Cavity Gyrotron Research Articles

A novel 4.5-MW electron gun for a coaxial cavity gyrotron

A novel 4.5-MW (90 kV, 50 A) electron gun for a 165-GHz coaxial cavity gyrotron operated in the transverse electric mode TE/sub 31, 3.7/ has been designed, fabricated, and operated in a gyrotron. The electrons are extracted toward the anode as in a conventional magnetron injection gun (MIG) and not toward the coaxial insert as in the previously used so-called inverse gun. The main advantage of this arrangement is the reduced overall radial size which becomes comparable to the size of a conventional gun with the same diameter of the emitter. The design and the technology of the electron gun fulfil the requirements for a high-power gyrotron operated at long pulses up to continuous wave. The coaxial insert is fully cooled and can be adjusted when the tube is completely assembled. The amplitude of mechanical vibrations has been measured to be less than 0.03 mm under operating conditions. This is sufficiently small for stable long-pulse operation. In operation at short pulses (ms) a microwave RF output power of 2.2 MW at 165 GHz has been obtained with a beam current of 84 A. The best operating conditions have been observed with an intermediate type of electron flow.

Frequency step-tunable (114–170 GHz) megawatt gyrotrons for plasma physics applications

Successful gyrotron experiments at FZK employing a conventional hollow cylindrical waveguide cavity, a quasi-optical mode converter with dimple-type launcher, a broadband silicon nitride Brewster window and a single-stage depressed collector (SDC), gave up to 1.6 MW output power at efficiencies between 48 and 60% for all operating mode series in the frequency range from 114 to 166 GHz. Frequency tuning in 3.7 GHz steps has been achieved by slow variation (minutes) of the magnetic field strength in the cavity. A specific hybrid magnet system for fast frequency tuning (1 s) is being manufactured. These experiments confirm the preliminary results achieved with a prototype fused quartz glass Brewster window. Up to now, the pulse duration has been 1–5 ms. However a water-edge-cooled silicon nitride composite Brewster window with its thermal conductivity of k=60 W/mK, permittivity ε r=7.85 ( θ Brewster=70.35°), tan δ=3.5×10 −4, excellent mechanical properties and clear window aperture of 100 mm diameter, will allow long-pulse operation (1 s) in the 1 MW power range. By increasing the beam current to 70 A (70% of the limiting current!), a maximum power of 2.14 MW, the highest value ever generated by a weakly relativistic gyrotron, has been obtained at 140 GHz with an efficiency of 34% (53% with SDC). With a TE 31,17 coaxial cavity gyrotron, also equipped with a quasi-optical mode converter and a SDC, a maximum RF-output power of 1.7 MW with an efficiency of 26.2% has been achieved at a beam current I b=69.7 A and a cathode voltage U c=93.2 kV. Around the nominal beam parameters ( I b=52.2 A, U c=91.8 kV, B cav=6.65 T) an RF-output power of 1.3 MW, with an efficiency of 27.2% (41% with SDC), has been measured. In experiments on frequency step tuning, also using the silicon nitride composite Brewster angle window, 19 different modes with ≈2.2 GHz frequency spacing have been excited with ≥1 MW power at frequencies in the range between 134 and 169.5 GHz.

Properties of a Second Harmonic Gyrotron Cavity with Internal Blazed Grating

Spectral domain analysis, scattering matrix analysis and PIC simulations are used here to design a second harmonic gyrotron cavity with an internal blazed grating which favors operation at the second harmonic of the electron cyclotron frequency rather than operation at the electron cyclotron frequency itself. Based on these simulations, the cavity design is optimized. The results show that a new gyrotron with higher power and higher frequency can be achieved by incorporating such a blazed grating.

Experimental investigation of a 140-GHz coaxial gyrotron oscillator

We report experimental results on a megawatt power level, 140-GHz coaxial gyrotron oscillator. The gyrotron has an inverted magnetron injection gun (IMIG) designed for operation at up to 95 kV and 88 A. The IMIG has an inner grounded anode which extends from the center of the gun down through the entire length of the tube including the cavity and collector. The IMIG was tested at up to 105 kV and 93 A in 3 /spl mu/s pulses, achieving an electron beam power of 10 MW. The output power from the coaxial gyrotron cavity was transported to an internal mode converter and a single mirror that coupled the power out transversely from the tube axis. A maximum output power of up to 1 MW was obtained in the TE/sub 27,11/ mode at 142 GHz at an efficiency of 16%, about one half of the design efficiency. The reduced efficiency was attributed to nonuniformity of the cathode emission and the sensitivity to the relative alignment of the electron gun, coaxial insert, and cavity. The cathode emission over the azimuthal angle was measured for two cathodes and was shown to be nonuniform due to both temperature and emitter work function nonuniformity. The gyrotron was also tested in two alternate configurations: 1) with the internal mode converter removed (axial output), and 2) with both the internal converter and the coaxial insert removed (empty cavity). In operation in the empty cavity configuration, which is equivalent to a conventional gyrotron oscillator, output power of up to 0.9 MW was observed.

