A Complex Mechanical Design in a Limited Volume

A field flattener is a piece of material that is placed in the beam path to give a more uniform dose over the desired field of view.  A typical plot of dose and angle is here first on the right.  It shows the typical forward peaked beam where the dose is highest in the direction the electrons were travelling before they were stopped in the target.  The dose falls off as the angle from centerline increases.

In some applications, it is desirable to have a more uniform dose over angle so a field flattener is introduced.  A field flattener is typically a piece of material, normally steel or copper, that attenuates the more intense beam center.   In the second image to the right, considering a +/- 10° field of view, one could construct a flattener that attenuated the beam center to 60% of the unflattened value.  The resultant profile would appear as shown in the third image where the cross hatched area is now attenuated in the flattener.

An initial estimate for the shape of a flattener is conical, with the base diameter set by the desired field of view, and the height set to attenuate to the desired level.  The axis of the cone is aligned with the beam centerline.

For the K15, with two operating energies, two separate flatteners are required in addition to having operating with no flattener in the beam path.

In addition to the flatteners themselves, I also designed a flattener wheel that aligned and positioned the flatteners.  The wheel was motorized and remotely driven through the control computer to any one of five positions. The entire assembly also included a retractable mirror for laser pointing and an ion chamber.