This paper presents a comprehensive theory of the cyclotron resonance maser (CRM) interaction in a circular waveguide. The kinetic theory is used to derive the dispersion relationships for both TE and TM modes. The TE mode case has been investigated by several authors, but there has been comparatively little work on the TM mode case. However, the TM mode interaction competes effectively with the TE mode interaction at relativistic electron energies. The conditions for maximum temporal and spatial growth rates are shown. The TM mode growth rates are found to vanish when the RF wave group velocity equals the beam axial velocity (‘grazing incidence’). The single particle theory is used to derive a compact set of self-consistent non-linear equations for the TE and TM mode interactions. These equations are particularly appropriate for the cyclotron auto-resonance maser (CARM) regime but applicability extends to other regimes as well. The conditions for optimum efficiency are investigated for oscillator and amp...

Linear and nonlinear theory of the Doppler-shifted cyclotron resonance maser based on TE and TM waveguide modes

This paper investigates the antenna waveguide (launcher), the main component of the quasi-optical mode converter of a 2-MW 170-GHz TE/sub 34,19/ coaxial cavity gyrotron, which is under development within the European Union. For coaxial gyrotrons operating in very high-order cavity modes like the TE/sub 34,19/, due to the ratio of the caustic to cavity radius of 0.323, the transformation of the high-order cavity mode into a nearly Gaussian distribution cannot be done as good as for gyrotron modes where the ratio of caustic to cavity radius is approximately 0.5. The simulation results for the TE/sub 34,19/ mode show that the average and peak values of the power density at the edges of the cuts of a conventional dimpled-wall launcher are approximately 32.3 W/cm/sup 2/ and 63.8, respectively, which will produce diffraction losses and reflection of power from the cuts. This paper reports on an advanced launcher for which average and peak values of power density of 1.9 and 5.4 W/cm/sup 2/ at the edges of the cuts are achieved, and a well-focused field at the aperture with a scalar Gaussian mode content of 94.8% is obtained.

Theoretical investigation of an advanced launcher for a 2-MW 170-GHz TE/sub 34,19/ coaxial cavity gyrotron

A coaxial Bragg reflector is suggested for the application in the cyclotron autoresonance masers (CARMs), where both the outer wall and inner rod are corrugated with weak axisymmetric-sinusoidal-ripple, and a concept design is presented. It is found that, compared to the cylindrical Bragg reflector, the coaxial Bragg reflector has larger dimensional size, narrower bandwidth, and larger eigenvalue intervals. These peculiarities are favorable to the mode selectivity, the ability of dissipating the ohmic heat on the wall, the oscillating startup of the operating mode, and machining manufacture, and therefore, this new kind of Bragg reflector can be developed into a coaxial resonator for the high-power CARMs

A Coaxial Bragg Reflector for Cyclotron Autoresonance Maser Oscillators

A S A TRIBUTE to individuals who have contributed significantly, and over many years, to the terahertz community, and as a guide and inspiration for those who are just beginning their professional association with this field of study, these transactions have included, on a regular basis, a series of biographical interviews with technical researchers who have appreciably impacted the THz community in a positive manner. In order to go beyond a strict technical review and to take better advantage of the information and commentary only available through a direct discussion, these articles take on a less formal style than the research articles that can be found within the remaining pages of the transactions. The Editor-in-Chief has taken some leeway in this regard, for the benefit of communicating more fully the character, experiences, and historic circumstances that have shaped our community and set the directions for our collective research. As a further means of assuring that the true flavor and circumstances of the contributions are expressed in the text, all of the articles are compiled after a face-to-face interview. The final text is shared with, and often helped considerably, by comments from the subject of the article. This month we travel to Japan to speak with Dr. Hiromasa Ito,1 a longterm affiliate of the Research Institute of Electrical Communication (RIEC), Tohoku University, Sendai, and a former Team Leader in Terahertz Photonics at Japan's premier science research institution, RIKEN (Institute of Physical and Chemical Research). Professor Ito is a link in a long chain of noted THz advocates and researchers at Tohoku. This chain began with Jun-Ichi Nishizawa2 who according to Ito, “predicted the possibility of using the resonance of phonons and molecular vibrations to generate THz electromagnetic radiation”3 in 1963. Nishizawa came to RIEC in 1953, and established

