Formation of Laminar Electron Flow for a High-Power Sub-THz Gyrotron

Abstract

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...

Analysis of oscillation characteristics and optimal conditions for high power operation of Gyrotron FU CW GIII

“…The electrode configurations were optimized by forming laminar electron trajectories, which resulted in a substantial reduction in the spread, Da, of the pitch factor (a) of the electron beam at the cavity entrance. 9,10 As a result, Da decreased from more than 30% in Gyrotron FU CW GII to 7% in Gyrotron FU CW GIII. 2,7 An inner mode converter composed of a Vlasov launcher 11 tunable and rotatable.…”

On optimization of sub-THz gyrotron parameters

“…First, let us carry out the qualitative treatment of the problem. Results of numerous studies of the dependence of the electron velocity spread on the cathode slant angle 10,13 allow us to approximate the dependence of the velocity spread on the angle h between the emitter surface and the magnetic force line there by the parabola…”

“…Consider as an example parameters of a 300 GHz gyrotron. 13 This gyrotron operates in the TE 14,2 -mode. The width of the emitter is 3.2 mm and the magnetic compression factor a B ¼ B res =B cath is equal to 53.2.…”

“…These data are based on calculations presented in Ref. 13 under the assumption that the beam orbital-to-axial velocity ratio is equal to 1.2. As follows from Fig.…”

“…(8), the relation between the ohmic and diffractive Qfactors defines the proportion between the power of outgoing radiation and the power of microwave losses in the circuit walls. The interaction efficiency in the case under study can be defined, in line with the definition given elsewhere, 13,14 as In Eq. (9), g ?…”