Abstract
A coaxial waveguideinteraction structure may be suitable for a gyrotron backwardwave oscillator (gyroBWO) operating in millimeter and submillimeter waves with good mode selectivity, frequency tunability, and high power. This study analyzes the linear and nonlinear behaviors of a coaxialwaveguide gyroBWO by using a singlemode, selfconsistent nonlinear code. Simulation results indicate that the coaxial gyroBWO exhibits features similar to those of a cylindricalwaveguide gyroBWO, such as nonlinear field contraction, the relation of startoscillation current to interaction length, and the difference value of the transit angle between adjacent axial modes. Additionally, the coaxial gyroBWO has distinctive characteristics due to its transverse geometrical parameter , i.e., the ratio of the outer radius to the inner radius. The beamwave coupling strength of the coaxial gyroBWO is a function of parameter . As a result, the startoscillation current of the coaxial gyroBWO varies as the value selected varies. The coaxial gyroBWOs with different values require different interaction lengths to reach the saturated state for the same beam current. Parameter also impacts the magnetic tuning bandwidth of the coaxial gyroBWO. However, maximum efficiency at saturation, which was obtained by tuning the magnetic field, is not highly dependent on the value of parameter .
The author would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract Nos. NSC 952221E346002 and NSC 972221E346001.
Professor Y. S. Yeh is thanked for his valuable discussions. Ted Knoy is appreciated for his editorial assistance.
I. INTRODUCTION
II. NUMERICAL METHOD AND SIMULATION MODEL
III. NUMERICAL RESULTS
IV. CONCLUSIONS
Key Topics
 Magnetic fields
 13.0
 Gyrotrons
 5.0
 Cyclotron resonances
 4.0
 Differential equations
 4.0
 Oscillators
 4.0
Figures
(a) Projection of electron orbit (circle) on the crosssectional plane of the waveguide in the presence of a uniform magnetic field. Point O is the center of the coaxial waveguide. and are the inner and outer radii of the coaxial waveguide, respectively. Point A is the guiding center of the gyrating electron. Point B is the instantaneous position of the electron. is the electron Larmor radius. (b) Drawings of the coaxial gyroBWO under study.
(a) Projection of electron orbit (circle) on the crosssectional plane of the waveguide in the presence of a uniform magnetic field. Point O is the center of the coaxial waveguide. and are the inner and outer radii of the coaxial waveguide, respectively. Point A is the guiding center of the gyrating electron. Point B is the instantaneous position of the electron. is the electron Larmor radius. (b) Drawings of the coaxial gyroBWO under study.
The diagram of a coaxial waveguide mode (parabola) and beamwave synchronism line (oblique line). The intersection in the backward wave region is the coaxial gyroBWO operating point. Parameters used are: beam voltage , velocity ratio , and . The inner radius and the outer radius of the coaxial waveguide are 0.45 and 0.945 cm, respectively.
The diagram of a coaxial waveguide mode (parabola) and beamwave synchronism line (oblique line). The intersection in the backward wave region is the coaxial gyroBWO operating point. Parameters used are: beam voltage , velocity ratio , and . The inner radius and the outer radius of the coaxial waveguide are 0.45 and 0.945 cm, respectively.
(a) Outer radius vs parameter . The cutoff frequency of the operating mode is fixed at 30.8856 GHz . (b) The coupling coefficient at the optimum (solid line) and at (dots) vs parameter when operating in mode at the fundamental harmonic. Parameters are: , , and .
(a) Outer radius vs parameter . The cutoff frequency of the operating mode is fixed at 30.8856 GHz . (b) The coupling coefficient at the optimum (solid line) and at (dots) vs parameter when operating in mode at the fundamental harmonic. Parameters are: , , and .
Axial profiles of field amplitude (solid line), phase angle (dotted line), and beam energy deposition rate (dashed line) for the first three axial modes at their startoscillation currents . Parameters used are: beam voltage , velocity ratio , , , and .
Axial profiles of field amplitude (solid line), phase angle (dotted line), and beam energy deposition rate (dashed line) for the first three axial modes at their startoscillation currents . Parameters used are: beam voltage , velocity ratio , , , and .
(a) Startoscillation currents vs interaction length . (b) Normalized startoscillation frequencies (solid line) and transit angles (dashed line) vs interaction length . Notably, is the cutoff frequency of the mode. The other parameters are the same as those in Fig. 4.
(a) Startoscillation currents vs interaction length . (b) Normalized startoscillation frequencies (solid line) and transit angles (dashed line) vs interaction length . Notably, is the cutoff frequency of the mode. The other parameters are the same as those in Fig. 4.
(a) Startoscillation currents vs magnetic field . (b) Normalized startoscillation frequencies (solid line) and transit angles (dashed line) vs the magnetic field . The other parameters are the same as those in Fig. 4.
(a) Startoscillation currents vs magnetic field . (b) Normalized startoscillation frequencies (solid line) and transit angles (dashed line) vs the magnetic field . The other parameters are the same as those in Fig. 4.
(a) Startoscillation currents vs parameter . (b) Normalized startoscillation frequencies (solid line) and transit angles (dashed line) vs parameter . The other parameters are the same as those in Fig. 4.
(a) Startoscillation currents vs parameter . (b) Normalized startoscillation frequencies (solid line) and transit angles (dashed line) vs parameter . The other parameters are the same as those in Fig. 4.
Axial field profiles of the first three axial modes for several values at their startoscillation currents . Solid curves, empty triangles and crosses mark calculated results for , , and , respectively. The other parameters are the same as those in Fig. 4.
Axial field profiles of the first three axial modes for several values at their startoscillation currents . Solid curves, empty triangles and crosses mark calculated results for , , and , respectively. The other parameters are the same as those in Fig. 4.
Efficiency (a) and normalized oscillation frequency (b) of the fundamental axial mode vs interaction length for (solid line), (dotted line), and (dashed line). The and other parameters are the same as those listed in Fig. 4.
Efficiency (a) and normalized oscillation frequency (b) of the fundamental axial mode vs interaction length for (solid line), (dotted line), and (dashed line). The and other parameters are the same as those listed in Fig. 4.
Axial profiles of field amplitude (solid line) and beam energy deposition rate (doted line) at the fundamental axial mode for (a) , (b) , (c) , and (d) . The , , and other parameters are the same as those listed in Fig. 4.
Axial profiles of field amplitude (solid line) and beam energy deposition rate (doted line) at the fundamental axial mode for (a) , (b) , (c) , and (d) . The , , and other parameters are the same as those listed in Fig. 4.
Saturated efficiency (a) and normalized oscillation frequency (b) of the fundamental axial mode vs the magnetic field for (solid line), (dotted line), and (dashed line). The and other parameters are the same as those listed in Fig. 4.
Saturated efficiency (a) and normalized oscillation frequency (b) of the fundamental axial mode vs the magnetic field for (solid line), (dotted line), and (dashed line). The and other parameters are the same as those listed in Fig. 4.
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