Bringing Superconducting Power Transmission for HL-LHC from concept to reality

Nb-Ti, MgB2 and ReBCO: a combination of conventional and novel superconductors for the powering of the HL-LHC magnets. Demo 2 and Demo 3 demonstration systems were successfully qualified at CERN in 2020.


Handling the 62.5 m long |110| kA MgB2 power transmission line

Handling the 62.5 m long |110| kA MgB2 power transmission line. (Image: CERN)

In December 2020, the development phase of the high-current MgB2 (magnesium diboride) power transmission lines for the High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) reached completion with the successful qualification of Demo 3, the demonstrator cold powering system for the HL-LHC Matching Sections. Demo 3 followed the validation, in autumn 2020, of Demo 2, the demonstrator cold powering system for the HL-LHC triplet magnets.

Demo 3 and Demo 2 each consist of a 62.5 m power transmission line made from a flexible, double-wall, corrugated cryostat, together with complex, final size MgB2 cable assemblies. There are up to nineteen cables in a single assembly, twisted together to form a compact bundle. The electrical insulation of each cable was tested at up to 15 kA in air. Each pair of cable will feed a different HL-LHC magnet circuit. The cables are cooled by an optimized forced-flow of helium gas generated and controlled within the system.

The MgB2 lines transferred record DC currents of up to about |120| kA (Demo 2) and |75| kA (Demo 3) at temperatures of up to 31 K. Each cold powering system included terminations of the type that will be used for HL-LHC: at one side, Nb-Ti cables – connected to MgB2 in a 4.5 K saturated liquid helium bath and, at the other side the ReBCO High Temperature Superconducting (HTS) part of the current leads, connected to MgB2 in a helium gas environment nominally at 20 K. The HTS ReBCO was operated at up to 65 K.

Demo 2 MgB2 and ReBCO cold powering system
Demo 2 MgB2 and ReBCO cold powering system. (Image: CERN)

Tests validated the current capability in nominal conditions and operation in transient modes, i.e. fast electrical discharges and combined powering of the various cables, according to different electrical scenarios; thermo-mechanical behaviour of the cold powering systems during cool-down and warm-up; feasibility of high-current low-resistance splices between MgB2 to ReBCO and MgB2 to Nb-Ti; global cryogenic and electrical performance; and novel ReBCO based HTS current leads. Handling, spooling and routing of the power transmission lines were also performed successfully for the first time.

While 2020 completed around a decade of R&D on MgB2 power transmission lines, 2021 started with the delivery of the first full-size series |120| kA MgB2 cable assembly for the HL-LHC Triplets. The cable was successfully assembled and tested at voltages of up to 15 kV in industry before being delivered to CERN spooled onto a large drum. In the meantime, 1150 kilometers of MgB2 wire were produced by an industrial company and delivered to CERN. This is the total needed for the HL-LHC cold powering systems, and it represents the first large-scale industrial production of MgB2 conductor.

For now, construction of all other series components of the HL-LHC cold powering systems has been launched and all is on track for assembling and testing the first full-scale series system at CERN, the type needed for the powering of the HL-LHC Triplets in early 2022.