Starting up the world’s largest particle accelerator

November 10, 2009 | 11:16 am

Over the next few weeks, scientists will use the Large Hadron Collider to accelerate subatomic particles to nearly the speed of light and collide them at unprecedented energies. The LHC is seventeen miles around, more than 300 feet underground, and contains more than 9000 magnets.  Making particles collide in this massive machine is no easy feat–dozens of scientists and engineers must ensure that every piece of equipment in the LHC operates in perfect harmony.

“Checking the accelerator is an unforgiving process,” explains Jim Strait of Fermi National Accelerator Laboratory. “You have to get all of the equipment and instrumentation to work together, all at the same time, before you can introduce a beam.”

Read on for an overview of the LHC’s start-up checklist, which takes months to complete and tests every system in the accelerator.

Check the LHC’s hardware. At the heart of the LHC are its superconducting magnets, which guide the particle beams around the ring and must be cooled to 1.9 K (-271.3 °C), just above absolute zero. The first systems tested are those that keep the magnets ultra-cold: the cryogenics system, which uses liquid helium to cool the accelerator; and the quench protection system, which prevents magnets from overheating. The hardware commissioning team next conducts magnet, helium, vacuum, and electrical tests on each of the LHC’s eight sectors.

Cool down the superconducting magnets. The process of cooling the LHC to 1.9 K takes about ten weeks for each LHC sector. When all sectors have been cooled, the LHC is the coldest place on the planet.

Power test the accelerator. The last task for the hardware commissioning team is checking the electrical circuitry in each sector. When the LHC is running at design energy, 11,700 amps of current will flow through each of the LHC’s 1232 main dipole magnets.

Make sure the whole machine works as one. Machine checkout, which tests the relationship between systems and sectors as a whole, takes about six weeks. During this period, the operations team uses specialized computer programs to examine the sequencing of all systems in a given sector. The final test is a dry run, when beam is simulated through the entire accelerator.

Check the beam removal system. A beam dump test ensures that the beam can be safely removed from the accelerator. When triggered, the system extracts the beam and sends it into a large graphite block where its energy is safely absorbed and distributed.

“The energy of one sector is equivalent to the energy of a fully loaded Boeing passenger jet,” notes Knud Dahlerup-Peterson, leader of the quench protection team.  Like quench protection, a functioning beam dump system protects the machine from itself.

Inject beams one at a time. Beam injection tests begin by accelerating a particle bunch in the PS and SPS, two smaller accelerators that ramp up a beam’s energy in preparation for the LHC. When the beam bunch has reached the appropriate energy, it is injected into the LHC.

The beam is then threaded through one sector at a time, limited by a temporary stop point so that accelerator and beam can be monitored in stages. Once all sectors have been tested and the beam has made a full turn around the LHC ring, it circulates a second time in case any obstruction went undetected. Injection tests are run for each of the LHC’s two beam pipes separately. These tests help determine how high the energy of the beams should be to maximize scientific potential while respecting the accelerator’s condition.

“The bottom line is protecting the machine,” explains Mike Lamont, leader of machine operations. “We have to be very, very careful.”

Guide the beams into collision. Once both beams are circulating, their energy is ramped up in stages. In an elaborate choreography, bunches are guided to take the correct size, energy, and distribution to collide at the interaction points in the center of each of the LHC’s four main experiments.

“Each LHC beam consists of bunches of 100,000 million protons. This could be compared to a cigarette in length, with a width corresponding to a human hair,” says Simon Mathieu White, one of the scientists who steers beams into collision. Almost 3000 of these tiny bunches make up an LHC beam, and ensuring that beams collide requires careful adjustments to and monitoring of the LHC’s 9593 magnets.

Starting up the LHC requires the orchestration of thousands of instruments to create hundreds of millions of collisions per second. With each collision, the LHC experiments have a chance to solve some of the mysteries of the universe.

by Daisy Yuhas

Symmetry Intern
Posted in LHC updates, Uncategorized |
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