A Fisher of Magnets
The naked truth about accelerators is that, without magnets, there would be no accelerators. Magnetic fields are used to steer, shape, focus, excite and otherwise modify the path of electrons and other subatomic particles throughout accelerator structures. Manipulating these beams of particles requires extreme precision, rendering even the slightest inconsistencies among otherwise identical magnets a big problem.
"Ideally, these should all be the same," says Andrew Fisher, a physicist working in SLAC's Magnetic Measurement Facility (MMF), while taking measurements on one of a series of electromagnets. "But in reality, they're all just a little different. What we're doing here is characterizing their fields."
Fisher and his colleagues at the MMF use a range of tools to map out and correct variations in a magnet's field strength and shape. The group currently spends a majority of time working with magnets destined for use in the Linac Coherent Light Source (LCLS). Recently, Fisher spent a few days measuring field distortions in a 5,000-pound dipole, a magnet designed for steering an electron beam. This particular dipole will soon be installed as part of the second of two bunch compressors for the LCLS.
To characterize the field within the magnet, Fisher took what is called a stretched-wire measurement. The bulky red dipole—so called because it's designed to have a single, uniform field with a north and a south pole—is made from a solid chunk of iron and contains opposing twin coils of hollow copper wire. The coils are separated by a gap of a few inches through which the beam pipe will pass. Easing a stretched wire back and forth between the poles along the same axis as the beam while the magnet is energized induces a tiny current that varies with field strength. Fisher records the variations in this current to infer how the magnetic field fluctuates to within 0.02%.
Many types of permanent magnets and electromagnets are used at SLAC. Fisher's stretched-wire experiments are just one of a range of tests used by MMF physicists to characterize the precise fields generated by these magnets.
Brad Plummer, SLAC Today, August 13, 2007
Above image: Andrew Fisher recently took measurements on a dipole destined for the second of two LCLS bunch compressors to be installed this fall.(Click image for larger version.)