure 1 shows the attendees of the first international review on the conceptual design of the HL-LHC magnet circuits, held in March 2016. The discussions during and the outcome of the reviews set a clear path for the detailed work that followed, and significantly helped reach a pivotal point in the lifecycle of the HL-LHC magnet circuits: their design validation in the HL‑LHC Inner Triplet (IT) String test facility.
Figure 1. Attendees of the Conceptual Design Review of the Magnet Circuits for the HL-LHC – March 2016. CERN
From its establishment in mid-2016, the Magnet Circuit Forum (MCF) has hosted technical discussions either in the form of general meetings or dedicated thematic discussions. In total, more than 200 meetings took place, involving experts from 11 of the 20 HL-LHC Work Packages (WPs). The concerned WPs include accelerator physics, magnets, the quench and machine protection systems, warm and cold powering systems, cryogenics, machine integration and installation, the IT String, HL‑LHC commissioning and HL-LHC infrastructure. The MCF has played, and continues to play, a key role in coordinating the discussions and decisions for topics crucial to the HL-LHC collaborations and for integrating the WP deliverables in coherent magnet circuits (described below on a few selected topics).
Coordinating Quench Performances
A reliable detection and activation of adequate mitigation measures is essential to preserve the integrity of the HL-LHC magnet circuits throughout the various operating scenarios during the HL‑LHC lifetime, and tests and simulations are the basis on which the overall quench performance is ensured. Extensive quench simulations were performed on all HL-LHC magnet circuits and in particular the Inner Triplet (IT) quadrupole circuit and its constituent MQXF magnets. The simulations aim to validate the different standalone test results in the diverse test benches at CERN, at the collaboration premises and at the HL‑LHC IT String. The assumptions and boundary conditions (e.g. the conductor parameter variations and system failure cases), and the circuits parameters (e.g. parameters of the warm powering), are systematically discussed in the MCF, and a coherent strategy is defined across the different HL‑LHC circuits to guarantee consistent and reliable magnet protection.
Figure 2 shows an example of the quench simulation results of the MQXFB magnet within the IT quadrupole circuit with the developed adiabatic hot spot temperature and the voltages to ground. In addition, the MCF is collecting the actual measurement data of the quench heater strip resistances and of the quench heater circuit parameters.
Figure 2: Adiabatic hot spot temperature and minimum/maximum voltages to ground in a simulated discharge after a quench at nominal current in MQXFB. Courtesy of E. Ravaioli
This to ensure that an adequate performance on the quench heaters is achieved on all the concerned magnets despite the variation of the measured resistance values.
Figure 3 shows an example of the MQXFA/B magnets with the calculated peak current and energy density as performance indicators, whereas figure 4 shows the modulable resistance in the quench protection system that allows the tuning of the quench heater circuits to account for the resistance variation of the individual quench heater strip.
Figure 3 (left). Quench heater strip resistance variations for the MQXFA/B magnets and the range of the quench heater circuit component parameters and the resulting performance indicators, HL-LHC circuit parameters table. Figure 4 (right). Modulable resistances to adapt to the strip resistance variation as a part of the quench heater circuits. CERN
The self-protected nature of the 120 A higher-order corrector magnets of the HL-LHC makes it possible to dispense of an active quench detection system and protection system, relying entirely on the power converter’s reactivity to switch off when a quench occurs. The quench simulations and the various failure scenarios (e.g. the power converter not switching off) have been considered and simulated by the collaborators at INFN, ensuring a robust powering scheme of the corrector magnets. Additional simulations have been performed jointly by CERN and INFN colleagues to simulate the quench transients in these corrector magnets with the 3D models shown on figure 5.
Figure 5. Simulated sextupole, octupole, decapole, and (skew) dodecapole HL-LHC magnets in the STEAM and LEDET framework. Courtesy of D. Mayr, E. Ravaioli, S. Mariotto, M. Prioli, M. Statera et al.
Magnet Circuit Instrumentation
Two CERN-internal reviews were fundamental to consolidate, optimise and propose a coherent magnet instrumentation scheme for the HL-LHC. The review comments and recommendations were thoroughly assessed in the MCF by stakeholders, leading to their broad implementation across system instrumentation.
