Analytic theory of a tapered gyrotron resonator

An analytic theory has been derived for determining the eigenfrequencies, RF-field distribution and Q of the TE_mpq modes of a gyrotron resonator consisting of a circular cylinder joined to a slowly tapered section. Explicit results are obtained for a linear taper. The cavity modes are found to have an RF-field distribution which is useful for prebunching the electron beam and enhancing efficiency. For high Q cavities, the cavity Q depends on axial mode number q as q^–2, which is important for mode discrimination. Proper selection of taper length is found to reduce the Q of high q modes, also aiding in mode discrimination. The present approach may be applied to other forms of weakly irregular cavities, such as cavities with nonlinear tapers.Work supported by U.S.D.O.E. Contract DE-AC02-78ET-51013Supported by U.S. Department of Energy     

Coaxial resonator for a megawatt 280 GHz gyrotron

To provide the required mode selectivity for a megawatt 280 GHz gyrotron, a coaxial resonator operating in a high order TE mode is considered. Mode discrimination is achieved both by exploring the differences in the transverse structures of the competing modes and investigating a suitable geometry for the coaxial insert. For modes with close eigenfrequencies the associated diffractionQ factors can be widely different in value, thereby ensuring an effective mode selection. In the resonator studied here, the frequency separation between the design mode TE_26,10,1 and its nearest competing mode TE_20,12,1 is about 0.6% and the ratio of the correspondingQ factors is as high as 6.5. Unlike the coaxial resonator, in the hollow cavity without the inner conductor the fundamental spectrum of eigenfrequencies is more dense, and all TE modes within the frequency interval 271–288 GHz have approximately the sameQ factor.     

Mode switching in a gyrotron with azimuthally corrugated resonator

The operation of a gyrotron having a cylindrical resonator with an azimuthally corrugated wall is analyzed. In such a device, wall corrugation cancels the degeneracy of the modes with azimuthally standing patterns. The coupling between these modes depends on the radius of electron beam. It is shown that such a gyrotron can be easily switched from one mode to another. When the switching is done with the repetition frequency equal to the rotational frequency of magnetic islands, this sort of operation can be used for suppression of neoclassical tearing modes in large-scale tokamaks and stellarators.     

Prebunched high-harmonic gyrotron

A scheme for harmonic wave generation using a prebunched electron beam has been demonstrated. The prebunched electron beam has been used to further increase the efficiency of the authors' axis-encircling high-harmonic gyrotron. The proof-of-principle experiment was performed at the third harmonic with a TE₃₁₂ mode at 27.7 GHz. The conversion power of 6.7 kW was significantly greater than that used in the nonprebunched experiment. Also, mode competition was effectively suppressed. As expected, the unsaturated output power is proportional to the square of the electron beam current and the start of oscillation current is essentially zero. A linear theory, derived by taking into account the spread of the guiding center and the spread of the axial velocity, gives good agreement with the experimental results     

Submillimeter-wave large-orbit gyrotron

We develop a thermionic-emission electron-optical system forming a dense beam of electrons moving along helical trajectories round the axis of a gyrotron cavity. The maximum beam current is 4 A and the pitch-factor of electrons is 1.0 for a particle energy of 250 keV and a pulse duration of 10 µs. Using such a beam in a gyrotron operated at the third cyclotron harmonic, we obtain single-mode oscillation with a power of 10 µs in the TE_3,8 and TE_3,9 modes with frequencies 371 and 414 GHz, respectively.     

Photonic-band-gap structures and guide modes in two-dimensional magnetic photonic crystal heterostructures

We first investigate the band gap structures of two-dimensional magnetic photonic crystals (MPC) composed of rectangular (square) magnetic cylinders embedded in a host dielectric material in the rectangular (square) lattice, and we then study guide modes at interface of MPC heterostructures (MPCHs) by use of plane wave expansion method in combination with supercell technique. We find that both the mirror-symmetric MPCHs and the mixed-type MPCHs composed of square cylinders in a square lattice can produce the TM guide modes even without any lattice distortions. This feature is quite different from that of the non-magnetic PC heterostructures, in which the occurrence of the guide modes requires the relatively longitudinal gliding or transverse displacement of lattices. It may provide a new way to generate guide modes and apply to the device of light wave guides.Received: 26 August 2003, Published online: 2 April 2004PACS: 42.70.Qs Photonic bandgap materials - 78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures - 63.20.Pw Localized modes - 42.79.Gn Optical waveguides and couplers     

