a synchrotron source
/ SIN-kroh-tron /
Picture a stadium-sized ring in which electrons race around at very nearly the speed of light, steered by powerful magnets. Any charged particle that is bent from a straight path radiates, and these electrons, whipped around the curve, hurl out an intense, tightly focused beam of X-rays tangent to the ring — rather like water flying off the rim of a spinning bicycle wheel. That beam is synchrotron radiation, and it can be millions to billions of times brighter than the best sealed tube.
The machine works in stages. Electrons are accelerated to near light speed and injected into a storage ring where they circulate for hours. Around the ring, bending magnets and special multi-magnet devices called wigglers and undulators shake the electrons and squeeze out radiation across a huge, continuous span of wavelengths. From this broad spectrum a monochromator selects whatever wavelength you want — so, unlike a fixed-target tube stuck with Cu K-alpha, a synchrotron beam is tunable. The beam is also tiny, intensely bright, and almost perfectly parallel.
Those qualities open experiments a lab tube cannot touch: vanishingly small or weakly scattering crystals, extremely fast time-resolved studies, high-resolution powder patterns, and — crucially for structure solving — tuning the wavelength to an element's absorption edge to exploit anomalous scattering, which helps crack the phase problem (the MAD method). The honest trade-off is access: a synchrotron is a large, shared national facility costing hundreds of millions, so you compete for scheduled beamtime rather than switching it on down the hall.
At a synchrotron beamline a crystallographer might dial the wavelength to 0.98 angstrom for routine data, then move it precisely onto a selenium absorption edge to collect anomalous data that phases a protein structure.
Tunable wavelength is the synchrotron's superpower — a sealed tube is stuck at one line.
Synchrotron radiation is not a different kind of X-ray, just an extraordinarily bright, tunable, parallel one. Its brilliance is a benefit but can also radiation-damage delicate samples during long exposures.