To guide electrons
Quadrupols like these are able to affect the movement of electrons by their magnetic field. This ability makes them very useful in high energy light sources like Bessy II or BERLinPro to control and guide the beam. For example, quadrupols play an important role in the recirculator of BERLinPro. This is the part where after a potential experiment the accelerated beam will be redirected back to the linac entry to recover the beam energy.
Photo: HZB/Michael Setzpfandt