Gyrotron internal mode converter reflector shaping from measured field intensity

Abstract

We present the formulation and experimental results of a new approach to designing internal mode converter reflectors for high-power gyrotrons. The method employs a numerical phase retrieval algorithm that reconstructs the field in the mode converter from intensity measurements, thus accounting for the true field structure in shaping the beam-forming reflectors. An outline for designing a four-reflector mode converter is presented and generalized to the case of an offset-fed shaped reflector antenna. The requisite phase retrieval and reflector shaping algorithms are also developed without reference to specific mode converter geometry. The design approach is applied to a 110 GHz internal mode converter that transforms the TE 22 6 gyrotron cavity mode into a Gaussian beam at the gyrotron window. Cold test experiment results of the mode converter show that a Gaussian beam with the desired amplitude and phase is formed at the window aperture. Subsequent high-power tests in a 1 MW gyrotron confirm the Gaussian beam observed in cold tests. The general development of the approach and its validation in a quasi-optical mode converter indicate that it is also applicable to other quasi-optical, microwave applications such as radio astronomy, free-space transmission lines, and mitre bends for overmoded waveguides.

Phase retrieval of gyrotron beams based on irradiance moments

“…This error-reduction approach, known as the iteration method (or "phase-retrieval method"), although numerically intensive, has proven to be successful in designing accurate internal phase correcting mirrors [6,12] and external mirrors [12,13,14].…”

“…In [15], the moments of infrared images of gyrotron radiation were calculated to determine the accuracy of the spatial alignment of the images. The iterative phase retrieval procedure and internal mirror synthesis [6] were employed using only the well-aligned images.…”

“…In the procedure of reflector synthesis, we can take advantage of the irradiance moment method approach's phasefront solution, which is in analytical form. Therefore, the numerical procedure of phase unwrapping, required for reflector synthesis using the iteration method [6], is not needed using irradiance moments. This improves the mirror shaping because phase unwrapping is ambiguous.…”

“…2 shows a schematic of a cold-test gyrotron experimental setup which may be used to design appropriate external phase correcting mirrors. A series of measurement planes, shown as dotted lines, lie immediately before and after the window location where the beam is the narrowest (the beam waist) [6]. In the actual cold-test, the window is omitted from the set-up.…”

Alignment Method of ECH Transmission Lines Based on the Moment and Phase Retrieval Method Using IR Images

“…This, however, is impossible to obtain on the Megawatt power level. The phase retrieval method is the one by which the phase information is reconstructed iteratively from only amplitude values acquired at several positions [4]. This method has been successfully used to design phase correction mirrors in the MOU.…”