Quench Latest LHC Setback

A magnet quench Friday that resulted in a large helium leak is the latest setback for the Large Hadron Collider (LHC), the world's largest particle accelerator, which was launched with great fanfare last week.

The LHC contains hundreds of magnets that need to be cooled to cryogenic temperatures with liquid helium in order to become superconducting and to keep the protons on course. During a magnet quench, the temperature of the magnet section becomes too high for it to be superconducting, resulting in a higher resistance that rapidly heats the current running through the magnet. That leads to even less superconductivity and more heat, which could damage the very thin wires that comprise the LHC electromagnets. The heat generated can also turn the liquid helium back into a gas, which can escape if the magnet begins to leak.

Each particle beam in the LHC is accelerated up to the record energy of 7 TeV. According to the LHC User's Web site, operators were commissioning the final LHC sector, sector 3-4, for 5 TeV operation when the quench occurred, resulting "in a large helium leak into the tunnel."

Citing a source at CERN, the organization hosting the collider, the PhysicsWorld Web site reported that 1 ton of helium escaped during the quench, which would represent about 1 percent of the helium needed to cool down the LHC and fill it for its first operation.

"I have no inside information, and I'm no more of an accelerator expert than you are, so the details aren't entirely clear to me; but it is pretty clear that around 11 a.m. today, the temperature in S34 went up by quite a bit, and it still isn't recovered completely. Past that, we don't know yet what is going on; I expect that the accelerator experts will make more details available once they have a handle on the situation," wrote Seth Zenz, a University of California graduate student working in Geneva with the Lawrence Berkeley National Laboratory group on the LHC's ATLAS experiment, in a blog on the US LHC Web site today.

"Quenches are expected to happen pretty routinely, and to be dealt with without any permanent damage," Zenz wrote.

The LHC is expected to reveal a world of unknown particles, and LHC experiments could explain why those particles exist and behave as they do. They could reveal the origins of mass, shed light on dark matter, uncover hidden symmetries of the universe and possibly find extra dimensions of space. Its 17-mile racetrack for accelerating and colliding particles at nearly light speed is located deep underground near Geneva, Switzerland, and straddles the Swiss and French borders. It cost more than $8 billion to build.

CERN celebrated the LHC's First Beam Day Sept. 10, reporting that the first beam of protons in the LHC was successfully steered around the track. What was kept quite for nearly a week was that a power transformer failed and temporarily shut down the LHC last weekend.

Yesterday CERN officials disclosed that a 30-ton power transformer on one of the surface points of the LHC switched off the main compressors of the cryogenics for two sectors of the machine, disrupting cryogenic conditions. The transformer was replaced last weekend, and the crygenics team spent much of this week recooling the magnets and preparing for beam operation, CERN said.

"Transformers are big and expensive, but compared with the LHC they are small potatoes, and quick to replace. Damage to the LHC magnets can be a more serious business, because replacing and repairing them involves heating a large portion of the accelerator up to room temperature, which can take months," Zenz wrote, adding that he's not worried.

"It’s far more likely that this is a minor glitch than a major one. We still have a lot to do to prepare the detectors for data taking, and we are continuing to work. Collissions (sic) will happen when they happen: probably not as soon as we hope, but soon enough in any case," Zenz wrote.

In another incident with the collider, earlier this week media outlets reported that, shortly after the LHC started up, hackers identifying themselves as the Greek Security Team accessed computers connected to the Compact Muon Solenoid (CMS) detector, one of the subsystems that monitors the LHC's proton collisions.

The hackers were stopped before they could access the Collider's central computer system, but were described by the UK's Daily Telegraph as being "one step away" from full control of the CMS.

CERN officials said that, other than having its Web site defaced, there was no damage done to its computer network.

Officials said yesterday that the LHC is still on course for first collisions "in a matter of weeks."

For more information, visit: www.cern.ch