pumping The specimen chamber sustaining the high-pressure gaseous environment is separated from the high vacuum of the electron optics column with at least two small orifices customarily referred to as pressure-limiting apertures (PLA). The gas leaking through the first aperture (PLA1) is quickly removed from the system with a pump that maintains a much lower pressure in the downstream region (i.e. immediately above the aperture). This is called differential pumping. Some gas escapes further from the low pressure region (stage 1) through a second pressure limiting aperture (PLA2) into the vacuum region of the column above, which constitutes a second stage differential pumping (stage 2). A schematic diagram shows the basic ESEM gas pressure stages including the specimen chamber, intermediate cavity and upper electron optics column. The corresponding pressures achieved are p0>>p1>>p2, which is a sufficient condition for a microscope employing a tungsten type of electron gun. Additional pumping stages may be added to achieve an even higher vacuum as required for a LaB6 and field emission type electron guns. The design and shape of a pressure limiting aperture are critical in obtaining the sharpest possible pressure gradient (transition) through it. This is achieved with an orifice made on a thin plate and tapered in the downstream direction as shown in the accompanying isodensity contours of a gas flowing through the PLA1. This was done with a computer simulation of the gas molecule collisions and movement through space in real time. We can immediately see in the figure of the isodensity contours of gas through aperture that the gas density decreases by about two orders of magnitude over the length of a few aperture radii. This is a quantitatively vivid demonstration of a first principle that enables the separation of the high-pressure specimen chamber from the low pressure and vacuum regions above. By such means, the gas flow fields have been studied in a variety of instrument situations, in which subsequently the electron beam transfer has been quantified. Electron beam transfer By the use of differential pumping, an electron beam is generated and propagated freely in the vacuum of the upper column, from the electron gun down to PLA2, from which point onwards the electron beam gradually loses electrons due to electron scattering by gas molecules. Initially, the amount of electron scattering is negligible inside the intermediate cavity, but as the beam encounters an increasingly denser gas jet formed by