Magnetron Injection Gun Research Articles

Analysis and Numerical Calculations of the Beam-Wave Interaction for Gyroklystron Amplifiers

A four-cavity gyroklystron was designed and optimized after analysis and calculation of RF system and magnetron injection gun, numerical simulations showed that the TE011 mode gyroklystron achieved 280kW peak output power, 38% efficiency, 35dB saturated gain with 250Mhz bandwidth centered at 34GHz for a 68 kV, 11A electron beam. The numerical simulation results were used to build a Ka band high power gyroklystron amplifier. In this paper, analysis and numerical calculation results of the beam-wave interaction are presented. The influences of electron beam, RF system parameters, magnetic field, and input RF signal on output power, efficiency, bandwidth and gain are discussed.

Frequency-Tunable CW Gyro-BWO With a Helically Rippled Operating Waveguide

Operation of a continuous wave gyrotron backward-wave oscillator (gyro-BWO) with a helically rippled operating waveguide has been experimentally studied. The gyro-BWO exploits a dc oil-cooled magnet with magnetic field up to 0.5 T and utilizes a weakly relativistic (20 keV) electron beam produced by a magnetron injection gun. Stable generation at the second cyclotron harmonic with a maximum power of 7 kW and an efficiency of 15% at a frequency of 24.7 GHz was achieved. Smooth oscillation frequency tuning by varying the magnetic field was measured to be as wide as 5% at the half-power level. The first gyro-BWO operation with a single-stage energy recovery system was realized. The use of a depressed collector provided an efficiency increase up to 23% and an opportunity for reduction of the main power supply voltage down to 10 kV.

Space-Charge Limited Magnetron Injection Guns for High-Power Gyrotrons

In this paper, we present the results of a space-charge-limited (SCL) magnetron injection gun (MIG) design, which is intended for use with a 500 kV, 500 A gyroklystron. The design performance is compared with that of a temperature-limited (TL) gun that was constructed for the same application. Both designs yield similar values for beam quality, namely, an axial velocity spread under 7% for an average perpendicular-to-parallel velocity ratio of 1.5. The peak electric field is 7% higher for the SCL design and the average cathode loading, at 7.6 A/cm/sup 2/, is about 27% higher than for the TL design. A disadvantage of the TL design is that realistic variations in emitter temperature or work function translate into azimuthal current variations that hinder the operation of the device. The SCL design should have a quite uniform azimuthal distribution of cathode loading and, therefore, should be a better electron beam source for many high-power gyrotrons.

Studies of the 1.5-MW 110-GHz Gyrotron Experiment

Results of a 1.5-MW 110-GHz short-pulse (3 /spl mu/s) gyrotron experiment are reported. The gyrotron magnetron injection gun operated at full voltage (96 kV) and current (40 A), producing up to 1.4 MW at 110 GHz in the TE/sub 22,6/ mode. The operation of the TE/sub 22,6/ mode, as well as nearby modes, was measured as a function of magnetic field at the cavity and at the electron gun to produce a mode map. Significant mode competition was found, but the measured efficiency of 37% in the TE/sub 22,6/ mode, without a depressed collector, is close to the design value of 39%. The beam alpha, the ratio of transverse to axial velocity in the electron beam, was measured with a probe. The alpha value was found to be 1.33 when the gyrotron was operating at conditions for achieving the highest output power level (1.4 MW.) This value of alpha is less than the design value of 1.4, possibly accounting for the slightly reduced experimental efficiency. The output power and efficiency, as a function of magnetic field, beam voltage, and beam current, are in good agreement with nonlinear theory and simulations with the MAGY code. These results are promising for the development of an industrial version of this gyrotron capable of long pulse or continuous-wave operation.

