Attention to Nanoscale Detail Leads to Brightere-Beams

Electron linear accelerators (linacs) used for the most demanding applications—x-ray free-electron lasers (FELs) and linear colliders—must deliver beams with high brightness. In such a beam, many electrons with a narrow spread of energies are bunched into a short pulse, resulting in a high peak current. Along each of the two axes transverse to the beam’s direction, the emittance, essentially the product of the size and the angular spread, must be small. The low transverse emittances allow the beam to be brought to a tight focus, while the high peak current enables coherent emission in an FEL or a high collision rate in a collider. Focusing a low-emittance beam might be compared to aiming the spray from a garden hose while sticking your thumb in the end.