During operation, a diffusion or turbo molecular pump maintains high vacuum in the column. A regulator valve meters gas into the sample chamber at a rate controlled by the operator. The chamber and column settle to equilibrium pressures, determined by various gas flows, and manipulated by the regulator valve. Gas flow through an aperture is proportional to the pressure difference and the area of the opening, for a given gas, temperature, and type of flow. At the maximum permitted PLA flow rate, the size of the aperture therefore determines the pressure differential between the chamber and the column and, consequently, the maximum permitted pressure in the chamber.
Smaller apertures permit higher pressures but reduce beam current.
Larger apertures permit higher beam current but reduce chamber pressure. The Low Vac CSEM must combine both optical and vacuum functions in a single aperture. Increasing the performance of the vacuum system is not as simple as increasing the size and speed of the pumps. Since the mechanical pump is isolated after initial evacuation, a faster pump improve pump down time, but does not effect the maximum operating pressure in the sample chamber. In the high vacuum system of the column, performance is determined by the combination of the pump capacity, and the pipe conductance between the pump and the column.