Increasing the potential
Electrical quality assurance testing (ELQA) of the LHC’s superconducting circuits is currently underway in sector 3-4 of the machine. The rigorous examination by the ELQA team covers secondary circuits as well as the main magnets.
ELQA tests are performed in sector 4-5 of the LHC tunnel. The mobile platform (in the red cart) was developed in collaboration with IFJ-PAN to test the LHC magnet circuits; it has been upgraded at the Institute especially for LS1.
The LHC’s superconducting magnets are connected in 1,612 electrical circuits, supplied from 52 electrical distribution feedboxes (DFBs) via no fewer than 3,286 current leads. Were you to add up the electrical currents fed into all the magnets through the current leads, in nominal accelerator operation, you would get 3.3 million amps. The LHC is, above all, a giant electrical installation. This is why quality assurance testing of the electrical systems is so important.
The ELQA team has been operating in the tunnel since 22 February. Their job is to test all the electrical circuits of the superconducting magnets and to certify their readiness for operation at high energy - 7 TeV per beam. “We’re testing the electric circuits of the magnets and the instrumentation. We do this by measuring their electrical parameters, including electrical continuity checks and tests of the insulation against earth,” explains Giorgio D’Angelo, an ELQA project engineer. As part of the testing, the ELQA team applies a high electrical potential to the circuits and checks that the resulting readings are within the specifications. "If we get an unexpected result, we continue troubleshooting until we find out exactly where the problem lies," adds D'Angelo.
A healthy electrical system is absolutely indispensable, as the slightest short-circuit, for example, could lead to a major breakdown and seriously damage the magnets. Of course, each circuit is also fitted with a protection system. In the event of a quench during operations, the protection system takes over, discharging the accumulated energy and bringing the current down to zero.
All these tests need to be done first on the machine in a cold state, corresponding to actual operating conditions in the LHC, and then on the machine warmed up to normal ambient temperature. This is because the LHC, when it goes from 1.9 to 300 Kelvin (see the article "Heatwave warning for the LHC" in Bulletin 12-14/2013), experiences thermal expansion, which can damage the electrical system. Also during Long Shutdown 1 (LS1), eighteen magnets (15 dipoles and 3 quadrupoles) will be replaced. Once they have been installed and connected, these will need to be thoroughly tested – another job for the ELQA team.
Another ELQA project engineer, Jaromir Ludwin, notes that: “For LS1, 25 experts from the Henryk Niewodniczański Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ-PAN) have come on board to bolster CERN’s ELQA team. CERN’s collaboration with IFJ-PAN – which trains electrical QA experts – dates back to 2005. Engineers from the institute were on hand during the construction of the LHC, and the expertise they have developed will be indispensable to the ELQA testing programme during LS1."
So far, cold tests have been done in sectors 1-2, 2-3, 4-5, 5-6, 6-7, 7-8 and 8-1. Testing should be completed by the end of April, when it will be time to restart the cycle, but this time at ambient temperature.
by Anaïs Schaeffer