Alignment of magnetic solenoid lenses for minimizing temporal distortions

An ubiquitous focusing element for charged particles is the magnetic solenoid lens. For the case of ultrashort electron pulses, we show here that misalignment of the lens, i.e. displacement or tilt, causes significant temporal aberrations on a femtosecond time scale. Pulse-lengthening is only minimized if the beam travels on the symmetry axis. We present an experimental procedure with periodic reversal of the magnetic field for aligning position and tilt with sufficient precision for reducing the aberrations to less than one femtosecond. This method will be instrumental for advancing ultrafast electron microscopy and diffraction to ultimate temporal resolutions.