Knocking on Heaven's Door - Lisa Randall [81]
I learned more about the backstory during a visit to CERN a few weeks after the mishap. Keep in mind that the ultimate goal for collisions is a center of mass energy of 14 TeV, or 14 trillion electron volts. The decision was made to keep the energy down to only about 2 TeV for the first run in order to ensure that everything functioned properly. Later the engineers planned to increase it to 10 TeV (5 TeV per beam) for the first actual data runs.
However, the plan became more ambitious following a small delay due to a transformer that broke on September 12. Scientists continued testing the tunnel’s eight sectors up to 5.5 TeV during the interval afforded by the short delay and had time to test seven out of the eight sectors. They verified those could run properly at higher energy, but they didn’t have the opportunity to test the eighth. They nonetheless decided to charge ahead and attempt higher-energy collisions since there didn’t seem to be any problem.
Everything worked fine until the engineers attempted to raise the energy of the last untested sector. The crippling accident occurred when its energy was being raised from about 4 to 5.5 TeV—which required between 7,000 and 9,300 amps of current. This was the last moment for something to go wrong, and it did.
During the year of the delay, everything was repaired at a cost of about $40 million. Although repairing the magnets and the beam took time, they were not impossible tasks. Enough spare magnets were on hand to replace the 39 dipole magnets that were beyond repair. In total, 53 magnets (14 quadrupole and 39 dipole) were replaced in the sector of the tunnel where the incident occurred. In addition, more than four kilometers of the vacuum beam tube were cleaned, a new restraining system for 100 quadrupole magnets was installed, and 900 new helium pressure release ports were added. In addition, 6,500 new detectors were added to the magnet protection system.
The bigger risk was the presence of 10,000 joints between magnets that could potentially cause the same problem. The danger had been identified, but how could anyone trust that this problem would not reemerge elsewhere in the ring? Mechanisms were needed to detect any similar problem before it could cause any harm. The engineers once again rose to the challenge. Their updated system now looks for minuscule voltage drops that might signal the presence of resistive joints, signaling a break in the closed system that houses the cryogenics that keeps the machine cold. Caution also dictated some delays to improve the helium release valve system and to further study the joints as well as the copper casings of the magnets themselves—which meant a delay in achieving the highest energies at which the LHC is designed to operate. Nonetheless, with all the new systems to monitor and stabilize the LHC, Lyn and others were confident that the kind of pressure buildups that caused the damage will be avoided.
In some sense, we are lucky that engineers and physicists were able to fix things before true operations began and filled the experiments with radiation. The explosion cost the LHC a year before they could even begin to test beams and aim for collisions again. That was a long time, but not so long on the scale of a quest for the underlying theory of matter that we have had for the last 40 years, and in many respects for thousands of years.
On October 21, 2008, the CERN administration did, however, stick to one piece of their initial plan. On that day, I joined 1,500 other physicists