Why do I need two magnetic field sensors?

Spicer Consulting magnetic field cancelling systems can be used with one or two sensors. Why provide the option and how does it work?

1. Why provide the 2 sensor option?

A magnetic field disturbs the image of an electron microscope by making unwanted deflections of the beam. The cancelling system cancels the field at the sensor location. Therefore the ideal sensor location is close to the electron beam. However, there are two reasons not to place the magnetic field sensor next to a TEM column.

a. A TEM illuminates the specimen with electrons and then projects an enlarged image of the specimen onto a phosphor screen (or camera) using a number of magnetic lenses. See the figure below. These lenses contain windings that generate magnetic flux at their pole pieces. The current in the windings is adjusted to change the strength of the lenses in order to set the magnification and focus of the microscope. When the magnification is changed, the lens currents are adjusted substantially and the field outside the column changes enough to overload the magnetic field sensor. Therefore we recommend that sensors are placed at least 40 cm away from a large TEM column. Similarly, sensors should be not be placed adjacentto the columns of some SEMs and dual beam systems that have operating modes requiring different lens currents.

b. A TEM column is effectively a large piece of iron that distorts the magnetic field around it. For example, take a uniform vertical field. When a TEM column is placed in this field, the high-permeability iron column attracts the field to go through it rather that the surrounding air. As a result, the field next to the column is lower than the original field. Also the field near the top and bottom is bent as the flux lines are concentrated by column. Therefore a sensor next to the column does not measure a true representation of the field to be cancelled. it is better to place the sensor some distance away from the column.

It is absolutely fine to place the sensor some distance from the column if the disturbing field and cancelling field are uniform, because in this case, the field will be cancelled fine at all locations. However, this is rarely the case. The purpose of the two sensor option is to allow the field to be cancelled at the column using two sensors, both of which are some distance away from the column, even if the field is somewhat non-uniform.

2. How does the two sensor option work?

The signals from the two sensors are sent to a mixer that adjusts the proportion of signal that comes from each sensor.The mixed field is thus a weighted combination of the sensor outputs, creating an adjustable “virtual sensor” between the two sensors. The mixed field is displayed on the control unit front panel and sent to the inputs of the cancelling amplifiers. When the two sensors are placed either side of the microscope column, the virtual sensor can be inside the column. The field is therefore cancelled at the column even though the sensors are some distance away.There are 3 controls to adjust the X, Y and Z channels independently.The optimum mixer settings are found by observing the microscope image and tuning for the best image improvement.