Theoretical Investigation of Iterative Phase Retrieval Algorithm for Quasi-Optical Millimeter-Wave RF Beams

In this paper, we present a detailed analysis of the iterative phase retrieval approach (TPRA) for determining the phase profile of the output microwave beam of a gyrotron from known intensity patterns emphasizing the field propagation techniques which are used to propagate the RF field of the microwave beam between known intensity planes. The propagation method, based on first Rayleigh-Sommerfeld diffraction integral (RSDI), is solved using fast Fourier transform (FFT) technique and zero padding. It is observed that the use of FFT and, therefore, the discretization of the RSDI propagation kernel introduce aberrations in the propagated field due to the superposition of the original field with its replicated versions. This problem is solved by approximations leading to the Huygens-Fresnel propagation method which further Imposes the restrictions on the distances of propagation depending on the size of the transverse plane used to discretize the intensity pattern. This constraint of the distance of propagation causes problem in the iterative phase retrieval approach (IPRA) when more than two intensity planes are used. A method based on interpolation is proposed to overcome this restriction. IPRA is then further discussed to optimize several parameters, such as plane separation, plane dimension, mesh size, and measurement accuracies, which become more of an issue during the measurements of infrared Intensity thermograms of the output microwave beam

Multimode calculation of frequency tunable gyrotrons for dynamic nuclear polarization applications

Multimode calculations of a low-power (above 10W) high-frequency (200 GHz -300 GHz) gyrotron for NMR (nuclear magnetic resonance) spectroscopy applications have been performed. Six transverse modes (TE-7, 2; TE7, 2; TE-4, 3; TE4, 3; TE-2, 4; TE2, 4) whose coupling factors are above 30% and most probably excited in the cavity were included into the calculation. The frequency fine tuning was obtained via the excitation of a sequence of longitudinal modes of TE-7, 2, q by varying the beam voltage from 15kV upward and the magnetic field from 9.6T to 9.77T. The diffractive quality factor of the cavity equals to 9965 and the ohmic quality factor of the cavity equals to 6414. The results show that the main mode TE-7, 2 is quite stable against the possible transverse mode competitors within this magnetic field range and a continuous frequency tuning range more than 800MHz (263.43 GHz - 264.28 GHz) has been achieved with alpha equaling 1.3, the beam radius and current being 1.33 and 100mA respectively

Infrared Measurements of the RF Output of 170-GHz/2-MW Coaxial Cavity Gyrotron and Its Phase Retrieval Analysis

We report the experimental results of the infrared measurements of output RF beam of the European 2-MW 170-GHz coaxial cavity gyrotron for ITER. The output beam profile is measured by the infrared thermographic technique using an infrared camera and a target material which is being irradiated by the RF output. The beam intensity was measured at several locations along the propagation direction. The data were processed for noise reduction and perspective correction and then used with the phase retrieval algorithm to yield an accurate estimate of the phase profile. The moments of the measured intensity arrays are calculated to improve the reliability of the measured infrared image data and data processing. The beam profile is also measured at specific locations in order to compare the phase calculated with the HuygensFresnel propagation method for fixed distance and RayleighSommerfeld diffraction integral method used for field propagation in the phase retrieval algorithm