These studies and detailed discussions have led to an optimised and coherent magnet circuit instrumentation, and detail-oriented layouts, to represent the instruments like the voltage pickups and the temperature sensors in the helium enclosure. The quench detection scheme was also elaborated and approved for the complete magnet circuits. The MCF has also drafted the general HL-LHC instrumentation layouts for the interaction region magnet circuits, and the quench detection representation layout. These layouts have proven to serve widespread teams such as the magnet manufacturers, the electrical quality assurance team, the cryogenics team and the quench detection experts. Fig. 6 depicts the routing of the voltage pickups from the magnets to the quench detection system as a collaboration among several WPs.
Figure 6. Fully integrated magnet circuit instrumentation scheme from the (a) magnets, capillaries, flanges, courtesy of H. Pri, (b) the Instrumentation Feedthrough System boxes, courtesy of G. D’Angelo, to the (c) the quench detection system patch panel with the ElQA ports, courtesy of J. Steckert.
Comprehensive Electrical Quality Assurance (ElQA) of the HL-LHC Magnet Circuits
Testing for the electrical quality of the HL-LHC magnet circuits is fundamental to ensure system robustness. Figure 7 shows an example of electrical verification tests being carried out in the IT String by the ElQA team. Nevertheless, ensuring electrical quality starts way before the final validation testing.
Assessing the behaviour of the circuits during quenches is one of the first steps in determining the values to be applied during ElQA dielectric verification tests. As a result of the collaboration in the MCF, a complete set of electrical design criteria documents (EDC) was approved for HL‑LHC – covering the magnets, the superconducting link, the superconducting bus bars and the systems connected to the magnet circuits operating at room temperature (i.e. DC cables, signal cables, circuit disconnector boxes, etc.). For example, since the superconducting link in the HL-LHC will operate in gaseous helium, a test voltage of 1.1 kV has been established under the same environment at room temperature (T = 24 ± 7 °C, p = 1.2 ± 0.1 bar) to ensure a proper safety margin for the cold powering system's operation.
Furthermore, ElQA includes instrumentation sanity checks and polarity verifications. These tests validate the different magnets and other components throughout their lifetime. These verification tests commence already during the interconnection activities, giving the go-ahead for the next activity and ensuring the components are ready for operation. These tests, which are coordinated within the MCF, have proven to be crucial to detect non-conformities in the circuit instrumentation, bus bar inversions during interconnection activities and other types of non-conformities with an impact on the quench detection and protection of the magnet circuits.
Figure 7: ElQA test set up for one of the quadrupole magnets in the IT String. CERN
Preparing HL-LHC Magnet Circuits for Commissioning
A final and important chapter in the study phase of magnet circuits is preparing them for the hardware commissioning in the HL-LHC as a part of the overall LHC machine. In collaboration with the different WPs and the LHC Magnet Circuits, Powering and Performance Panel (MP3), the MCF has been very active in producing the hardware commissioning procedures for the different magnet circuits, with a total of 5 powering procedures being released for the new HL-LHC circuit families (HL-LHC Inner Triplet - RQX, HL‑LHC Individually Powered Dipoles - RD1 and RD2, HL-LHC 2 kA Corrector Circuits - RCBX, HL-LHC 200 A Circuit RQSX3 and the HL-LHC 120 A circuits the High-Order Correctors). These procedures and related documentation will be implemented on the HL-LHC IT String circuits to gather initial feedback on acceptance criteria for electrical performance, cryogenics, quench behaviour, and other factors, providing a solid foundation for hardware commissioning of the HL-LHC magnet circuits.
Final Comments
HL-LHC magnet circuits span diverse technical domains and teams. Nearly a decade of MCF activity, involving dozens of experts, has been key in integrating WP deliverables into coherent HL-LHC magnet circuits through studies, testing, and technical discussions. This transverse work is starting to show its fruits with the finalisation of installations in the HL-LHC IT String and the upcoming IT String magnet cooldown and powering in 2025 and 2026.
The MCF has been already preparing for this exciting next chapter for years, and the forum is planned to remain mobilised until the full HL-LHC magnet circuit implementation at the end of Long Shutdown 3.