The present Master’s Thesis is part of the Master’s degree in Nuclear Engineering of the Universitat Politècnica de Catalunya (UPC) and the ENDESA Escuela de Energía, and it was developed during the internship in the Technische Universiteit Eindhoven (TU/e). The project shows the determination of the magnetic axis over time in the fusion reactor KSTAR. During the project, different data is analysed, obtained from the diagnostic method Motional Stark Effect, and verified with EFIT. The data evaluation is done with the Interactive Data Language (IDL) code. To start, an introduction to fusion is done by explaining some basic concepts, as what plasma is and how it is confined so energy will be generated, among other concepts. The characteristic parameters that define the KSTAR reactor are given as well. Next, the physic principle of the MSE system is explained, as well as the instrumentation and its configuration and the data treatment. Later, a brief explanation of the code development is done, trying to explain what was done for the data analysis and verification, and what possible further development can be done. Finally, it is studied how the change of different parameters affects the magnetic axis over time in terms of the position, as the plasma current, the magnetic field or the plasma temperature. This is done by analysing different “shots”. The outcome of all is the demonstration of the MSE system as a reliable, strong and flexible tool for the control of the plasma equilibrium.