Cold test measurements on components of the 1 MW, 140 GHz, CW gyrotron for the stellarator Wendelstein 7-X

For the development of a 1 MW, 140 GHz gyrotron for CW operation which will be installed at the stellarator facility Wendelstein 7-X at IPP Greifswald, a collaboration between different European research institutes and an industrial company has been established. In order to prove the proper functioning of the millimeter wave components installed in the gyrotron — such as the cavity, the waveguide taper and the quasioptical mode converter — these components should be cold tested, preferably before installation. However, due to lack of time as well as long delivery times, this was not possible. Therefore, two units of the quasioptical mode converter and the cavity were fabricated with identical geometry, one of those being used for measurements on the low power test device. To perform these cold tests for tapers and mode converters, the gyrotron cavity output mode has to be simulated. This means that a high order rotating mode (TE 28,8 mode) must be generated at low power. This can be achieved by means of a mode generator consisting of two mirrors and a coaxial cavity with a perforated outer wall. Before applying the mode generator to the components, its proper behavior and the accurate alignment of the system must be verified either by radiation pattern measurements or k-spectrometer measurements. As the coupling through the holes of the k-spectrometer is extremely low, a special vector network analyzer with a dynamic range of at least 100 dB had to be developed. This has been achieved by integration of a phase locked backward-wave oscillator with a line width of 100 Hz and an output power of 10 mW. A non-destructive measurement of the resonance frequency and the quality factor of the cavity does not seem possible. The second cavity will be prepared for the cold measurement by drilling a small radial hole into its wall in the plane of the field maximum. This hole is then used for the input coupling. The accuracy required for this hole is rather critical. The coupling coefficient must be high for sufficient excitation of the rf field, but on the other hand it must neither change the frequency nor the quality factor strongly. The transmission is measured by a probe at the output of the uptaper.

Possible operation of a 1.5-2-MW, CW conventional cavity gyrotron at 140 GHz

We present a study of the feasibility of continuous wave (CW) operation of a 140-GHz conventional cavity gyrotron at high power levels operating in the TE/sub 31,8/- and TE/sub 25,10/-modes. The question of mode selection is discussed, and a possible design of such a gyrotron with beam energy up to 90 keV and a current of 60-70 A is given. We find that it should be possible to operate a 140-GHz gyrotron at power up to 2 MW if a sufficiently high-order mode is used, although CW power may be somewhat lower.

Step-frequency operation of a coaxial cavity gyrotron from 134 to 169.5 GHz

Experiments on frequency step tuning have been performed in a coaxial cavity gyrotron equipped with a Brewster window. In single-mode operation, 19 different modes have been excited with high power In a frequency range between 134-169.5 GHz. In all modes, the radio frequency (RF) power was efficiently transmitted through the output window with only slightly varying RF power distribution. The step-frequency tuning is restricted by the coaxial insert. In particular, the strong increase of the quality factor for small values of the parameter w, which describes the corrugation impedance, limits the maximum frequency range. Furthermore, the increase of ohmic losses at the insert for modes with reduced caustic radius may restrict the selection of usable modes.

A new approach for a multistage depressed collector for gyrotrons

A feasibility study for a two-stage depressed gyrotron collector has been performed. A new approach for an adiabatic magnetic decompression of the hollow electron beam has been used. It permits control of the radius of the constant magnetic flux surface, which determines the radial extension of the electron beam. Independent of the value of the magnetic field around the beam. For this purpose, either solenoidal coils or a ferromagnetic insert can be placed inside the hollow electron beam. Thus, the radial dimensions of a multistage depressed collector of a high-power high-frequency gyrotron can be kept within limits given by technological constraints. The energy sorting of the electron beam is improved by using electrodes inside the hollow electron beam for controlling the potential distribution. The additional control electrodes make it possible to eliminate almost all of the effect of secondary electrons on the operation of the collector. In order to demonstrate the proposed approach, a compact two-stage depressed collector has been designed for a 1.5-MW coaxial cavity gyrotron operating at 165 GHz in the transverse electric (TE)/sub 31,17/ mode, which is under development at FZK, Karlsruhe, Germany. Including the effect due to secondary electrons, a collector efficiency of 73% has been calculated with an average and peak heat dissipation density of about 240 W/cm/sup 2/ and 500 W/cm/sup 2/, respectively. This results in an increase of the output gyrotron efficiency from 36.5% to 62.6% when internal radio frequency (RF)-losses inside the gyrotron tube of 15% are taken into account.

