duration, whereby 1e and 2e pulses exhibit a duration of around 10 fs. This trend continues in figure 4(c), whereby the 100e pulses approach a minimum of only 100 fs.

The shifting z-position of the minimum duration with increasing pulse charge is a consequence of the spatio-temporal variation of the charge density, influencing convergence and vice versa, with the outcome a varying compressivity. The peak electric field also directly defines the position of the temporal focus: increasing the peak field moves the temporal focus close to the compressor. This is analogous to the spatial focusing of the electron beam with magnetic lenses discussed earlier. We select a field strength of 5 MV/m, which is an achievable compromise between compressor performance and mechanical feasibility.

Fields in excess of 10 MV/m could compress pulses to shorter durations within the restriction placed by the breakdown limit of resonator material. Simulations of alternative resonator geometries have shown field enhancement factors which would facilitate such field strengths [@Bagiante2015]. However the electron optics required to collimate or focus the diverging beam onto the sample constricts the distance between the resonator and sample, necessitating a heavily modified fs-TEM geometry. Consideration must also be given to the laser 