When a charged particle beam passes through a device, the size and shape of the beam vary at any given distance along the axis. The smallest diameter of a circle around the beam axis that covers all the beam is termed the Spot Size. For example, if the beam cross section is circular, the spot size is equal to the diameter of the circle. If the cross section is elliptical, the spot size will correspond to the major axis of the ellipse.
Analyzing the spot size allows for measurement and control of the beam envelope as it passes through the device. A large beam spread is usually undesirable since it may result in rays which pass outside the uniform field region, and/or exceed the limits of the device.
As shown in the magnetic quadruple model, launching rays in LORENTZ from the perimeter of the emitter surface provides excellent visualization of the beam spot size. At any distance along the beam axis, the spot size can also be calculated to include a certain percentage of the beam so that a few stray rays will not have a large effect. The beam parameters and quadruple characteristics, such as charged particle type and energy, emitter shape and size, and current and spacing of the magnets, can easily be varied in LORENTZ to carry out detailed analysis.
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