of the spatio-temporal distribution of the electron pulses [@Paarmann2012]. The reduction of the DC field decreases the longitudinal momentum acquired during acceleration (see figure 1(c) & (d)), increasing the time required to reach the sample and increasing propagation induced temporal broadening. The reduction of the DC field gradient reduces the transverse momentum of electrons and hence reduces the path length differences for electrons emitted across differing angles with respect to the z axis. These two processes cannot be varied independently. Typically, the DC electric field would be set as high as possible while suppressing field emission. With $V_{tip} = -70$ kV, $V_{sup}$ would be set such that the surface field of the NSMT does not exceed 1.5 GV/m [@Yanasigawa2010], hence optical field enhancement would only permit electron tunnelling during the presence of the driving laser pulse. In practice, this would depend on the microscopic shape of the NSMT, and would be the first parameter varied when identifying operating conditions.

The region between the NSMT and the anode is shown in figure 1(e). The inclusion of a magnetic lens allows the divergence of the trajectories from the NSMT to be controlled independently of $\Delta V$. This lens focusses the electron pulse 