A multifrequency gyrotron

A theory of a gyrotron that generates at frequencies that are multiples of the cyclotron frequency is considered. In the steady-state regime, this radiation appears as a nonsinusoidal electromagnetic oscillation whose waveform depends on the amplitudes of its harmonics. The theory is developed for the weakly relativistic case and is based on known transverse momentum equations for electrons moving in an electromagnetic field. Under optimal conditions, the single-harmonic emission of a multifrequency gyrotron is more efficient than that of a single-frequency device.     

Explosive cathode gyrotron experiments

Experiments have investigated the behavior of explosive emission cathode gyrotrons using cylindrically symmetric coaxial diodes with centrally located knife-edged cathode and conical anode. Voltage probes and current shunts were exploited to monitor the diode electrical behavior when driven by a 3 μs 1/e time Marx power supply (up to 150 kV). Temporal reference signals permitted comparison of the diode's electrical evolution with the simultaneously measured microwave output signal from the gyrotron. The microwave output pulse duration was found to depend on the diode gap spacing (in the range 16-27 mm), the cavity magnetic field (range 1.3-3.3 T) and the cathode material, the termination being caused by the decay of the accelerating potential across the diode and the disruption of the diode geometry. The ratios of the cathode flare expansion velocities for graphite, copper and stainless steel cathodes were estimated as (1:(1.3±0.2):(2.2±0.5)) with a corresponding change in the duration of the microwave output signal [((590±17):(414±25):(325±10)) ns at 2.6 T]. Preliminary results are reported of recent experiments which have compared the optical emissions from the diode region during pulsed gyrotron operation with the simultaneously recorded diode electrical behavior and the gyrotron microwave output     

Submillimeter-wave harmonic gyrotron experiment

A theoretical and experimental investigation of the operation of a harmonic gyrotron at submillimeter wavelengths is reported. Using a waveguide cavity with an iris at the output end of the straight section, 14 different second-harmonic modes were observed with frequencies of 301-503 GHz, output powers of 1-22 kW, and a 12-MHz emission frequency bandwidth. The highest output power was 22 kW, with a total efficiency of 3.5% at 467 GHz, and an output power of 15 kW with a 6% efficiency was obtained at 417 GHz. Research was conducted using a 65-75 kV up to 10-A electron gun with a 1/1.5-μs pulse length and a 4-Hz repetition rate, which produced a helical electron beam in magnetic fields of up to 14 T. These results represent the first operation of a high-power harmonic gyrotron in the submillimeter region     

Axisymmetric quasi-optical gyrotron oscillator

An axisymmetric quasi-optical gyrotron (ASQUOTRON) is considered to realize a 10 MW, 150 GHz, CW oscillator required for an electron cyclotron resonance heating of a fusion plasma. The gyrotron has an axisymmetric mirror to be used as its optical cavity. It is shown that the axisymmetric mirror of relatively small radius (20 cm) can be used in producing the 10 MW continuous wave with a tolerable mirror heat load (0.5 kW/cm²). Considerations are also made on wave transmissions through the mirror and to a target.     

Double-beam gyrotron electron-optical systems

Geometry and practicability are considered for various forms of double-beam magnetron injector guns, which produce spiral beams for high-power gyrotrons. Path analyses are considered for double-beam electron-optical systems, which confirm that one can obtain acceptable parameters for both beams.Applied Physics Institute, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 2, pp. 205–210, February, 1991.     

Multifrequency operation of a gyrotron

One specific example with the parameters close to the existing coaxial cavity gyrotron at the Forschungszentrum Karlsruhe (FZK) is analyzed in detail. Mode competition calculations are performed using the time dependent and self-consistent codes developed both at the FZK and the Institute of Applied Physics (IAP). The results are compared with the calculations performed earlier by one of the authors in the cold cavity approximation. Significant qualitative differences are found and the conclusion is drawn that it is absolutely necessary to use the self-consistent approach in analyzing advanced operation regimes of a gyrotron     

Radiation structure of single-mode gyrotron

The stationary structure of the radiation field of a gyrotron with an axisymmetric resonator and electron beam is examined. The study is based on the inhomogeneous-string equations for the field and the equation of electron motion for the electron transverse momentum. It is shown that the gyrotron has evanescent modes and modes that travel through the resonator.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 1, pp. 88–96, January, 1993.     

