Position Control of Electron Energy Loss Spectrum in Case of Accelerating-Voltage Fluctuation

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Abstract
In the parallel detection EELS, the fluctuation effect of the accelerating voltage is compensated both by the open loop and negative feedback controls. The fundamental frequency components of the AC power line and the excitation of Cockcroft-Walton circuit are canceled by applying the inverse-phase signal to the deflection coil, while the effect from the drift and the higher-order ripples is controlled by the negative feedback. The energy resolution is increased from 9.3 eV to 5.7 eV by the open loop control and can be improved up to 3.8 eV by the negative feedback. As the loop gain increases, the resolution increases at first, but then decreases. This decrease results from the shot noise of electron beam and the phase shift of the feedback loop. It is theoretically shown that the optimum condition to the loop gain and the bandwidth exists for the feedback control.

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