Stopping Stray Electrons in Their Tracks
Set out before an experiment, the collimators resemble a miniature skateboard park with sloping ramps and blocks with killer angles. But unlike at the skateboard park, the collimators do not welcome mavericks. Their job is to stop the rebels: electrons that refuse to travel in tight formation with the rest of the electrons in a beam of particles.
Collimators are pieces of metal fitted inside a beam pipe, allowing the focused core of the beam through while blocking stray electrons (or positrons). The job of the International Linear Collider (ILC) group is to develop and test collimators that effectively deal with the nonconformists without disrupting the rest of the beam.
"Even during perfect operations, you have a halo of particles—a big fuzz of low charge that will reduce efficiency and increase noise if it gets into the detector. For ILC, we need collimators that can also absorb the full energy of the beam if something goes wrong, to keep the beam from damaging the detector," said Steve Molloy, a postdoc in the ILC Experiments and Prototypes group.
In a series of experiments in End Station A with a test beam that simulates certain ILC conditions, Molloy and his colleagues are testing collimators with different shapes, different material compositions, and different degrees of surface polishing.
One challenge is to learn why equations and simulations don't predict the experimental results. For example, calculations said a collimator with a gradual slope up to a peaked "roof" would cause less trouble to the core beam than a collimator with an abrupt right angle before the roof. But initial results suggest that these collimators have the same effect on the beam.
Collimators change the geometry of the beam pipe and thus alter the electromagnetic field that travels with an electron bunch. The changes—which can "kick" the beam off angle—are difficult to simulate.
"We want to get confident that we can simulate the same answer that our measurements give us," Molloy said. "Then we'll better understand different geometries and materials, and be able to design collimators that will work for ILC."
—Heather Rock Woods, May 7, 2007
Above image: This collimator box folds in half in the middle, forming slots with a different collimator in each. To take measurements, experimenters move the box to give each collimator a turn in the beam's path.