Formation and Diagnostics of Intense Relativistic Helical Electron Beams for Gyrotrons

We develop a magnetron-injection gun for a 1-cm wavelength gyrotron, which allows us to increase the operating current up to 50% of the Langmuir current of the gun by reducing the time required to bring electrons to the high-potential region. To measure the velocity distributions in helical electron beams for energies of several hundreds of keV, we propose, develop, and study a new electron-velocity distribution analyzer based on the phenomenon of electron reflection from an adiabatic magnetic mirror. The results of our study demonstrate that the gun is capable of forming high-quality intense beams suitable for generating microsecond microwave radiation with power higher than 10 MW in gyrotrons.

A 35-ghz low-voltage third-harmonic gyrotron with a permanent magnet system

A systematic theoretical and experimental study on a 35-GHz 45-kV third-harmonic gyrotron with a permanent magnet system is presented in this paper. A complex cavity with gradual transition and a diode magnetron injection gun (MIG) are employed in the gyrotron. A self-consistent field nonlinear theoretical investigation and numerical simulation for electron beam interaction with RF fields are given. The diode MIG is simulated numerically utilizing our code in detail. The permanent magnet system provided the maximum axial magnetic field of about 4.5 kG in the cavity region of the gyrotron. The Ka band third-harmonic complex cavity gyrotron with a permanent magnet system has been designed, constructed, and tested. A pulse output power of 147.3 kW was obtained at a beam voltage of 45 kV with beam current of 32.2 A, corresponding to an efficiency of 10.2%.

Design and emission uniformity studies of a 1.5-MW gyrotron electron gun

We present the design and initial operation of a 96-kV 40-A magnetron injection gun for a 1.5-MW 110-GHz gyrotron. A critical parameter for the successful application of this electron gun is the uniformity of electron emission. The current-voltage curve of emission, at a series of temperatures, is measured. Analysis indicates that the work function of the emitter is 1.6 eV with a (total) spread of 0.07 /spl plusmn/ 0.01 eV. Measurement of the azimuthal emission uniformity with a rotating probe indicates that the work function variation around the azimuth, the global spread, is 0.04 /spl plusmn/ 0.02 eV. The spread due to local (microscopic scale) work function variations is then calculated to be 0.06 /spl plusmn/ 0.02 eV. Temperature variation can be ruled out as the cause of the observed emission nonuniformity.

Cusp gun TE/sub 21/ second-harmonic ka-band gyro-twt amplifier

A second-harmonic TE/sub 21/ gyrotron traveling-wave tube (gyro-TWT) amplifier with an axis-encircling electron beam is being constructed at the University of California-Davis that is predicted to double the efficiency of our previous 200-kW 12%-efficient magnetron injection gun (MIG) TE/sub 21//sup (2)/ gyro-TWT. Both devices employ a sliced mode-selective circuit to suppress the odd-order azimuthal modes. The new device will avoid the loss in efficiency that is due to off-axis electrons interacting with the resulting linearly polarized mode. The amplifier is predicted by our large signal code to produce 50 kW in Ka-band with 20% efficiency, 30 dB saturated gain, and 3% bandwidth. The 70-kV 3.5-A axis-encircling electron beam with /spl upsi//sub /spl perp////spl upsi//sub z/ = 1.2 and /spl Delta//spl upsi//sub z///spl upsi//sub z/ = 7% will be produced by a Cusp electron gun delivered by Northrop Grumman. The amplifier has been designed using linear theory to provide a 30% safety margin from absolute instability. Distributed loss is added to the first 30.5 cm of the circuit to ensure the device is stable to harmonic gyro-BWO. The last 11.5 cm of the circuit is standard copper to avoid attenuating the wave at high power levels.