Design of rapid-frequency step-tunable powerful coaxial-cavity harmonic gyrotrons

A design of a new powerful high-frequency short-pulse coaxial-cavity harmonic gyrotron operating at first, second, and third harmonic is presented. The design includes the analysis of mode competition between the fundamental and higher harmonic modes, as well as the investigation of the possibility of the corresponding fast frequency tuning of the gyrotron by changing the accelerating voltage. The possibilities of modification of the existing coaxial cavity gyrotron operating at present at the Forschungszentrum Karlsruhe (FZK) for operation at the second harmonic are discussed. Potentialities of such gyrotrons for plasma physics applications are noted.

A NONLINEAR SIMULATION ON BEAM-WAVE INTERACTION FOR HIGH-HARMONIC COMPLEX CAVITY GYROTRON WITH RADUAL TRANSITION

Starting from the general transmission line equation with an electron beam, a se lf-consistent nonlinear analysis on complex cavity gyrotron with gradual transit ion is presented, in which the multiple modes interacting with an electron beam and the mode coupling are taken into account. The interaction between the electr on beam and H51-H52 RF field for third-harmonic gyrotron i s simulated numerically. The influences of multiple modes on the interaction are analyzed.

Eigenvalue equations and numerical analysis of a coaxial cavity with misaligned inner rod

Based on the Helmholtz equation, the superposition of cylindrical wave functions, and coordinates transformation, the eigenvalue equation is derived rigorously for a coaxial gyrotron cavity with a misaligned inner rod. It is shown that, due to the existence of the structural misalignment, any single normal mode of a perfect coaxial structure (i.e., without misalignment) no longer simultaneously satisfies both the outer and inner boundary conditions; consequently, the superposition of cylindrical wave functions must be taken into account. A numerical approach of solving the eigenvalue equation is proposed in this paper. As a practical application, analysis is given to the higher mode coaxial cavity employed in a 140-GHz/1.5-MW gyrotron device at the Forschungszentrum Karlsruhe, Karlsruhe, Germany. Result shows that the eigenvalue of the operating mode in a misaligned coaxial cavity is affected noticeably by the structural misalignment.

Gyrokinetic description of the structural eccentricity influence on the starting current of a coaxial-cavity gyrotron

Based on the gyrokinetics of free-electron masers, the structural eccentricity influence on the starting current of a 1.5 MW/140 GHz, TE28,16,1 coaxial-cavity gyrotron oscillator is studied. Results show that the structural eccentricity substantially increases the starting current. The stable term of the instability, which results from the contribution of the guiding-center distribution, is of significance in a coaxial-cavity gyrotron, although it has usually been neglected in a cylindrical-cavity gyrotron. It is found that the influence of the structural eccentricity on the performance of the coaxial cavity gyrotron is more serious than that of the beam misalignment. The influences of the operating magnetic field and the beam energy on the starting current are also taken into account.

Study of a 35-GHz third-harmonic low-voltage complex cavity gyrotron

A self consistent nonlinear theoretical model for a complex cavity gyrotron with abrupt transitions is presented in this paper. The model accounts for mode conversion in the transition region of the complex cavity through the general theory of modal expansion techniques. The interaction between the electron beam and TE/sub 61//TE/sub 62/ RF field in the step cavity for a third-harmonic gyrotron is simulated; many calculations are carried out under different electron beam parameters.

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.

165 GHz, 1.5 MW-coaxial cavity gyrotron with depressed collector

A further step in the development of a coaxial-cavity gyrotron operated in the transverse electric TE/sub -31,17/ mode at 165 GHz is presented. The gyrotron has been equipped with a quasi-optical output system consisting of a Vlasov launcher with a single cut and two mirrors, one with a quasi-elliptic and the other with a nonquadratic phase correcting surface. The radio frequency (RF) power is transmitted through a single output window. A maximum output power of 1.7 MW has been achieved. At the nominal operational parameters an RF power of 1.3 MW with an efficiency of 27.3% has been measured. The efficiency increases to 41% in operation with a single-stage depressed collector.

