Mode content analysis of the RF output of a gyrotron based on the astigmatic Gaussian beam of higher order

An efficient intermodal decomposition technique is developed for an accurate reconstruction of the RF field obeying a paraxial approximation such as the output of a gyrotron. This method requires complete RF beam information i.e. its amplitude, measured using infrared thermography technique, and phase, which is calculated by an accurate phase retrieval method using complete scalar diffraction integral. The complex RF field evaluated at several positions along the direction of propagation is then used in our modal decomposition method which is based on minimizing the error between the original field and the optimized field. The minimization method is implemented such that it evaluates the optimized field by calculating the beam parameters and the complex coefficient of each mode simultaneously. During the beam propagation, after phase retrieval method, it is observed that in several cases the output RF beam cannot be analyzed using simple Gaussian beam equation and therefore, we extended our method to a more general case of astigmatic Gaussian beam of higher order modes rotating along the optic axis of propagation.