On the frequency scaling of coaxial gyroklystrons

In this paper, we present scaling arguments for coaxial gyroklystron systems in order to assess their ultimate peak power capabilities as a function of output frequency. After reviewing the tradeoff equations for magnetron injection guns as they relate to frequency scaling, we present an analysis of the scaling properties and limitations of coaxial gyroklystron circuits. We then present concrete designs of both electron guns and microwave circuits to demonstrate the validity of the scaling arguments. The examples are derived from high-energy physics applications, where amplifiers with peak powers in excess of 100 MW are envisioned to be required in the millimeter wave range of frequencies. Concrete designs of coaxial systems predicted to produce about 100 MW in Ku-Band, Ka-Band, and W-Band are presented and compared. We demonstrate that there are a number of potential limitations to high-frequency operation.

Design of a 10-MW, 91.4-GHz frequency-doubling gyroklystron for advanced accelerator applications

A 10-MW, 91.4-GHz gyroklystron is under development for W-Band accelerator applications. The device is expected to generate 1-/spl mu/s pulses at up to 120 Hz, with efficiencies near 38%, with a gain above 57 dB. A double-anode, magnetron injection gun operating at 500 kV will provide a 55-Ampere beam for interaction with a six-cavity circuit in a magnetic field of about 27.9 kG. The beam will have an average perpendicular-to-parallel velocity ratio of approximately 1.6 with a parallel velocity spread below 3.3%. The first two cavities will operate in the TE/sub 011/ mode at the drive frequency near 45.7 GHz, while the remaining cavities will operate in the TE/sub 021/ mode at twice the drive frequency. The output cavity will have smooth radial wall transitions to minimize mode conversion and pulse heating, the latter of which is shown via thermal modeling to be within acceptable limits. The output signal will be in a TE/sub 01//TE/sub 02/ mode mixture for easier power handling and to facilitate possible depressed collector operation, which could result in wall plug efficiencies in excess of 50%. A complete description of the system design and modeling is presented.

Design of a large orbit gyrotron with a permanent magnet system

In this paper, we present results of a conceptual design study of a prospective gyro-device with a permanent magnet system and an axis-encircling electron beam. Computer-aided electron-optical design of the tube has been performed by the new version of the software package GUN-MIG named GUN-MIG/CUSP. It is based on a self-consistent relativistic physical model and is developed as a problem-oriented tool for analysis of electron-optical systems with conventional magnetron injection guns (MIG) and electron guns with field reversal (cusp guns), forming axis-encircling beams. Results of numerical experiments predict satisfactory performance of the gun which is expected to form high-quality axis-encircling electron beams with small ripple and dispersion of the velocities. Parameters of the beams (current, radius, velocity ratio, etc.) are appropriate for weakly relativistic harmonic large orbit gyrotron (LOG). The cavity was optimized for excitation of TE 41 mode at the fourth harmonic of the cyclotron frequency. The results of this feasibility study will be used as a basis for development of a novel LOG with a permanent magnet system. The target parameters of such a prospective device are frequency of generated microwave radiation about 104 GHz, output power near 1 kW and efficiency of several percent.

Analytic formulas for current density profiles in small-orbit spiralling electron beams

Analytic expressions for the current density as a function of radius in small-orbit, spiralling electron beams, including guiding center spread, are derived using probability theory. The expressions are valid for beams with a small spread in Larmor radius compared to the guiding center spread and a space charge depression below 10%, which are typical for beams produced by modern magnetron injection guns. The current density profile is shown to exhibit a number of functional forms, depending on the relative size of the spread in guiding center radii compared to the average Larmor radius. Several applications for the new expressions are explored, including the calculation of the thermal loading experienced by beam-intercepting metal objects, as well as the computation of average electron density per unit volume (used in predicting the growth of electrostatic cyclotron instabilities). The analytic results are extended, in the vicinity of the beam edges, to include the effects of Larmor radius spread. The edge expressions are employed to predict the amount of current intercepted by a tunnel that closely surrounds the beam, which can be useful in selecting radial clearances.

Generation of high-power electron beams by magnetron injection guns with secondary-emission cathodes

The generation of high-power electron beams by magnetron injection guns using a secondary-emission cold cathode is studied experimentally. Stable operation at an output power of up to 8 MW is achieved.

