If you're an avid follower of High-Luminosity LHC (HL-LHC) news, you will no doubt already have heard about "the python", the new superconducting link developed at CERN. It is a component of the new cold powering system that will power the HL-LHC inner triplet magnets, which will focus proton beams more tightly around the ATLAS and CMS collision points.
This new system is packed with novel superconducting technologies: MgB2 superconducting cables, twisted together to form a compact bundle of about 9 centimetres in diameter, are inserted into a 22-centimetre-diameter flexible cryostat, with vacuum insulation and flowing helium gas. The MgB2 cables operate in the helium gas at temperatures from about 4.5 K (-268.7 °C) to 20 K (-253.2 °C). The REBCO high-temperature superconducting cables then transfer the current from 20 K to 50 K (-223.2 °C) and, finally, current leads provide the transition from 50 K to room temperature. This system can carry a direct electrical current (DC) of around 120 kA over the required distance of 85 metres.
While the superconducting cables of the LHC magnets have to be maintained in superfluid helium (at 1.9 K (-272.2 °C)) or in liquid helium (at 4.5 K), the new superconducting part of the system is capable of operating at a temperature of up to 60 K (-213.2 °C) at its highest, qualifying it as "high temperature" in superconductivity terms. "One of the beauties of this new system is that it operates in helium gas. The cryogenic cooling of the superconducting link is at zero cost, because it transfers the helium gas that in any case is needed to cool the current leads. This is one of the benefits of using high-temperature superconductors," explains Amalia Ballarino, leader of the HL-LHC Cold Powering Work Package.
The superconducting link and its flexible cryostat can be spooled onto a large drum and transported like conventional power transmission cables. This new type of superconducting system has enormous potential for future accelerators and in areas beyond accelerator technology where large transfer of current is needed, or for the development of clean aviation.
The first HL-LHC cold powering system has just passed its first tests in the SM18 test facility. While the python was fully qualified in the previous R&D phases, this is the first time that a full power transmission system, transferring current from room temperature to the liquid helium environment via MgB2 and REBCO superconducting technology, has been constructed and successfully validated in final operating conditions. The complexity of the system is enhanced by the multiplicity of the circuits it contains. "The 19 superconducting cables and current leads, rated at currents ranging from 2 kA to 18 kA, transported a total DC current of 94 kA, the maximum current that could be delivered by the test station," adds Ballarino. "Electromagnetic compatibility among circuits was validated, and high-voltage insulation tests were successfully accomplished. This great success is the result of ten years of R&D."
The next steps will take place in early summer, when the cold powering system will be transported to the HL-LHC IT String where the collective behaviour of the inner triplet magnet system will be tested prior to installation underground in the LHC during the next long technical stop (LS3), scheduled to begin in 2026.
