Abstract
This study analyzes the characteristics of a gyrotron backwardwave oscillator (gyroBWO) with a longitudinally tapered coaxialwaveguide by using a singlemode, selfconsistent nonlinear code. Simulation results indicate that although tapering the inner wall or the outer wall can significantly raise the startoscillation current, the former is more suitable for mode selection than the latter because an increase of the startoscillation current by a tapered inner wall heavily depends on the chosen value (i.e., the average ratio of the outer radius to the inner radius over the axial waveguide length). Selective suppression of the competing mode by tapering the inner wall is numerically demonstrated. Moreover, efficiency of the coaxial gyroBWO is increased by tapering the outer wall. Properly downtapering the outer wall ensures that the coaxial gyroBWO can reach a maximum efficiency over twice that with a uniform one.
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract Nos. NSC 992923M007005MY3 and NSC 992221E346003.
I. INTRODUCTION
II. NUMERICAL METHOD AND SIMULATION MODEL
III. NUMERICAL RESULTS
IV. CONCLUSIONS
Key Topics
 Magnetic fields
 11.0
 Resistive wall mode
 6.0
 Differential equations
 4.0
 Frequency analyzers
 4.0
 Oscillators
 4.0
Figures
Schematic drawings of the coaxial gyroBWO with a slightly tapered inner radius and slightly tapered outer radius. Radii and are the inner and outer radii at the position (the midpoint point between the upstream end and the downstream end ), respectively. Additionally, angles and represent the taper angle of the inner wall and the taper angle of the outer wall, respectively.
Schematic drawings of the coaxial gyroBWO with a slightly tapered inner radius and slightly tapered outer radius. Radii and are the inner and outer radii at the position (the midpoint point between the upstream end and the downstream end ), respectively. Additionally, angles and represent the taper angle of the inner wall and the taper angle of the outer wall, respectively.
Root (solid line) and (dashed line) of mode versus the parameter , where is the ratio of the outer radius to the inner radius at the position .
Root (solid line) and (dashed line) of mode versus the parameter , where is the ratio of the outer radius to the inner radius at the position .
Dependence of coupling strength on normalized guidingcenter radius at different values for (a) and (b) modes.
Dependence of coupling strength on normalized guidingcenter radius at different values for (a) and (b) modes.
(a) and (b) Profiles of inner wall radii and the equivalent hollow waveguide radii. (c)(e) Field amplitudes (solid line) and beam energy deposition rates (dotted line) of mode at their startoscillation currents for inner taper angles = , , and . Parameters used are = 100 kV, = 13.0 kG, = 1.0, = 0.54, = 2.5, = 1.625 cm, = = 1.0 cm, = 10 cm, = , and = = (=1.72 × 10^{−8} Ωm).
(a) and (b) Profiles of inner wall radii and the equivalent hollow waveguide radii. (c)(e) Field amplitudes (solid line) and beam energy deposition rates (dotted line) of mode at their startoscillation currents for inner taper angles = , , and . Parameters used are = 100 kV, = 13.0 kG, = 1.0, = 0.54, = 2.5, = 1.625 cm, = = 1.0 cm, = 10 cm, = , and = = (=1.72 × 10^{−8} Ωm).
Startoscillation currents (a) and startoscillation frequencies (b) versus the taper angle of the inner wall for = 13.0 kG, 13.5 kG, and 14.0 kG. The other parameters are the same as those used in Fig. 4.
Startoscillation currents (a) and startoscillation frequencies (b) versus the taper angle of the inner wall for = 13.0 kG, 13.5 kG, and 14.0 kG. The other parameters are the same as those used in Fig. 4.
Startoscillation currents versus magnetic field for (a) several inner taper angles and (b) several outer taper angles . The outer taper angle = in (a) and the inner taper angle = in (b). Other parameters are the same as those used in Fig. 4.
Startoscillation currents versus magnetic field for (a) several inner taper angles and (b) several outer taper angles . The outer taper angle = in (a) and the inner taper angle = in (b). Other parameters are the same as those used in Fig. 4.
Startoscillation currents versus parameter for (a) several inner taper angles and (b) several outer taper angles . = 1.12 , where is the grazing magnetic of mode. The outer taper angle = in (a) and the inner taper angle = in (b). Other parameters are the same as those used in Fig. 4.
Startoscillation currents versus parameter for (a) several inner taper angles and (b) several outer taper angles . = 1.12 , where is the grazing magnetic of mode. The outer taper angle = in (a) and the inner taper angle = in (b). Other parameters are the same as those used in Fig. 4.
Startoscillation currents of and modes versus parameter for inner taper angles = and . The magnetic field = 13.24 kG and other parameters are the same as those used in Fig. 4.
Startoscillation currents of and modes versus parameter for inner taper angles = and . The magnetic field = 13.24 kG and other parameters are the same as those used in Fig. 4.
(a) Profile of outer wall radius. (b) Field amplitudes and (c) beam energy deposition rates of the operating mode at = 10 A for outer taper angles = , , and . Parameters used are = 100 kV, = 13.4 kG, = 1.0, = 0.54, = 2.6, = 1.625 cm, = = 1.0 cm, = 10 cm, = , and = = ( = 1.72 × 10^{−8} Ωm).
(a) Profile of outer wall radius. (b) Field amplitudes and (c) beam energy deposition rates of the operating mode at = 10 A for outer taper angles = , , and . Parameters used are = 100 kV, = 13.4 kG, = 1.0, = 0.54, = 2.6, = 1.625 cm, = = 1.0 cm, = 10 cm, = , and = = ( = 1.72 × 10^{−8} Ωm).
Calculated (a) efficiency and (b) oscillation frequency versus magnetic field for several outer taper angles . Other parameters are the same as those used in Fig. 9.
Calculated (a) efficiency and (b) oscillation frequency versus magnetic field for several outer taper angles . Other parameters are the same as those used in Fig. 9.
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