3.1.3. Beam Regulation

The beam regulator, which is located between the upper two condenser lenses, is used to maintain a stable beam current. The emission from a tungsten filament can vary by up to 1% in 10 minutes, an unacceptable amount for analysis. The beam regulator operates by sensing changes in the current impinging on a pair of apertures and adjusting the condenser lens current accordingly (Figure 3.1.3a).

Figure 3.1.3a. The electron beam is shown (greatly expanded) in red. The beam regulation system has two apertures: limiting (2) and probe-forming lens or PFL (3). The amount of current impinging on the PFL is amplified (5) and used to control the condenser lens power supply (4). The effects of the condenser lens (1) and PFL (objective lens) are shown schematically by divergent black arrows. The probe sample point is located at 6.