## Electron guns ![[Drawing 2023-09-30 14.16.35.excalidraw.svg]] ### Filament Typical filaments include Tungsten wire, LaB6, and field emission gun (Schottky, cold). The brightness, lifetime, coherence, tip size change from worst to best from left to right. Tungsten wire gets lower price through, and lower vacuum condition required (typically $10^{-5}$ mbar). While cold FEG requires $10^{-11}$ mbar vacuum (UHV regime) to ensure the gun tip does not gets contaminated. Such contamination would greatly decrease the current stability. The high coherence of FEG enables (only) the phase contrast to be investigate for tiny features, like lattice fringes, so they are applied in HRTEM. The circuit connected to filament (as a cathode) ensures the gun continuedly emitting electron toward all directions. To have a better convergence, Wehnelt is installed. ### Wehnelt Wehnelt cap is connected to the filament and small negative bias is applied, in order to provide smaller divergence angle. At the optimal bias level, a crossover point of small diameter is produced (or say, virtual gun source), this makes the system has fewer current fluctuation, better spatial coherency, and higher brightness (since almost no electrons get "wasted"). >[!Note] >Brightness $B$ is defined as the beam current per unit area per solid angle, i.e., >$B=\frac{4i}{(\pi d \alpha)^2}$ >here $i$ is beam current, $d$ is diameter of the source (crossover), and $\alpha$ is the semi-angle of beam divergence. ### Anode The upper part -- filament and Wehnelt create a concentrated source for electron, while the velocity of electron is quite slow. These electrons get accelerated by the field created by the anode, which is connected to the high tension (high voltage). This value is adjustable in many TEM. A higher high tension makes the electron gain more energy, but also requires more strict vacuum level. This high tension is connected to very big power supply systems, typically an external equipment. Turn on/off high tension takes quite some time. Therefore, in many occasions, when we start the TEM, the high tension is already on, while we only control the emission on/off of the beam.