detailed above is also apparent from the flight time. In the case of holding the NSMT at 70 kV, the total acceleration time is around 400 ps.

The influence of space charge on the 2$\sigma$(t) pulse duration is further shown in figure 1(h). The duration of electron release is 2$\sigma$(t) = 25 fs, which including geometric effects, is comparable to that induced by a 20 fs FWHM laser pulse. A charge of one to five electrons increases 2$\sigma$(t) from 25 to 40 fs. A charge of tens of electrons results in a major temporal stretch, exceeding 100 fs with fifty electrons per pulse. With expected temporal compression ratios of the order of 10, exceeding tens of electrons per pulse by a large margin will result in a pulse that will be difficult to compress to low tens of femtoseconds duration.

fs-TEM electron optics configuration
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Defining design criteria for a high performance fs-TEM source is relatively straightforward, being low or controllable divergence, short pulse duration, minimal space-charge distortion, high coherence and excellent beam quality. Defining similar criteria for the electron optics between the source and sample requires a series of often contradictory conditions to be met simultaneously. Our design criteria 