at the aperture of the anode, ensuring distortions induced by fringing fields are minimal, also reducing spherical aberrations. Geometric temporal expansion of the pulse is also minimized by reducing path length differences across electron trajectories.

Electron pulse rotation about the propagation direction (z-axis) induced by a magnetic lens depends on the longitudinal velocity. In the acceleration region, the subsequent focusing is coupled to the DC field gradient, and the best performance is varied is found when the electron bunch has reached a significant proportion of its final velocity. This is shown through a comparison between figures 1(e) and (f). For a final beam energy of 70 keV, a maximum radius of 2$\sigma$(r) = 55 $\mu$m is achieved, where $\sigma$ is the standard deviation of the parameter in parenthesis, and is used throughout. The optimal balance between minimized 2$\sigma$(r) and a small temporal geometric stretch is found when a strong magnetic field is applied in the first few millimetres of electron flight. The convergence angle of 2.5 mrad observed in figure 1(e) is at least an order of magnitude smaller than the divergence angle when no source magnetic field is present.

Comparing figures 1(c) and 1(f) at small z position (z 