Experimental observation of the effect of aftercavity interaction in a depressed collector gyrotron oscillator

This paper presents the experimental observation of the effect of an aftercavity interaction (ACI) in a depressed collector gyrotron oscillator. The gyrotron generates an output power of 1.5 MW at 110 GHz in 3 ??s pulses with a 96 kV and 40 A electron beam and has a single-stage depressed collector. The ACI arises from an unintended cyclotron resonant interaction between the microwave beam traveling out from the cavity and the gyrating electron beam. The interaction occurs in the uptaper of the launcher, immediately downstream from the cavity, where the magnetic field is slightly lower than its value in the cavity region. The ACI results in a reduction in efficiency since the electron beam tends to extract power from the wave. There is also a broadening of the spent beam energy profile, which reduces the effectiveness of the depressed collector and in turn limits the overall efficiency of a gyrotron. Measurements of the maximum depression voltage of the collector vs beam current at 96 kV are compared with simulations from the MAGY code [M. Botton, IEEE Trans. Plasma Sci. 26, 882 (1998)]. Excellent agreement is obtained between theory and experiment but only if the ACI is included. In the present experiment, it is estimated that the observed efficiency of 50% would have been about 60% in the absence of the ACI. These results verify the role of the ACI in reducing the efficiency of the gyrotron interaction.open151

Imaging of atmospheric air breakdown caused by a high-power 110-GHz pulsed Gaussian beam

We present the images of regular filamentary plasma arrays produced upon the breakdown of air at atmospheric pressure at the focal region of a high-power 110-GHz pulsed Gaussian beam. The source of the millimeter wave beam is a gyrotron that can generate up to 1.5-MW output power with 3-mu s pulselength. This unique plasma structure exists only at high pressures. With decreasing pressure, the structure changes into layers of curved plasma sheets and into more familiar diffuse plasma. A main cause of the formation of the regular array structure appears to be the reflection from filaments. The successive generation of conductive filaments modifies the incident field pattern and creates local hot spots upstream of the existing filaments with regular spacing of roughly a quarter wavelength.close81

Efficiency enhancement of a 1.5-MW, 110-GHz gyrotron with a single-stage depressed collector

We report new experimental results from a 1.5-MW, 110-GHz gyrotron with a single-stage depressed collector. The,gyrotron was operated in the TE22.6 mode with 3-mu s pulse duration. An internal mode converter, which consists of a launcher and four mirrors, has been installed and tested A highly Gaussian-like output beam was observed A single-stage depressed collector has been operated for the study of efficiency enhancement using the same cavity V-2005 as was used in a previous experiment in the axial configuration, in which the output microwave beam propagated through a circular waveguide that also served as a collector. Output power of 1.5 MW, corresponding to 50% efficiency, was measured at 97 kV of beam voltage and 42 A of beam current at 25 kV of collector depression voltage. The results are compared between the axial configuration and the internal mode converter configuration.open101

Erratum: Experimental Results on a 1.5 MW, 110 GHz Gyrotron with a Smooth Mirror Mode Converter

We present an internal mode converter (IMC) design for a 1.5 MW, 110 GHz gyrotron operating in the TE 22,6 mode. The launcher, designed using the codes Surf3d and LOT, converts the cavity waveguide mode into a nearly pure Gaussian beam. The Gaussian beam output from the launcher is shaped by a series of 4 smooth, curved mirrors to provide a circular output beam with a flat phase front at the gyrotron window. By employing smooth mirrors rather than mirrors with phase correcting surfaces, such an IMC is less sensitive to alignment issues and can more reliably operate with high efficiency. The IMC performance was verified by both cold test and hot test experiments. Beam pattern measurements in each case were in good agreement with theoretical predictions. The output beam was of high quality with calculations showing that the Gaussian Beam content was 95.8 ?? 0.5% in both hot and cold test.close0

Experimental Results on a 1.5??MW, 110??GHz Gyrotron with a Smooth Mirror Mode Converter

We present an internal mode converter (IMC) design for a 1.5 MW, 110 GHz gyrotron operating in the TE(22,6) mode. The launcher, designed using the codes Surf3d and LOT, converts the cavity waveguide mode into a nearly pure Gaussian beam. The Gaussian beam output from the launcher is shaped by a series of 4 smooth, curved mirrors to provide a circular output beam with a flat phase front at the gyrotron window. By employing smooth mirrors rather than mirrors with phase correcting surfaces, such an IMC is less sensitive to alignment issues and can more reliably operate with high efficiency. The IMC performance was verified by both cold test and hot test experiments. Beam pattern measurements in each case were in good agreement with theoretical predictions. The output beam was of high quality with calculations showing that the Gaussian Beam content was 95.8 +/- 0.5% in both hot and cold test

Development of the Full Package of Gyrotron Simulation Code

A complete code-package for gyrotron simulation to analyze its performance is under development in UNIST, Korea. We first time report the present status of the code-package named as UNIST Gyrotron Design Tool (UGDT). It can perform design simulations for gyrotron's interaction cavity, RF window, and the essential mode calculations including the study of mode competition. We will discuss about its salient features, theory, numerical implementation, and its calculation result for 95 GHz UNIST Gyrotron. Moreover, we will validate its capability to perform the mode competition calculation for fundamental and second harmonic modes

A comprehensive analysis of a TE11 to HE11 mode converter for an oversized F-band corrugated waveguide

We have investigated the mode transition behavior from approximately half- to quarter-wavelength in typical F-band (90-140 GHz) TE11 to HE11 converters of varying corrugation depths. Simulations in which the number of slots in the corrugation is varied indicate that the optimal number of slots should be greater than two, which is in excellent agreement with the experimental results. The mode content was also analyzed with and without a quarter-wavelength depth-corrugated tapered section. The electric field pattern at the exit of a quarter-wavelength tapered transition connected to a half-wavelength to quarter-wavelength TE11 to HE11 mode converter is compared with that without a converter. Electric field patterns both with and without a half- to quarter-wavelength TE11 to HE11 mode converter show a highly Gaussian field pattern with less than 35 dB of cross-polarization. Simulated field patterns are in good agreement with the experimentally measured field patterns, with the measured and simulated electric field distributions matching approximately 96.5 and 97.5%, respectively, to a pure Gaussian distribution.close2