New magnets aim to meet positron demands of 91km collider

Researchers at the Paul Scherrer Institute (PSI) in Switzerland are developing a new magnet design utilizing high-temperature superconductors to address a critical challenge for future particle physics. The technology aims to generate the intense positron beams required for CERN’s planned 91 km Future Circular Collider, a facility intended to succeed the 27-km Large Hadron Collider as it nears the end of operations in the 2040s.

“The FCC-ee needs a much larger number of positrons than current technology can efficiently produce and capture,” says Paolo Craievich of the PSI Center for Accelerator Science and Engineering. This advance focuses stray positrons with a peak magnetic field of 12.7 tesla, exceeding the capabilities of existing sources.

FCC-ee Demands Drive Positron Sources

The newly tested magnet design at the Paul Scherrer Institute (PSI) achieves a peak magnetic field of 12.7 tesla, a substantial increase over the 3.5 tesla currently attainable in operational positron sources like Japan’s SuperKEKB collider. This leap in field strength directly addresses a critical bottleneck in preparing for the Future Circular Collider (FCC), a proposed 91 km circumference successor to the 27-km Large Hadron Collider.

The FCC-ee configuration, prioritized by the CERN Council in May 2026, requires significantly more positrons than existing technology can efficiently deliver for high-luminosity electron-positron collisions. Achieving these collision rates is essential to allow the FCC-ee to generate an abundance of Higgs bosons for detailed study. The experimental setup at PSI utilizes high-temperature superconductors, specifically ReBCO (rare-earth barium copper oxide) materials, to construct the capture magnet.

After capture by the ReBCO solenoid, the positrons proceed through radio-frequency cavities designed to accelerate the particles and organize them into tightly packed bunches. These initial results provide experimental validation of the baseline approach and its potential to meet the stringent requirements of the FCC-ee, a project that will necessitate positron beams with an intensity of 10¹³ positrons per second. While current positron sources rely on bombarding a target with electrons to generate positrons, the resulting shower of particles scatters in multiple directions.

The PSI design focuses on collimating this shower using an intense magnetic field, improving capture efficiency, a key factor identified by Iryna Chaikovska of the University of Paris-Saclay, who notes, “The real challenge for a positron source is not only to produce enough positrons but also to capture enough of them.” The team’s preliminary measurements show that this technology can be scaled up.

“An important next step will be to benchmark the measurements in detail and to demonstrate how this promising approach can be further optimized and scaled toward the full FCC-ee requirements,” Craievich explains, outlining the immediate priorities for continued development. The need for these advancements is becoming increasingly urgent as the Large Hadron Collider nears the end of its operational lifespan, with the 2040s currently projected as its decommissioning timeframe.

This timeline necessitates a functional successor facility like the FCC, and the successful development of a high-efficiency positron source is paramount to its realization. The work at PSI represents a critical step in ensuring the FCC-ee can deliver on its promise of precision in particle physics, enabling researchers to probe the fundamental laws of nature with greater accuracy than ever before.

ReBCO Superconductors Enable High-Field Positron Capture

The completed P³ system at the Paul Scherrer Institute has demonstrated a functional positron beam, validating a new approach to particle beam generation. This achievement relies on a 12.7 tesla solenoid constructed from ReBCO (rare-earth barium copper oxide) superconducting tape, a material chosen for its ability to maintain superconductivity at relatively high magnetic fields and temperatures. While conventional superconducting magnets require operation at temperatures near absolute zero, the PSI team designed their coil to operate at 15 K, significantly simplifying cooling requirements and increasing current capacity.

ReBCO’s performance at high fields addresses a critical limitation of existing positron sources. Current facilities, such as Japan’s SuperKEKB collider, utilize capture magnets reaching 3.5 tesla, insufficient for the demands of the proposed 91 km Future Circular Collider (FCC).

The P³ experiment’s design focuses on collimating the positron shower produced by an electron beam impacting a tungsten target, improving capture efficiency with this intensified magnetic field. This process prepares the beam for injection into a circular accelerator, representing a complete system validation. The team previously tested a prototype of the solenoid at SwissFEL, PSI’s x-ray free-electron laser facility in 2025, building towards this integrated demonstration.

The 2024 study led by Nicolas Vallis at PSI detailed the solenoid’s design, emphasizing the need for a stronger magnetic field than conventional superconductors could deliver. ReBCO’s higher critical temperature, below 93 K, allowed for operation at a warmer 15 K, reducing cooling complexity while maintaining the necessary superconducting properties. This innovation extends ReBCO’s existing use in LHC cooling systems to a new application: actively steering and focusing particle beams. The team’s work establishes that this technology can be scaled up, offering a pathway toward meeting the escalating demands of particle colliders.

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