requirement for laser access.

![image](fig1.jpg){width="\textwidth"}

We apply a 25 $\mu$m fillet to the inner circumference of the suppressor to prevent high field gradients causing field emission. The tip-anode distance is fixed at 2.5 cm throughout, such that the surface fields on all electrodes are below 5 MV/m. The nanotip potential ($V_{tip}$) and suppressor potential ($V_{sup}$) define the surface fields at the emission site. With the suppressor set at a potential comparable to the tip ($\Delta V = V_{sup} - V_{tip} \approx 0$) there is a pronounced bunching of the equipotentials around the apex, shown in figures 1(a) & (b), indicating a strong DC gradient. The dominant gradient lies along the z-axis towards the anode and the on-axis field E$_z$(z) is explored as a function of $\Delta V$, shown in figure 1(c) & (d). Increasing $\Delta V$ moves the saddle point (highlighted in the equipotential lines of figure 1(a) in dark magenta), towards the apex of the tip and ultimately into the z $\geq$ 0 region. This causes field reversal and an extinction of the photo-electron current, present in figure 1(c) & (d) for $\Delta V$ $>$ 400 V.

Changing $\Delta V$ modifies the electrostatic landscape around the NSMT, facilitating adjustment 