Give+a+brief+description+about+Electron+Source

The electron beam comes from a filament, made of various types of materials. The most common is the Tungsten hairpin gun. “This filament is a loop of tungsten which functions as the cathode. A voltage is applied to the loop, causing it to heat up. The anode, which is positive with respect to the filament, forms powerful attractive forces for electrons.” This causes electrons to accelerate toward the anode. Some accelerate right by the anode and on down the column, to the sample. There are other examples of filaments, Lanthanum Hex boride filaments and field emission guns.

= This figure will help you to understand much easier how SEM works. http://mse.iastate.edu/microscopy/source.html