Most of the present-day millimeter-wave gyrotrons developed for plasma experiments in controlled fusion reactors utilize cylindrical cavities operating in high-order modes. The choice of modes should obey certain restrictions dictated by the achievable mode selection and the maximum admissible level of the density of microwave ohmic losses in the cavity walls. Even with these restrictions, developers have successfully manufactured quasi-continuous-wave gyrotrons operating in the short millimeter wavelength bands that are capable of delivering microwave power on the order of 1 MW. To upgrade gyrotron power to the level of several megawatts, more complicated coaxial microwave circuits should be used. This statement is also valid for relativistic gyroklystrons, which are currently under development for driving future linear accelerators. This paper presents an overview of the history of the development of coaxial gyrodevices, a discussion of the physics-based issues which are the most important for their operation, a description of the state of the art in the development of coaxial gyrodevices for the above-mentioned applications, and a brief forecast for their future.

Coaxial gyrotrons: past, present, and future (review)

Do you design and build vacuum electron devices, or work with the systems that use them? Quickly develop a solid understanding of how these devices work with this authoritative guide, written by an author with over fifty years of experience in the field. Rigorous in its approach, it focuses on the theory and design of commercially significant types of gridded, linear-beam, crossed-field and fast-wave tubes. Essential components such as waveguides, resonators, slow-wave structures, electron guns, beams, magnets and collectors are also covered, as well as the integration and reliable operation of devices in microwave and RF systems. Complex mathematical analysis is kept to a minimum, and Mathcad worksheets supporting the book online aid understanding of key concepts and connect the theory with practice. Including coverage of primary sources and current research trends, this is essential reading for researchers, practitioners and graduate students working on vacuum electron devices.

Microwave and RF Vacuum Electronic Power Sources

The design of a 1.5-MW, 140-GHz, TE-/sub 28,16/-coaxial cavity gyrotron is presented and results of experimental operation are given. A cavity with a cylindrical outer wall and a radially tapered inner rod with longitudinal corrugations was used. A maximum output power of 1.17 MW has been measured in the design mode with an efficiency of 27.2%. Single-mode operation has been found over a wide range of operating parameters. The experimental values agree well with the results of multimode calculations. Frequency-step tuning has been performed between 115.6 and 164.2 GHz. In particular, an output power of 0.9 MW has ben measured in the TE/sub 25,14/ mode at 123.0 GHz and 1.16 MW in the TE/sub 32,18/ mode at 158.9 GHz. At frequencies its with strong window reflections the parameter range for which stable operation is possible is reduced significantly. In order to obtain results relevant for a technical realization of a continuously operated gyrotron, a tube with a radial radio frequency (RF)-beam output through two output windows and a single-stage depressed collector has been designed and is under fabrication. A two-step mode conversion scheme-TE-/sub 28,16/ to Te/sub +76.2/ to TEM/sub 00/-which generates two narrowly directed (60/spl deg/ at the launcher) output wavebeams has been chosen for a quasioptical (q,o) mode converter system. A conversion efficiency of 94% is expected.

A 1.5-MW, 140-GHz, TE/sub 28,16/-coaxial cavity gyrotron

To realize continuous-wave (CW) operation of millimeter-wave gyrotrons at megawatt (MW)-power levels, these devices must operate in very high-order modes. To excite such an operating mode and to drive it into the regime of MW-level operation with high efficiency requires careful consideration of the startup scenario through which the operating parameters of the device are brought to their nominal values. In the present paper, several common startup scenarios and the most important physical effects associated with them are discussed. Then, the paper presents the results of startup simulations for a 140-GHz, MW-class gyrotron developed by Communications and Power Industries (CPI) for electron-cyclotron plasma heating and current drive experiments on the "Wendelstein 7-X" stellarator. The simulations were done with MAGY, a multifrequency, self-consistent code developed at the University of Maryland. Simulations tracking six competing modes show that, with a proper choice of operating parameters, stable excitation of the desired TE/sub 28,7/-mode at 1 MW level can be realized, despite the presence of dangerous parasites in the resonator spectrum. These results are in approximate agreement with experimental tests, in which the gyrotron demonstrated reliable operation at power levels up to 900 kW. PACS numbers: 84.40.Ik, 52.75 Ms.

Startup scenarios in high-power gyrotrons

The development of a coaxial-cavity gyrotron operating in TE/sub 31,17/ mode at 165 GHz is presented. The selection of the operating frequency and mode are based on the limitations imposed by the maximum held of the superconducting (sc) magnet at Forschungzentrum Karlsruhe, Institut fur Technische Physik (FZK), the use of the inverse-magnetron injection gun (IMIG) of the 140-GHz, TE/sub 28,16/ coaxial gyrotron and the possibility of transforming the cavity mode to a whispering gallery mode (WGM) appropriate for the dual-beam quasioptical (q.o.) output coupler and the two output windows, which are foreseen for the next lateral output version of the tube. The tube with axial output has been tested at FZK to deliver maximum output power of 1.17 MW in the designed TE/sub 31,17/ mode with 26.7% efficiency at 164.98 GHz. Maximum efficiency of 28.2% was achieved at 0.9-MW output power. The design operating point with output power 1.36 MW and 36.7% efficiency was net accessible because of beam instabilities at high electron-velocity ratio /spl alpha/, presumably caused due to high electron-velocity spread. Power at higher frequencies was also detected: 1.02 MW at 167.16 GHz in TE/sub 32,17/ mode with 26.88 efficiency, 0.63 MW at 169.46 GHz in TE/sub 33,17/ mode with 18% efficiency, and 0.35 MW at 171.80 GHz in TE/sub 31,17/ mode with 13.3% efficiency.

Design and experimental operation of a 165-GHz, 1.5-MW, coaxial-cavity gyrotron with axial RF output

A design for a 3-MW 140-GHz gyrotron based on the use of a coaxial cavity is given. The cavity mode is TE/sub 21,13/, chosen so that the ohmic heating on both the inner and outer conductors would be low enough for CW operation. The mode selection process, nonlinear, multimode and time-dependent modeling of the beam wave interaction, and gun design are discussed in detail. An inverted magnetron injection gun (MIG) is used to accommodate the inner conductor. The radiation is coupled out via a quasi-optical mode converter, consisting of an irregular cylindrical waveguide section followed by a step-cut launching aperture and a single near-parabolic mirror. The design of these components is also described.

Design of a 3-MW 140-GHz gyrotron with a coaxial cavity

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Design of a 3-MW 140-GHz gyrotron with a coaxial cavity