Third-harmonic complex cavity gyrotron self-consistent nonlinearanalysis

Starting from general transmission line equations with an electron beam source, a self-consistent nonlinear theoretical model for a complex cavity gyrotron with gradual transitions is presented in this paper. The model accounts for mode coupling in the transition region of the complex cavity. The interaction between the electron beam and H₆₁-H ₆₂ RF field in the complex cavity for a third-harmonic gyrotron is simulated; many calculations are carried out under different cavity dimensions and electron beam parameters     

Determination of gyrotron wave beam parameters

The output radiation of a gyrotron has the form close to the Gaussian beam. Knowing parameters of the wave beam along its way from a gyrotron to an object permits one to optimize the transmission losses. By this, the structure of the wave beam can be recorded with an infrared camera in several cross-sections of the beam. Real gyrotron wave beam is approximated with high precision by the Gaussian beam.     

Dielectric materials for gyrotron output windows

The dielectric parameters of materials for gyrotron output windows measured in the range 65–300 GHz are given. The special attention is paid to the spread of values of the refractive index and loss tangent of dielectrics produced by different firms. The refractive index and loss tangent are measured for sapphire at the cryogenic temperatures and for boron nitride and silica glass at the temperatures 300–1000K.     

206 GHz光子带隙谐振腔回旋管

分析了光子晶体谐振腔的模式选择功能,实现光子晶体谐振腔回旋管振荡器高阶电磁模与高次电子回旋模的有效耦合,并成功抑制了模式竞争。通过对光子晶体谐振腔禁带特性的分析,定出了工作模式为TE23模,还建立了光子晶体谐振腔回旋管的等效半径的概念,设计了自洽非线性理论和相关的计算机数值模拟程序。研究发现TE23模能有效地与电子的二次回旋谐波相互作用,其耦合频率为206 GHz,并极大地降低了对工作磁场的要求。在考虑诸多物理因素影响的情况下,对该二次谐波光子带隙谐振腔(PBGC)回旋管振荡器进行了参数优化,得到了电压40 kV、电流4.2 A、磁场3.925 T、输出功率35 kW、互作用效率21%的二次谐波TE23模PBGC回旋管振荡器。     

A 0.33-THz second-harmonic frequency-tunable gyrotron

Dynamics of the axial mode transition process in a 0.33-THz second-harmonic gyrotron is investigated to reveal the physical mechanism of realizing broadband frequency tuning in an open cavity circuit. A new interaction mechanism about propagating waves, featured by wave competition and wave cooperation, is presented and provides a new insight into the beam-wave interaction. The two different features revealed in the two different operation regions of low-order axial modes(LOAMs) and high-order axial modes(HOAMs) respectively determine the characteristic of the overall performance of the device essentially. The device performance is obtained by the simulation based on the time-domain nonlinear theory and shows that using a 12-kV/150_(-mA) electron beam and TE_(-3,4) mode, the second harmonic gyrotron can generate terahertz radiations with frequency-tuning ranges of about 0.85 GHz and 0.60 GHz via magnetic field and beam voltage tuning,respectively. Additionally, some non-stationary phenomena in the mode startup process are also analyzed. The investigation in this paper presents guidance for future developing high-performance frequency-tunable gyrotrons toward terahertz applications.     

Operation of a 32 GHz gyrotron

The design and operation of a 32 GHz pulsed gyrotron are reported. The device is step-tuned between the TE_1,2 (24.16 GHz) and TE_0,2 (31.78 GHz) modes with cathode voltages ranging from 30 to 40kV and beam current up to 5.0A. Experimental frequencies are in close agreement with the self-consistent calculated values and in the TE_2,2 resonator mode an output peak power of 6kW corresponding to an 18% efficiency was measured by using a fast response calorimeter with a thermal sensitivity of 0.1°C/Wmin.