PhD Thesis Final Defense to be held on January 8, 2020, at 12:30
Photo Credit: Grigorios Koutantos
The examination is open to anyone who wishes to attend (Multimedia Room, Central Library of NTUA).
Thesis Title: Modeling Methodology of Low Frequency Electric Fields for Space Missions requiring Electric Cleanliness
Abstract: In the present PhD thesis, specific issues in the scientific field regarding electromagnetic cleanliness, electromagnetic interference and electromagnetic compatibility in a space equipment are identified and investigated. The aim is to develop methods and techniques for the characterization of electromagnetic behavior of a random space structure and achieving the maximum cleanliness conditions based on the current equipment. Space missions carry payload which consists of various scientific instruments in order to measure electromagnetic field and particle populations in space plasma. Such missions include instruments that are necessarily sensitive to magnetic and electric fields and set strict requirements on electro-magnetic cleanliness and compatibility. Focusing on electrostatic cleanliness and LF electric cleanliness, the instruments are meant to measure slow time - variant electric fields corresponding to frequencies below 200 KHz. Electric field variations show frequency dependence, so these variations are useful to be characterized, measured and modelled.
To manage this, the proposed methodology in chapter 2 describes the modeling using equivalent electric dipoles (EDM-Electric Dipole Modeling) for the EUT’s characterization and validated both via simulated and real measurements by employing sources with well-known electromagnetic behavior. The main target is the extraction of dipole models for all the space subsystems and the correct extrapolation of the field in greater distances. Specifically, every source is represented by one electric dipole. In chapter 2, the basic principles of electromagnetism and the mathematical background supporting this analysis are analyzed, identifying possible assumptions, limitations and approximations in the modeling approach. Basically, the correlation of electric field with the quantities of electric moment and relative distance is demonstrated. Different measurement setups are elaborated for the electric field calculations in the various points to support the EDM methodology. Moreover, two optimization algorithms are proposed in order to solve the inverse problem which is the correct identification of the source that produces the electric field. The first one is the Particle Swarm Optimization (PSO) and the other one is the Differential Evolution Algorithm (DE). For every useful frequency component a different unique dipole is assigned. In the main part of the chapter the results of the dipole modeling procedure are presented using different scenarios integrating real phenomena such as noise and uncertainty. Repeating the abovementioned process for the whole frequency range, a completed spectral model for each EUT is formed.
In the third chapter, the EDM methodology is applied in real space equipment via measurements conducted from Thales Alenia in the context of THOR mission. More specifically, the models produced from the measurements based on the proposed methodology are compared with the real sources. Field comparison in greater distances is elaborated for validation purposes.
In chapter 4, a study and evaluation about uncertainty factors is established in MFEDM methodology (Multi Frequency Electric Dipole Modeling). Basic parameter in the methodology is the position accuracy of both EUT’s (and by extent the equivalent models) and measurement points. During the experiment all the moving parts of the setup may lead to untrustworthy models and altered electric field values in both measurement points and extrapolation distances. Using statistical distributions and other tools, the position errors are simulated to investigate how affect the electric field values and afterwards the equivalent models. After extended simulations it is clearly indicated that even in the worst case scenario (maximum mismatch in position) either measuring instrument or source under test, the field is in good agreement in comparison to the field in the ideal case.
In the last part of the thesis, an effective methodology for a common electromagnetic problem is presented. This concern is the attainment of electromagnetic cleanliness in space or point inner and outer the space equipment and especially in the place where measuring sensor probes are located. The proposed methodology exploits the models from the MFEDM method and assures that with proper equipment ordinance, when necessary, the electromagnetic emissions are reduced significantly. For further improvement of electromagnetic conditions an auxiliary source with known characteristics can be utilized. The algorithm uses heuristic procedures with successive iterations in order to find the optimistic combination of the sources under the given equipment. There are specific and well defined criteria to decide if the total field distribution is tolerable and ensures electromagnetic cleanliness. These criteria correlate the total field distribution with the contribution of each device separately. Depending on the kind of cleanliness (spatial or point) every case differs.
Key Words: Electromagnetic Compatibility, Electric Cleanliness, Electric Dipole Modeling, Differential Evolution Optimization Algorithm, Low Frequency Electric Field, Uncertainty, Equivalent Dipole Model
Supervisor: Christos Capsalis, Professor
PhD student: Grigorios Koutantos