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We study a disk cathode in the weakly relativistic region less than 100 kV by comparing the conventional tubular cathode. The disk cathode is an explosive electron emission cathode made of bare metal. The beam generated by the disk cathode has the more uniform distribution and has the higher current density compared with the tubular cathode with velvet. For the uniform emission, some surface roughness of 1 ����m order is required on the emission surface. Metals such as aluminum, copper and stainless steel are tested as the cathode. Up to now, the best result is obtained by a copper cathode. In view of application to the weakly relativistic oversized backward wave oscillator, the disk cathode is superior to the tubular cathode and is effective for the high-power operation.

An electric brake by a three-phase short-circuit system used as the brake equipment of a small wind turbine damages the rotor blades by a rapid revolution stopping of the generator. Moreover, the generators windings may also be damaged by a large short-circuit current. In this paper, the electric braking circuit using the NTC thermistors (negative temperature coefficient resistors) is proposed as a braking system for a cheaper and safe stop of the small wind turbine. The effect under the natural wind condition is examined by the field test using a 500W small wind turbine.

The Cherenkov instability used in slow-wave devices has been well studied in the literature. However, in previous analyses, the beam motion is restricted to the longitudinal direction assuming an infinitely strong magnetic field. For the finite strength magnetic field, the transverse beam perturbation cannot be ignored and leads to the slow cyclotron instability. Recently, a new version of self-consistent field theory considering three-dimensional perturbation has been developed based on a solid beam, in which the effect of the transverse perturbation appears as a surface charge at a fixed boundary. In the case of a thin annular beam, the boundary is modulated and is essentially different from the solid beam case. We propose a self-consistent field theory considering the moving modified boundary surface. The slow cyclotron instability due to the modulation of an infinitesimally thin annular electron beam is presented.

Backward wave oscillators (BWOs) have been studied as a candidate high-power microwave source. To increase the operation frequency, an oversized slow-wave structure (SWS) is used. The operation at reduced voltage is preferable for practical applications. This work is aimed at numerically examining the operation mode of a weakly relativistic oversized BWO. We examine not only the axisymmetric transverse magnetic mode but also the non-axisymmetric hybrid modes of the oversized SWS. Both of them are candidates for the operation modes. These modes are surface waves whose fields are concentrated near the SWS wall. They overlap in frequency and are not separated by stop-bands. For an efficient beam interaction, the injected electron beam needs to be controlledmore accurately\nthan in the non-oversized SWS case.

Oscillation starting condition of a K-band oversized backward wave oscillator (BWO) is investigated experimentally. Periodically corrugated slow-wave structure (SWS) is used, which is designed for K-band operation in a weakly relativistic region, i.e. 30-80 kV. The beam current is in the range of 50- 00 A. It is shown experimentally that there exists a critical beam energy (starting energy) for the meaningful radiation. The starting energy is more critical than the starting current in the oversized BWO. It decreases by increasing the SWS length. Above the starting energy, the output owers up to about 100 kW are obtained. The typical frequency observed is about 23 GHz. Nonaxisymmetric hybrid HE11 mode as well as axisymmetric TM01 mode is observed. The operation mode is able to be controlled by the SWS length and the magnetic field strength.

The previous studies of backward wave oscillators (BWOs) have been restricted in the operation at the fundamental axisymmetric TM mode. owever, nonaxisymmetric operations of BWO are observed in the experiments and become important. In this work, the Rayleigh-Bessel (R-B) method is improved in order to analyze the nonaxisymmetric as well as axisymmetric fields in a periodic slow-wave structure (SWS). This method is based on the Rayleigh hypothesis, whose limit has been argued extensively. For the X-band SWS, the modulation depth is about 4 times of the theoretically established limit of the Rayleigh hypothesis. We compare the R-B method with another one based on a numerical integration of Maxwell equations, which is free from the Rayleigh hypothesis. The field properties derived by the R-B method show some singularities inside the corrugation due to the Rayleigh hypothesis. However, regardless the singularities, the dispersion characteristics and the fields outside the corrugation are in very good agreement with those by the numerical integration. The validity of the R-B method is discussed.

Normal mode and their high-frequency instabilities are analyzed self-consistently considering electron beams propagating along a magnetic field in unbounded and bounded systems. For unbounded dielectric system, the beam couples with electromagnetic waves corresponding to X and O modes, resulting in the slow cyclotron and Cherenkov instabilities, respectively. For cylindrically bounded system, a self-consistent field theory is developed. Axisymmetric and nonaxisymmetric normal modes obtained are hybrid modes. Cherenkov instability for the EH mode and slow cyclotron instability for EH and HE modes are confirmed in a cylindrical slow wave structure system.