SC24 Magnetic Field Cancelling
Electron microscopes have to operate in an ambient magnetic field comprising the earth’s field and fields radiated by electric power networks and electrical machines. When the ambient field changes, the electron beam in the microscope is deflected, causing loss of resolution and image distortion. The SC24 system stabilises the field by dynamically creating nearly equal and opposite field changes, so that the microscope performance is much improved.
The SC24 system comprises a magnetic field control unit, one or two magnetic field sensors and three orthogonal axis multicore cables, that are installed in the room where the field is to be cancelled. The SC24 can drive single loop and double loop room size cables. It also works with Helmholtz cables that are attached to a frame around the column. Three power amplifiers in the control unit drive currents through the cables to create a field of the opposite sign to the change in ambient field. The magnetic field sensor measures the resulting field and real time negative feedback reduces the ambient field by the loop gain of the system. The system is dynamic, automatically responding to field changes within 100 µs.