LHC – Take 2

By John Conway | December 9, 2008 12:30 pm


CERN has issued a press release detailing the extent of the damage from the September 19 incident, the plans for retrofitting the magnets to ensure it does not happen again, and a new schedule. The skinny: the machine should be back up and running in July of next year, resuming the commissioning that was underway when the incident occurred. There are now some impressive photos of the damaged components, showing what tens of megajoules of energy can accomplish in a few seconds.

Six tons of helium was released in the incident, and is presumably gone into outer space now, since at atmospheric temperatures the helium atoms readily attain escape velocity. (Helium is not a renewable resource, per se, though it is continually being produced within the earth from radioactive decay.)

But what caused the incident to begin with? Here is a quote from a talk by Robert Aymar, the outgoing CERN Director General:

On 19 September 2008 morning, the current was being ramped up to 9.3 kA in the main dipole circuit at the nominal rate of 10 A/s, when at a value of 8.7 kA, a resistive zone developed in the electrical bus in the region between dipole C24 and quadrupole Q24. No resistive voltage appeared on the dipoles of the circuit, so that the quench of any magnet can be excluded as initial event. In less than 1s, when the resistive voltage had grown to 1 V and the power converter, unable to maintain the current ramp, tripped off, the energy discharge switch opened, inserting dump resistors in the circuit to produce a fast power abort. In this sequence of events, the quench detection, power converter and energy discharge systems behaved as expected.

Since the interconnect bus subsequently vaporized, it has been difficult to tell exactly what triggered the event. The retrofits to the magnets will improve the quench detection and helium pressure relief systems, but I’ve not heard what, if anything, CERN will do to prevent other interconnects from “developing resistive zones.” Hence the question: will it happen again? It is clear that the new systems will help prevent serious damage to the machine, but there are thousands of these interconnects, so…

Anyway, quite a number of magnets have been removed from the tunnel and are being refurbished and will be reinstalled in the coming months. The experiment I work on, CMS, is in the “open” state now with the end caps pulled back, allowing access to the inner detectors. Our team on the forward pixel detectors plans to remove and reinstall our detectors, making minor electrical and cooling repairs and modifications, starting in February. The photo of the detectors being installed last summer shows the beam pipe, the installation fixtures, and the detector half-cylinder going in, like a ship in a bottle.

Once more, from the top, this time with feeling!


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