http://ieeexplore.ieee.org/iel7/5503871/6759772/06759793.pdf

Terahertz Pioneers

A circular waveguide loaded with dielectric and metal discs was chosen to evaluate its dispersion characteristics and dispersion shaping with change of structure parameters for wideband coalescence of beam- and waveguide-mode dispersion characteristics for wideband gyro-TWT performance. The azimuthally symmetric TE-mode analysis of the structure was carried out in fleld matching technique by considering the propagating wave in cylindrical free-space region having radius equal to the hole-radius of metal disc, and the stationary waves in free-space and dielectric regions between two consecutive metal discs. The dispersion relation and, in accordance, a computer code were developed. Further, the roots of the dispersion relation for various sets of the structure parameters were obtained using the developed computer code; the dispersion characteristics were plotted; and the dispersion shaping was projected for typically chosen TE01-, TE02- and TE03-modes. The analytical results were validated against those obtained for the conventional and earlier published structures, and also those obtained using commercially available simulation tool. Finally, a study on azimuthal electric fleld available over the radial coordinate was carried out to show the control of structure parameter on the gyrating electron beam position for the chosen operating mode of a dielectric and metal discs loaded gyro-TWT.

/pdf/analysis-of-a-circular-waveguide-loaded-with-dielectric-and-53o3eogt26.pdf

Analysis of a circular waveguide loaded with dielectric and metal discs

This paper presents the investigation of a mirror system of a quasi-optical (QO) mode converter for a high-power coaxial-cavity gyrotron. The mirror system consists of three mirrors. The first mirror is a quasi-elliptical one. Based on the Katsenelenbaum-Semenov Algorithm (KSA), the second and the third mirrors are iteratively optimized as adapted phase-correcting mirrors to transform the outgoing wave beam into a fundamental Gaussian structure. The investigation shows that the focal length of the quasi-elliptical mirror has a great influence on the optimized conversion efficiency, and, hence, it should be chosen to match the asymptotic beam growth (ABG) angle well in order to obtain high conversion efficiency. The design of a mirror system has been performed for a 2 MW, continuous wave (CW), 170 GHz, and TE34,19 -mode coaxial-cavity gyrotron, which is under development at Forschungszentrum Karlsruhe, Germany. Taking into account the size of the mirrors and the conversion efficiency, a mirror system has been designed with a conversion efficiency of 98.3%

Quasi-Optical Mode Converter/Mirror System for a High-Power Coaxial-Cavity Gyrotron

Based on the gyrokinetics of free‐electron masers, the influence of the electron‐beam misalignment on the energy‐transfer rate and starting current of a coaxial cavity gyrotron oscillator is studied. As a practical example, a numerical analysis is given to the 1.5 MW/140 GHz coaxial‐cavity gyrotron that is being constructed at the Forschungszentrum Karlsruhe (FZK) [M. Thumm, Fusion Eng. Design 30, 139 (1995)]. Results show that the electron‐beam misalignment will decrease the energy‐transfer rate and increase the starting current, and the dependence of the output power on the magnetic field may become more sensitive due to a poor alignment of the electron‐beam.

Kinetic description of the influence of electron‐beam misalignment on the performance of a coaxial‐cavity gyrotron

This note demonstrates the existence and possible suppression of the residual side-lobes in a coaxial Bragg reflector by employing Blackman window distribution, no matter if the phase difference between the outer and inner corrugations is 0, or π/2, or π. Physical explanation is qualitatively given to the effect of the window distribution.

Suppression of Residual Side-lobes in a Coaxial Bragg Reflector

In practice a coaxial Bragg structure always has an eccentricity between the outer-wall and inner-rod axes. Numerical simulations are carried out to analyze the effect of the eccentricity on the transmission in a coaxial Bragg structure. Results demonstrate that the effect of the eccentricity is minimized and becomes negligible when the phase difference between the outer and inner corrugations is π, no matter if the eccentricity is parallel or oblique.

Shi-Chang Zhang

Papers

Quasi-Optical Mode Converter/Mirror System for a High-Power Coaxial-Cavity Gyrotron

A Coaxial Bragg Reflector for Cyclotron Autoresonance Maser Oscillators

Kinetic description of the influence of electron‐beam misalignment on the performance of a coaxial‐cavity gyrotron

Suppression of Residual Side-lobes in a Coaxial Bragg Reflector

Effect of Eccentricity on Transmission in a Coaxial Bragg Structure