It’s the magnets, stupid: Why the LHC succeeded where the SSC failed
Soon the Large Hadron Collider (LHC) will attempt to reach collision energies of 7 teraelectronvolts (TeV). So, despite the early setbacks of 2008, the LHC is marching on.
It’s worth thinking about how far physics would have come had the Superconducting Super Collider (SSC) been completed. It’s even more important to think about why the SSC never got built. Ironically, it was because the SSC’s designers were not ambitious enough, at least in certain aspects of accelerator design.
The Superconducting Super Collider (SSC) was designed to achieve energies of about 40 TeV, and the tunnel to house it was to be 87 kilometers in circumference. The site chosen was near Waxahachie, in northeast Texas, about 30 miles south of Dallas. The ambitious plan was approved in 1987 by President Ronald Reagan, who used a popular sports metaphor from American football to rally the physicists (a tribe that really needs no such encouragement). “Throw deep,” he said.
So, where was the lack of ambition? Surely, 40 TeV energies and an 87-kilometre-long tunnel were ambitious enough.
To make sense of why the SSC lacked ambition, we need to look at a key aspect of particle accelerators: the magnets that create the magnetic fields to keep the particles (such as electrons and protons) confined to the beam pipe. The particles that are being accelerated want to go straight, and their trajectories have to be bent precisely by the magnetic fields, so that the particles can go round and round the tunnel. Steering a beam is not unlike driving a Formula One racing car. The faster the car, the harder it is for the driver to tackle tight bends. F1 drivers train hard to build up their arm and (especially) neck muscles to handle the turns. Magnets are the muscles of the collider world. The tighter the curve of the tunnel, the stronger the magnetic muscles need to be.
The SSC played safe when it came to the magnets. Although they chose superconducting magnets, the technology was already well-tested and not innovative. Had they designed magnets with fields that were twice as strong, they could have halved the circumference of the tunnel.
The other mistake they made was in designing a separate set of magnets for each beam, one going clockwise and the other counter-clockwise. This meant that the tunnel’s bore had to be correspondingly large, at a staggering 4.25 metres. Ultimately, it wasn’t any fancy technology that proved the SSC’s undoing. It was the cost of the civil engineering. By 1993, the cost estimates for building the SSC had ballooned from $4.4 billion to more than $12 billion. The U.S. Congress canned the project, leaving behind a 22.5 kilometer stretch of completed tunnel that now lies derelict.
So, what did the folks at CERN do when it came to the LHC? They decided to reuse the 27-kilometre-long tunnel that housed the Large Electron Positron (LEP) machine. The LEP, at its peak, achieved energies of 200 GeV. The LHC was being designed for 14 TeV collisions. How could such energetic particles be confined to such a tight orbit around the old tunnel? It all comes down to magnets. Of the more than 9,000 superconducting magnets inside the LHC tunnel, 1,232 need special mention. These are dipole magnets, and they are the machine’s neck muscles. Each weighs 35 tons, and the entire lot has to be cooled down to 1.9ºK, the temperature of superfluid liquid helium (the SSC, in contrast, used simple liquid helium at 4.5 K). It’s the immense magnetic fields created by these giant magnets at the LHC that keeps the protons confined to the beam pipe.
There was another innovation. The LEP tunnel was only 3.8-metres wide. The LHC could not afford to use two sets of cryogenically-cooled superconducting magnets — they wouldn’t fit inside the tunnel. So, the magnets for LHC were designed such that the same cryostat could house two magnets, one for the clockwise beam and the other for the counter-clockwise beam. It was a tight fit, but it worked.