A compact size hybrid fractal antenna is proposed for the application in wideband frequency range. The proposed antenna structure is the combination of Koch curve and self-affine fractal geometries. The Koch curve and self-affine geometries are optimized to achieve a wide bandwidth. The feed circuit is a microstrip line with a matching section over a rectangular ground plane. The measured impedance matching fractal bandwidth (S11 ≤ −10 dB) is 72.37% from 1.6 to 3.4 GHz. An acceptable agreement is obtained from the simulated and measured antenna performance parameters.

This paper describes the design of a triode magnetron injection gun for use in a 200 kW, 300 GHz gyrotron. As power and frequency increase, the performance of the gyrotron becomes quite sensitive to the quality of the electron beam. Formation of a laminar electron flow is essential for the realization of a high quality beam with a small velocity spread. In this study, a new method is developed for a quantitative evaluation of the laminarity and is applied to optimize the electrode design. The laminarity depends not only on conventional design parameters such as the cathode slant angle, but also on the spatial distribution of the electric field along the beam trajectory. In the optimized design, the velocity pitch factors, α, larger than 1.2 are obtained at 65 kV, 10 A with spreads, Δα, less than 5%.

This paper describes the design of a magnetron-injection gun for a 100 kW, 300 GHz gyrotron. With an increase in power and frequency, performance of the gyrotron becomes quite sensitive to the quality of the electron beam. Formation of a laminar electron flow is essential for the realization of a high quality beam with small velocity spread. In this study, a new method is proposed for the evaluation of the laminarity, and applied to the design optimization of the electrodes. It is found that the laminarity depends not only on the conventional design parameter of the cathode slant angle, but also on the spatial distribution of the electric field inside the beam. Gyrotrons are capable of providing high-powers at millimeter and sub-millimeter wave lengths, and widely used in various fields of physics study and technological use. In the field of nuclear fusion research, MW class tubes with the frequencies from 28 to 170 GHz have been already utilized [1,2]. Recently, development of a high power source in the sub-THz region is expected for the measurement of collective Thomson scattering (CTS) in the fusion plasmas. For use in the CTS diagnostics, an oscillation power of more than 100 kW is required at the frequency range of 300 ∼ 400 GHz [3,4]. In FIR, Univ. of Fukui, development of a high power tube is currently in progress. The frequency is set at 295 GHz with a magnetic field strength of 11.4 T in the cavity. In this study, the design consideration of a magnetron-injection gun (MIG) is performed by use of an electron trajectory code EGUN [5].In order to achieve the output power of over 100 kW, an electron beam voltage of 65 kV, and current more than 10 A are required with the velocity pitch-factor α (the ratio of perpendicular to parallel velocities to the magnetic field line) of 1.2. The spread of α,should be minimized because it decreases the oscillation efficiency and may cause the magnetic mirror reflection of a part of the electrons. With an increase in power and frequency, a large-current beam is compressed by an extremely strong magnetic field near the cavity. Then the beam will have a larger-current density, which enhances the influence of the space charge force on α spread . Formation of a laminar electron flow is essential for reduction of the space charge effect and for realization of a high quality author's e-mail: y-yama@fir.u-fukui.ac.jp beam with small α spread [6]. A triode structure is adopted to control the beam properties without changing the total beam energy. Figure 1 represents the schematic drawings of the electrodes which are used for the numerical computation of the beam trajectory. The radius of the cavity is 3.4 mm. In order to couple to the selected mode of TE 14,2 , the electron beam should be injected into the cavity with the radius R b of 2.42 mm. The radius of the emitting ring is set to 17.6 mm, from which the guiding-center of the electron arrives at R b = 2.42 mm in the cavity. In this geometrical design, the magnetic compression ratio between the cavity and the emitting ring be...

For application of high frequency gyrotron to high power THz technology, Gyrotron FU CW series is being developed in FIR FU. Gyrotron FU CW CI is developed as one of sub-THz gyrotrons included in the series. The advantage of the gyrotron is compactness using a compact superconducting magnet and compact power supply system, which makes the accesses of the gyrotron to applied large-scale devices easier and extends the applications of gyrotron to wider fields. The designed frequency and cavity mode are 394.5 GHz and TE 26 mode for application to the 600 MHz DNP-NMR spectroscopy. As the operation results, the frequency and the output power were 394.03 GHz and around 30 W, respectively, which are available for the application to the 600 MHz DNP-NMR measurement. In addition, this gyrotron can operate at many other frequencies and cavity modes for application to high power THz technologies in wide fields. In this paper, the design and the operation results including long pulse or CW mode are presented.

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