Officials at the European particle physics lab, CERN, near Geneva, Switzerland, still don't know how long it will take to fix an electrical short in the world's largest atom smasher, the Large Hadron Collider (LHC), but engineers have now homed in on the fault.
As ScienceInsider reported earlier this week, the short has delayed the restart of the LHC, which was expected this week, after 2 years of downtime for repairs. During preparatory tests of the LHC's systems on 21 March, a short developed in an electrical connection to one of the 1232 superconducting dipole magnets—each measuring 15 meters in length and weighing 35 tonnes—that steer particles around the LHC's 27-kilometer ring. Researchers suspect that a wayward fragment of metal has caused the problem, and using standard electrical diagnostics, engineers have located the metal scrap to within 10 centimeters, according to a statement on the CERN website.
The fragment is inside the tube that connects the magnet to its "diode box." The diode would protect the magnet in the event of a quench—a loss of superconductivity—by safely shunting the 12,000 amps of current out of the magnet before it heats up. The offending metal is likely making a connection between a cable inside the box and the tube, causing a short to ground. X-ray images have shown evidence of debris in the suspected region, but the area is difficult to image and results of those tests are still inconclusive.
Even once the piece of metal is found, the fix may not be quick. The diode box operates at a temperature of 1.9 K and is cooled with frigid liquid helium by the accelerator's cryogenic systems. CERN engineers have three options to try to fix the problem. They can try to melt away the metal by injecting a pulse of current. Or they can try to dislodge it with a blast of helium gas sent through the machine's cryogenic system. If neither of those measures proves practical or succeeds, then workers will have to open the box and directly remove the metal. But that approach would require that the area be fully warmed up and then cooled down again. That could add about 6 weeks to the restart schedule. First collisions were originally expected in late May.