Sextupoles
In the sections of the beamline where dispersion is significant (for instance after bends), sextupole magnets are used to control 2nd order transport matrix T elements, especially those governing for longitudinal motion. UCLA-designed-and-built sextupoles are indispensable to experiments now underway at Neptune (the dogleg compressor) and BNL ATF (VISA II). At Neptune, sextupoles are used to correctly produce very short, ramped beam pulses. At VISA II, they are employed to allow a nearly linear chirping of the beam's longitudinal phase space as it enters the undulator. In both cases, the main longitudinal aberration (T566) is suppressed by the sextupoles.
