Active electronic compensation of ambient magnetic fields for electron optical columns

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Abstract
In order to reduce ac magnetic field interference with electron beam instruments, a relatively simple and inexpensive electronic field compensation system has been built. The system has a bandwidth extending from ∼8–200 Hz and reduces field strength at the sensor by a factor of about 50 (60 Hz). Because of inhomogeneities in the noise field, the compensated field strength increases with distance from the sensor, but is typically reduced by a factor between 10 and 50 at a distance of 10 cm for the case of a nonmagnetic environment. System performance in a magnetic environment, as in the application to an electron beam instrument, depends on sensor position. Fields within a simulated column consisting of a mu-metal cylinder are reduced by a factor of 5 or more with sensor position optimized. ac magnetic field interference with an electron beam instrument was reduced by a similar factor.

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