Beam Characteristics of Mechanically Tunable Magnetron Injection Guns

Gyrotron has received extensive attention owing to its high-power capability, particularly when the wavelength shrinks below the millimeter-wave range. The electron beam of a gyrotron is typically generated by a magnetron injection gun (MIG), and a mechanically tunable MIG is employed to enhance the beam quality. However, a slight adjustment in the position of the center electrode of the mechanically tunable MIG can cause significant modification of the electric-field profile. Consequently, the cathode current of the mechanically tunable MIG is not only dependent on the cathode temperature, but also the relative position of center electrode. To accurately determine the beam characteristics of the mechanically tunable MIG, an improved computer program is employed to simulate electron trajectories. Simulation and measurement results indicate that although the magnetic field and compression ratio do not influence the beam current of the mechanically tunable MIG, but the beam current of the MIG depends on the cathode temperature and relative position of the center electrode. Finally, an improved mechanically tunable MIG design is developed to enhanced an beam quality while minimizing beam current variation.

The Hobetron-a high power vacuum electronic switch

This paper describes a new high-power, hollow electron beam, current-regulating switch tube that we call the Hobetron. Experimental data on the Litton L-6026 Hobetron, an engineering prototype designed to stand off 125 kV dc and switch 200 amps, has demonstrated that Hobetron performance is in excellent agreement with computer predictions over the range of test. Further, the data confirmed that L-6026 switch power loss is less than 20% that of standard magnetron injection gun (MIG) switch tubes (for similar currents through each). Various potential Hobetron applications are presented throughout the paper. It is possible to include an additional electrode in the basic Hobetron configuration for even higher voltage standoff and switch current capability, and the paper presents an arrangement capable of standing off 500 kV dc and switching 500 amps. Simulations of a superpower Hobetron-Plus are presented, and the potential of this device as an efficient, 50 MW peak, relatively compact, VHF power amplifier for advanced heavy-particle (e.g., muon) colliders, is evaluated.

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.

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.

Design of a Ka-band gyro-TWT for radar applications

The design of a Ka-band gyrotron traveling wave tube (TWT) amplifier capable of operating over a wide range of peak powers and bandwidths is presented. The amplifier operates in the TE/sub 11/ mode at the fundamental cyclotron harmonic. Instantaneous bandwidths in the range of 2.5 to 3.9 GHz (at 1 dB below saturation) with corresponding peak powers between 92.2 and 57.9 kW can be achieved by simply adjusting the mod-anode voltage of the electron gun. The corresponding gains range from 57.1 to 36.0 dB. The design performance is obtained with a high quality, 6 A, 70 kV electron beam generated with a double-anode magnetron injection gun. This wide range performance capability in the design is achieved via the use of the distributed loss approach in the interacting circuit, which also guarantees zero-drive stability. A diffractive loading scheme is employed in the lossy section of the circuit to ensure high average power operation of the amplifier.

Design of an Electron Gun for a 42 GHz, 200 kW, TE52 Mode Gyrotron using the BFCRAY code

This paper describes the design of a magnetron injection gun (MIG) for a 42GHz, 200 kW CW gyrotron operating in the TE5,2 mode. The design was performed using the BFCRAY code. The basic motivation is discussed and the design of the MIG is presented.

Dynamical chaos in a charged-particle flow produced by a magnetron injection gun: Numerical simulation and experiment

A theoretical and experimental study of spatial and temporal current oscillations in a magnetron injection gun is presented. Basic features of the device operation are ascertained. Complicated system dynamics, namely transitions from regular to chaotic oscillations in response to changing the system parameters, is revealed. The conclusions are drawn based on the analysis of the computed electron trajectories and output-current waveforms and spectra. Experimentally, the spectra of the beam-current oscillations, noise-intensity spectral density, etc., are obtained. Strong broadband microwave oscillations of the output current are observed for a wide range of the lengths of the emitting and the nonemitting portions of the cathode.