Douglas R. Denison

Gyrotron internal mode converter reflector shaping from measured field intensity

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/sub 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

We present the formulation of the moment method applied to the determination of phase profiles of microwave beams from known amplitudes. While traditional approaches to this problem employ an iterative error-reduction algorithm, the irradiance moment technique calculates a two-dimensional polynomial phasefront based on the moments of weighted intensity measurements. This novel formulation has the very important advantage of quantifying measurement error, thus allowing for its possible reduction. The validity of the irradiance moment approach is tested and confirmed by examining a simple case of an ideal Gaussian beam with and without measurement errors. The effectiveness of this approach is further demonstrated by applying intensity measurements from cold-test gyrotron data to produce a phasefront solution calculated via the irradiance moment technique. The accuracy of these results is shown to be comparable with that obtained from the previously developed iteration method.

Design of correcting mirrors for a gyrotron used at Large Helical Device

Abstract We report the design of a mirror system used at the Large Helical Device (LHD) to convert the output radiation of the CPI 84-GHz gyrotron into the HE 11 mode of a corrugated waveguide transmission line. The radiation pattern of the gyrotron is measured using an IR camera at LHD. The measured data is analyzed at Massachusetts Institute of Technology (MIT) to retrieve the phase distribution of the radiation. The moments of the measured amplitude arrays are calculated to improve the reliability of the IR image data. The retrieved radiation structure is treated to synthesize the correcting mirrors.

NEARLY ISOTROPIC PHOTONIC BANDGAP STRUCTURES IN TWO DIMENSIONS

It is suggested that a material with a novel spatial dielectric distribution can exhibit a bandgap that is approximately independent of propagation angle. This independence is accomplished by development of the dielectric constant from reflection vectors of equal strength that are ideally equispaced in angle.

Multimegawatt gyrotrons for plasma heating

Summary form only given, as follows. The gyrotron is under development as a high power source for plasma heating at electron cyclotron resonance. For heating large scale plasmas, such as the DIII-D machine at General Atomics, it is advantageous to have high unit power heating sources to reduce the cost and complexity of the system. We will present preliminary designs of 1.5 and 2 MW gyrotrons at a frequency of 110 GHz. The gyrotron designs are based on previous successful results at the 1 MW level at frequencies from 110 to 170 GHz. The baseline design is for a TE/sub 28,8/ mode cavity with an electron beam of 80 to 110 kV and a current of up to 80 A. The expected efficiency exceeds 30% but it should increase to over 50% with a depressed collector. The output beam will be a Gaussian TEM/sub 00/ mode in free space. The gyrotron will be investigated experimentally in short pulse operation (/spl sim/3 microseconds) at MIT and, if successful, will be developed in a 10 s pulsed or CW version by industry. There are two competing approaches for the design of multimegawatt gyrotrons: conventional, cylindrical cavity gyrotrons and coaxial cavity gyrotrons. The conventional cavity approach is being considered as an extension of present day gyrotron research at 110 GHz. The coaxial cavity gyrotron is under investigation at MIT with the goal of output powers of 3 MW at 140 GHz. Recent experimental results from the coaxial cavity gyrotron at power levels in excess of 1 MW will be presented.

Phase retrieval of gyrotron beams from intensity measurements using the moment method

Summary form only given. We present the formulation of the moment method applied to the determination of phase profiles of microwave beams from known amplitudes. Although used in optics, this numerical method has never before been applied to the phase retrieval of a microwave beam. While traditional approaches to this problem employ an iterative error-reduction algorithm, the moment method calculates an initial two-dimensional polynomial phasefront based on weighted moments of intensity measurements. Since this novel formulation calculates the moments of quasi-optical Gaussian-like beams, the moment method has the very important advantage of quantifying and compensating for measurement error. A brief introduction of the theory behind the moment method is presented. The validity of the moment method is confirmed by examining a simple case of an ideal Gaussian beam. A manufactured example to measurement error is then introduced to highlight the advantages of retrieving the phase using the moment method. The effectiveness of the approach is further demonstrated by applying intensity measurement from cold-test gyrotron data to calculate a phasefront solution via the moment method. The accuracy of these results is shown to be comparable with that obtained from the previously developed iteration method. Future developments for the moment method are discussed.

Phase characterization of gyrotron beams from intensity measurements using the moment method

We present the formulation of the moment method applied to the determination of phase profiles of microwave beams from known amplitudes. While traditional approaches to this problem employ an iterative error-reduction algorithm, the moment method calculates a two-dimensional polynomial phasefront based on weighted moments of intensity measurements. Since this novel formulation calculates the moments of quasioptical Gaussian-like beams, the moment method has the very important advantage of quantifying and compensating for measurement error. The validity of the moment method is tested and confirmed by examining a simple case of an ideal Gaussian beam. The effectiveness of this approach is further demonstrated by applying intensity measurements from cold-test gyrotron data to produce a phasefront solution calculated via the moment method. The accuracy of these results is shown to be comparable with that obtained from the previously developed iteration method.

Internal mode converter mirror shaping from measured field intensity

Summary form only given. The simulation and experimental results for a pair of gyrotron internal mode converter phase-correcting mirrors designed from intensity measurements of the feed system will be presented The feed system consists of a rippled-wall launcher that converts a 110 GHz, TE(22,6) rotating cylindrical waveguide mode into a quasi-Gaussian free-space beam and two toroidal mirrors that shape the beam. The field intensity of the beam after the second toroidal mirror was measured over several consecutive planes, and the phase of this beam was retrieved from the measurements to recover the full field structure. This reconstructed field was used to shape a pair of phase-correcting mirrors which transform the feed field into an ideal Gaussian beam suitable for transmission through a two-inch clear aperture diamond gyrotron window. An independent physical optics numerical electromagnetics code was used to propagate the feed field through the two-mirror system to the gyrotron window aperture, and the system produced the desired Gaussian beam amplitude and phase on the window, with 99.5% of the beam power passing through the aperture. Several new insights concerning the current design approach, derived from these simulations, will be presented. The shaped mirrors were manufactured from solid aluminum and installed on the mode converter in a cold test experiment. Measurements were made of the radiated field pattern before, on, and after the window plane, and the measured beam is indeed a Gaussian with parameters that match those of the theoretical design. A detailed analysis of these encouraging results will be presented.

Edge effects in finite gratings of cylindrical scatterers

Edge effects in finite arrays are studied via the analysis and interpretation of one canonical geometry—a linear grating of thin circular cylinders. The partial field scattered by each element in a finite array is decomposed into three components: (1) the contribution of the infinite or periodic array, (2) a component due to the presence of the left edge of the array, and (3) a component due to the right edge of the array. The scattered field due to either array edge is derived from a separate analysis of the appropriate semi‐infinite array and is interpreted in terms of a decaying wave that is launched from the array end. This solution to the well‐posed boundary value problem is numerically verified via boundary condition satisfaction, where the accuracy of the edge‐wave decomposition persists for all tested values of element spacing and plane wave incidence angles. [Work supported by NSF.]