2.3. Simulation of Mini-Applicator’s Magnetic Field

Non-uniformity of magnetic fields, and non-uniform spatial distribution, make the prediction of particles’ paths very difficult based on Lorentz equations. An effective method to solve such complex problems is the finite element method. To calculate the magnetic field and estimate the path of electrons, CST Studio Suite 2015 simulator software was used. This is among the most powerful software in the field of numerical simulation of electro-magnetic fields from low frequency to high frequency regions. This software is extraordinary for designing, analysis, 3D observation and particle tracking (12). The geometry of applicator components include aluminum holders, peripheral frame, cerrobend shield that limits the radiation field size, standard lead frame and also, protective plastic layers were modeled in the software. Density, electrical conductivity coefficient, magnetic permeability and permittivity of materials entered the software and an appropriate spatial mesh was considered. Properties of the magnet were supplied by the manufacturer (13). Magnetic remanence, Br, i.e. the maximum magnetic field that a magnet can produce in a closed cycle, is an important parameter in calculation of intensity of the magnetic field. Br varies among various grades of the magnet and also in various samples of a grade. Simulation was conducted using a default value for magnetic remanence, Br, (1.23T) based on information provided by the manufacturer (Figure 1 above), and magnetic field curve was extracted in orthogonal directions. The value of magnetic remanence changed frequently to determine the best Br value for which the results of calculations were matched with practical measurements.

A schematic of simulated non-uniform magnetic field of mini-applicator in CST Studio software. Equi-intensity curves of magnetic field were plotted (above). 12MeV electron beam’s tracking and its deflection are displayed in the model (below). Colors indicate differences in the electrons’ velocities.