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The electron chamber is divided into two parts; one is the electron gun and the other is the column chamber consisting of many magnetic lenses. Electron beams are generated in the electron chamber by a filament heated to high temperature (thermionic emission) or kept at strong electric field (field emission). The electrons emerging from the filament are then directed through a small spot before being accelerated in the electron gun toward the column chamber. The accelerating voltage is typically in the range of hundreds to tens of thousands of volts. The electron gun must be operated in very high vacuum ($10^{-7} \space Torr$) conditions to minimize scattering. The electron beams come in the column chamber as divergent and broadened beams; therefore, condenser lenses are required to converge and focus the beam into a small crossover and toward the two environmental chambers.   \subsection{Environmental Chambers and PLAs}  One of the key development in ESEM is the chamber design to keep high vacuum at the electron gun and relatively low vacuum in the sample chamber at the same time. The pressure gradient from $10^{-7}$ to $30 \space Torr$ is achieved by multiple stage of chambers with tiny opening to other stages. Such stages are the two environmental chambers (EC1 ad EC2) which separate the column chamber from the sample chamber, functioning as buffer zones to minimized the pressure fluctuation. The chambers are connected by two pressure limiting apertures (PLA's). The PLA has a tiny opening for electron beams, through which the gaseous molecules will leak out from high-pressure side to the other. Even though the opening is designed as small as possible, serious leakage takes place without the differential vacuum pumping. As the gaseous molecules leak through one PLA from pressure $P_2$ into the gap between two PLAs maintained at pressure $P_1$, they will be removed from the gap region by a vacuum pump. Therefore the pressure $P_1$ remains intact. However, the molecules in the gap will also leak into the chamber with pressure $P_0$ through the second PLA. Another more powerful pump is required for $P_0$. The flux $\Phi$ of the gaseous molecules is the number of molecules $dN$ flowing through an area $dA$ within certain period of time. time $dt$.  The flux in the opening of a PLA is proportional to the pressure difference between the two sides of a PLA. \begin{equation}  \frac{} \Phi = \frac{dN}{dA dt} \propto \frac{P_H-P_L}{P_L} = \frac{\Delta P}{P_L}  \end{equation}