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The quality of an ion beam is described by the brightness, which is defined as particle density in phase space (in amperes per square millimeter milliradian). This density should remain constant along the accelerator if normalized with respect to the actual particle velocity. Any degradation of phase space due to nonlinear forces will decrease the quality of the ion beam. For that reason, highest interest should be paid to improve the extraction of an ion beam from the ion source plasma as much as possible, because here, the initial value of brightness is determined. For understanding the process of extraction of an ion beam from a plasma generator and for possible improvements of the extraction system, computer simulation codes are helpful; however, a correct model has to be used. The physical model for ECR ion sources which has been used in this 3-D simulation using KOBRA3-INP will be described. Results of simulation achieved with these boundary conditions are then compared with experimentally obtained results to verify the validity of the applied models. Viewing targets, metal targets coated with KaB, BaF, or CsI, have been used to measure the beam profile. The ion beam profile has been measured behind extraction and behind the first active optical element, a solenoid, which can be used for a further diagnostic measurement: the dynamic beam profile.