The making and tending of heavy ion beams for the LHC
This week the Large Hadron Collider began heavy ion physics, the process of colliding lead ions to learn about conditions in the primordial universe.
The accelerator is expected to perform five to 10 times better than it did in its first run of these collisions last November. Although the heavy ion program will last only from now until CERN’s annual winter shutdown just after the first week of December, operators started preparations months in advance. Here symmetry breaking examines what it really takes to put lead beams in the LHC.
Making heavy ions is more complicated than preparing the protons used in regular LHC collisions, which come from hydrogen gas. Since hydrogen atoms have only one proton and one electron each, applying a voltage to them is sufficient to rip off their electrons, leaving a load of beam-ready, positively charged protons. But the source for heavy ions, enriched lead, starts with 82 electrons. Physicists do not have miracle flypaper to grab that many subatomic particles at once, so the process takes a few steps.
Meet Detlef Kuchler, a heavy-ion expert who tends the lead source, the first part of the heavy-ion acceleration process, by hand. He helped develop the method of extracting lead ions decades ago and can explain from memory its hundreds of associated, unlabeled diagrams. Although several people work on the source, a flowchart of what to do when things go wrong at this stage dead-ends everywhere with, “Call the expert.” It may as well say, “Call Detlef.” He spends a lot of nights and weekends at CERN during heavy ion season.
The first thing Kuchler does when it’s time to make heavy ions is prepare the oven, a palm-sized cylinder on a long pole that evaporates metallic lead. The whole thing fits into another machine that begins a chain of hand-offs from the source to the LHC. The lead must heat slowly in the oven or fragments spill out. Kuchler refills the oven once every two weeks.
The lab’s 10-gram stock of enriched lead costs $12,000. But so little is used at a time -- about 500 milligrams per oven fill -- that the bill equates to roughly $2 per hour. Not bad, in the scheme of accelerator operating costs.
Kuchler carefully unscrews the oven, takes it apart, cleans its seals with ethanol, measures and fills it with lead, puts it back together, enters the correct settings and turns it on. When symmetry visited, he searched with gloved hands for a leak around water tubes used for cooling and requested new O-rings to fix a vacuum connection somewhere. He usually finishes all the physical tweaks in less than half an hour.
“I know every screw of this machine,” he said. “It’s fun.”
After lead evaporates in the oven, it moves into a chamber of plasma heated by microwaves. The lead gas loses some electrons when it hits the plasma. In 30 milliseconds, the ions inside have each lost varying amounts of the negatively charged particles, but the majority have lost 29, making their charge 29+. As they leave the chamber, the 29+ ions are separated from the rest and collected into a beam.
A newborn beam of lead ions spends its first hours in testing. Here, the human-machine interactions become less TLC and more CPU. Kuchler adjusts computer settings to send a bit of beam a few meters into a small, straight section of accelerator, where it hits a diagnostic device called a Faraday cup. Kuchler sees how he did from what the cup tells him, makes adjustments accordingly, heats a little more lead in the oven and repeats. Eventually, when the beam looks good enough, he retracts the Faraday cup from the path so that the beam may continue its tour of CERN’s accelerator complex on the way to the LHC.
Electronics that control every aspect of source machine settings sit in shelves all around the oven and the accelerator. “During the day I keep an eye on [the machine],” Kuchler said. He can easily pop down from his office any time. During the weeks of heavy-ion physics, he elects not to take any personal holidays so that even when he’s not at work, he’ll always be nearby. He and dozens of other experts work day and night and remain on-call after hours in case anything goes wrong.
In August, eight weeks after Kuchler began working on the source, he began letting the beam through to the Low Energy Ion Ring, where other people took over its testing and fine-tuning.
On the way to the ion ring, the beam passes briefly through an area where it encounters a number of 300-nanometer-wide stripping foils. These take off more electrons, increasing the ions’ charges to 54+. Next, at the LEIR, a process called beam cooling begins to reshape and intensify the beam, which the machine accelerates.
The newly narrowed beam travels from there to another accelerator, the Proton Synchroton, behind a wall in the same building. This accelerator takes the beam up to a higher energy and sends it through another stripping foil. Ions leave here at the charge with which they will collide: 82+.
The last checkpoint before the LHC is the second biggest accelerator at CERN: the Super Proton Synchroton. This machine further accelerates the ions. The Proton Synchrotron accelerator started sending beam to the SPS in September. Over the course of eight weeks, a team of physicists spent hours refining the machine’s settings in order to optimize the beam and prepare it for its final destination: the LHC.
During the few days of tightly-scheduled heavy-ion commissioning, many additional experts join the usual operators at the LHC controls. They help troubleshoot to make sure the beams reach the precise conditions needed for physics. Lead beams here accelerate to 1.38 TeV per single proton or neutron inside the ion.
Three minutes after midnight on Sunday, Nov. 6, LHC accelerator physicist John Jowett captured an image of one moment in the process that always thrills him. When the lead beams in the LHC reach about twice their injection energy, they begin emitting enough radiation, called synchrotron light, to be seen by a special telescope and camera system in the accelerator.
“We see a shimmering spot on a screen,” Jowett said. “It’s as close as we get to seeing the beams of lead nuclei with our own eyes.”
At the end of that week, on Saturday, Nov. 12, at 6:41 a.m., operators declared “stable beams,” the machine term that indicates the LHC is ready for physics.