Rigorous computation of time-dependent electromagnetic fields in gyrotron cavities excited by internal sources

Computation in frequency, as well as in time domain of the electromagnetic field in aperture-coupled cavities which are excited by electron beams, requires an accurate representation of the field. Furthermore, a fast tool for simulation of beam-field interaction in electron tubes is desirable. Application of the modal expansion method, which utilizes both the solenoidal and the irrotational eigenfunctions of the equivalent short-circuited cavity, is generally rigorous but numerically inefficient. In this contribution, three main steps towards a more accurate and simultaneously more efficient analysis are presented. First, it is shown how the irrotational magnetic eigenfunctions can be eliminated from the analysis. Furthermore, some poorly convergent series in the frequency domain analysis as well as in the time-domain analysis are replaced by analytic expressions. Finally, the modal analysis is directly formulated in time domain using rigorous boundary conditions. Numerical results are presented for idealized structures with impressed current density and for self-consistent calculations which are compared to analytical or to numerical results, respectively. Thus, excellent accuracy of the developed method is proved and significant simplifications are justified. For weakly inhomogeneous cavities, the influence of mode conversion on field profile and on numerical aspects is also discussed.

Gyrotron internal mode converter reflector shaping from measured field intensity

We present the formulation and experimental results of a new approach to designing internal mode converter reflectors for high-power gyrotrons. The method employs a numerical phase retrieval algorithm that reconstructs the field in the mode converter from intensity measurements, thus accounting for the true field structure in shaping the beam-forming reflectors. An outline for designing a four-reflector mode converter is presented and generalized to the case of an offset-fed shaped reflector antenna. The requisite phase retrieval and reflector shaping algorithms are also developed without reference to specific mode converter geometry. The design approach is applied to a 110 GHz internal mode converter that transforms the TE/sub 22,6/ gyrotron cavity mode into a Gaussian beam at the gyrotron window. Cold test experiment results of the mode converter show that a Gaussian beam with the desired amplitude and phase is formed at the window aperture. Subsequent high-power tests in a 1 MW gyrotron confirm the Gaussian beam observed in cold tests. The general development of the approach and its validation in a quasi-optical mode converter indicate that it is also applicable to other quasi-optical, microwave applications such as radio astronomy, free-space transmission lines, and mitre bends for overmoded waveguides.

Numerical simulation of processes in a 170 GHZ/1 MW CW gyrotron cavity with operating mode TE 25.10 for iter

We consider a gyrotron for the ECR plasma heating complex of ITER. Gyrotron cavity parameters are optimized with allowance for ohmic losses, reradiation of waves due to profile inhomogeneities, potential depression, and velocity spread of the electron beam. We study the effect of ion compensation for the space charge, the onset of oscillations after switching on the gyrotron, and the competition between operating and spurious modes. The possibility of obtaining 35–39% efficiency without electron-beam energy recovery and 55–60% efficiency with a one-stage depressed-potential collector is shown for cavity RF ohmic loss power less than 2 kW/cm2.

Gyrokinetics of Coaxial-Cavity Electron Cyclotron Maser

Based on the gyrokinetics of free-electron masers, the energy-transfer rate and starting current are derived for a coaxial-cavity electron cyclotron maser (gyrotron oscillator) with the misalignment of the inner-rod axis to the outer-cavity axis taken into account. The effect of the finite cavity length is included in the derivation. The stable term of the instability which results from the contribution of the guiding-center distribution is of significance, although it is usually neglected in a cylindrical-cavity gyrotron. It is found that the structure misalignment has more serious influence on the performance of the coaxial cavity gyrotron than the beam misalignment.

Expansion of a frequency tuning band in a gyrotron with coupled cavities

An electron frequency tuning band of a conventional high-Q cavity gyrotron approximately coincides with the unloaded cavity bandpass which usually does not exceed 0.1%. To expand the frequency tuning band we introduce a gyrotron with an RF circuit that consists of two coupled circular cavities separated by a thin iris. A nonlinear self-consistent gyrotron theory predicts a saturated efficiency of 18% and a tuning band of 1.1%. An experimental study of an X-band tube operating in the TE011 mode was carried out to test the gyrotron with the coupled cavities concept. Measurements of gyrotron performance showed a 0.96% tuning band—a tenfold increase over that of an ordinary gyrotron. Simulation results are in a good agreement with the experimental data.