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.     

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.     

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     

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.     

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     

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.     

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     

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     

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.     

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.     

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     

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.     

Chaos and hyperchaos in a gyrotron

We study oscillation in a gyrotron with allowance for reflections from an output horn. Regions with different system behaviors, such as stationary oscillation, self-modulation, and complex-dynamics regimes are found in the parameter plane. The scenarios of appearance of chaotic oscillations are considered. It is shown that they can emerge via either a sequence of period-doubling bifurcations or destruction of quasiperiodic motion. For chaotic attractors, Lyapunov exponents are calculated and their dimensions are estimated on the basis of the Kaplan-Yorke formula. The dimension values turn out to be anomalously large, which is stipulated by the presence of a large number of high-Q eigenmodes in the gyrotron cavity due to operation near the cutoff frequency of an electrodynamic system. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 10, pp. 887–899, October 2006.     

Electron-optical system for a large-orbit gyrotron

An electron-optical system generating a rectilinear or helical 250 keV/4 A/10 μs electron beam with a high compression factor is developed. For the former beam, a compression factor as high as 4400 and a current density of 25 kA/cm₂ are achieved. In the process of forming the helical beam, the electrons rotating about the system’s axis (paraxial beam) acquire an initial velocity in a transverse magnetic field produced by a kicker. Their pitch factor is increased to a desired (operating) value in an adiabatically growing magnetic field. In tentative experiments with the helical beam in a large-orbit gyrotron, generation was obtained at the second cyclotron harmonic (223 GHz).     

Technological gyrotron with low accelerating voltage

We consider the design of a gyrotron with an operating frequency of 24 GHz and an accelerating voltage of about 5 kV and describe a nonadiabatic electron gun forming an electron beam with a pitch factor of 1.8 and a velocity spread of about 10%. Longitudinal distribution of high-frequency field in a cavity with a step is optimized by choosing the step height and length. The design value of the gyrotron output efficiency is 35% at the second gyrofrequency harmonic.     

Window materials for high power gyrotron

The room temperature application of sapphire as window material at higher frequencies is not feasible since its absorption coefficient increases almost linearly with increasing frequency in the millimeter wavelength region. At cryogenic temperature the absorption coefficient value decreases only by a few factors (factor of 2 to 3) in the 90 – 200 GHz region. The earlier reported temperature squared dependence (decrease) in the absorption coefficient or the loss tangent value is totally absent in our broad band continuous wave data we are reporting here (at 6.5 K, 35K, 77K and 300K) and one we reported at conferences earlier. Our results are verified by another technique. We utilize our precision millimeter wave dispersive Fourier transform spectroscopic techniques at room temperature and at cryogenic temperatures The extra high resistivity single crystal compensated silicon is no doubt the lowest loss material available at room temperature in the entire millimeter wavelength region At higher millimeter wave frequencies an extra high resistivity silicon window or an window made with extra high resistivity silicon coated with diamond film would certainly make a better candidate in the future. A single free standing synthetic diamond window seems to have higher absorption coefficient values at millimeter wavelength region at this time although it is claimed that it possesses good mechanical strength and higher thermal conductivity characteristics. It certainly does not rule out the use of diamond film on a single crystal high resistivity silicon to improve its mechanical strength and thermal conductivityThis research program was supported by the U.S. Department of Energy, Office of Fusion Research. Authors acknowledge the loan of a synthetic diamond specimen from Dr. Kevin Gray of Norton Diamond Company     

Axisymmetric optical system for quasi-optical gyrotron

An analysis is carried out with a profile of hollow Gaussian radiation beam propagating in an optical system composed of coaxial annular curved mirrors. The result will be a useful tool in designing an axisymmetric quasi-optical gyrotron oscillator (ASQUOTRON).