Axis-encircling Electron Beam Research Articles

Development of a Magnetic Cusp Gun for Terahertz Harmonic Gyrodevices

A magnetic cusp gun (MCG) is being developed to generate an axis-encircling electron beam, which is called the large orbit beam, which is going to drive a 0.396-THz fourth-harmonic gyrotron. Developing an MCG imposes crucial challenges on a simultaneously minimizing guiding center deviation and velocity spread of the electron beam, particularly because an ultrahigh magnetic compression ratio is unavoidable, as is the case for a terahertz (THz) gyrotron. The study of the electron dynamics in the MCG reveals that, close to the emitter, a pair of focusing electrodes are employed to construct a special focusing and accelerating electric field as a way to balance the space-charge influence and guiding center deviation. Investigation indicates that both the electron-beam generalized-angular-momentum spread and the guiding center distribution are the critical factors contributing to beam parameter spread. Intensive optimization generates a high-power MCG with a pitch factor of 1.5, the highest magnetic field of 4 T, minimum transverse velocity spread of 1.1%, and a beam current of 2 A. The key parameters exhibit excellent stability tuning over a wide range of beam current and magnetic field. These merits enable the harmonic gyrotrons or even the frequency-tunable THz gyrotrons to be developed.

Numerical Analysis of a Ka-Band Third-Harmonic Magnetron-Type Slotted Peniotron

In this paper, we have numerically investigated the operating characteristics of a ka-band magnetron-type slotted peniotron. The third-harmonic four-vane peniotron operates in a circularly polarized 2π mode. The MAGIC3D simulation shows that, with proper acceleration voltage, beam current, and axis-encircling electron beam, the peniotron is predicted to generate a pure 2π-mode electromagnetic wave with output power of 45 kW at 29.7 GHz and conversion efficiency of up to 66.7%. Critical characteristics such as the start oscillation time, output power, and conversion efficiency have been also well examined; and it is shown that a careful control of the beam current and the acceleration voltage is important for the device performance.

Generation of large-orbit electron beam using magnetron type injection gun

The generation of axis-encircling large-orbit electron beam using magnetron type injection gun is studied theoretically and numerically. Since the cathode emission band is parallel not only to the system’s axis but also to the magnetic field lines, it eliminates the initial angular momentum spread at the cathode and helps to reduce the velocity spread greatly. Through numerical simulation by EGUN, a large-orbit electron beam with axial velocity spread of 1.82%, guiding center deviation of 6.1%, and velocity ratio of 1.67 is obtained. By adjusting the voltages of the two anodes, the beam quality can be modulated easily to meet the specific requirements of different gyro-devices. The scheme provides a new electron gun candidate for the large-orbit devices.

Design and Numerical Optimization of an 11th-Harmonic Slotted Peniotron

An 11th-harmonic slotted peniotron has been designed for reducing the external magnetic field and maintaining relatively high efficiency. Small-signal theory is used to get the start-oscillation conditions for an axis-encircling electron beam. The device employs a 30-kV 1-A axis-encircling beam to interact with the 2π modes of a slotted 11-vane circuit. As an oscillator, the 11th-harmonic slotted peniotron is predicated to yield 8.6 kW of output power with an electron efficiency of 28.7% at 99 GHz.

Smooth magnetic cusp profile calculation for axis-encircling electron beam generation

The calculation method of a smooth magnetic cusp profile has been introduced to obtain an axis-encircling electron beam in a fixed electrostatic field distribution. Using the calculated smooth magnetic cusp profile, an axis-encircling electron beam of current 1 A and voltage 30 kV can be obtained with a velocity ratio 1.8 and low beam ripples. Theoretical calculation and 3D simulations agree well, which show that this method can be widely used in other axis-encircling electron gun designing.

A cusp electron gun for millimeter wave gyrodevices

The experimental results of a thermionic cusp electron gun, to drive millimeter and submillimeter wave harmonic gyrodevices, are reported in this paper. Using a “smooth” magnetic field reversal formed by two coils this gun generated an annular-shaped, axis-encircling electron beam with 1.5 A current, and an adjustable velocity ratio α of up to 1.56 at a beam voltage of 40 kV. The beam cross-sectional shape and transported beam current were measured by a witness plate technique and Faraday cup, respectively. These measured results were found to be in excellent agreement with the simulated results using the three-dimensional code MAGIC.

Design and Numerical Optimization of a Cusp-Gun-Based Electron Beam for Millimeter-Wave Gyro-Devices

A novel thermionic cusp electron gun operating in the temperature-limited regime that produces a large-orbit electron beam through a nonadiabatic magnetic-field reversal was designed, analyzed, and optimized to give an electron-beam ideal for driving gyro-devices, particularly in the millimeter-to-submillimeter-wavelength range due to its small cross-sectional size. The annular-shaped axis-encircling electron beam had a beam current of 1.5 A at an acceleration potential of 40 kV, a tunable velocity ratio alpha (= vperp/vz) between one and three, an optimized axial velocity spread A.vx/vz of ~8%, and a relative alpha spread Deltakappa/alpha of ~10% at an alpha value of 1.65.

Design of a Third-Harmonic Gyrotron Oscillator Using a Photonic Crystal Cavity

A photonic crystal cavity of the transverse electric (TE) mode is designed to support the third harmonic interaction with a 30 keV–0.7 A axis-encircling electron beam at the operating frequency of the Ka-band, where a magnetic field of 0.38 T is used. The lattice parameters of a square lattice of alumina rods are determined to fit the TE301-like mode in the photonic-band-gap spectral region, and the output signal of a fundamental waveguide mode is conveniently coupled to a conventional waveguide. A particle-in-cell code simulation predicts the single mode operation with the output power of concerning 2.9 kW, corresponding with an efficiency rating of 14%. This scheme can be extended to a n-th harmonic gyrotron using the rectangular cavity of a TEm01-like mode.

Large-Orbit Gyrotron Operation in the Terahertz Frequency Range

Coherent terahertz high-harmonic radiation has been obtained in a gyrotron with an axis-encircling electron beam. An electron-optical system with a cusp gun and a following drift section of adiabatic magnetic compression with an area factor of 3000 provides the formation of an 80-keV/0.7-A beam of gyrating electrons in a wide range of voltages and magnetic fields. Stable single-mode generation with a power of 0.3-1.8 kW in microsecond pulses is detected at four frequencies in the range 0.55-1.00 THz at resonant magnetic fields 10.5-14 T.

Axis-encircling electron beam generation using a smooth magnetic cusp for gyrodevices

The generation of an annular-shaped axis-encircling electron beam using a smooth magnetic cusp was studied through numerical simulations and experiments for harmonic operation of a gyrodevice. Two magnetic coils were used to form a magnetic cusp located just downstream from the velvet cathode of an accelerator diode. An electron beam of current 34 A and voltage 130 kV with an adjustable velocity ratio α up to 1.2 was fully transported to the downstream uniform magnetic field region and used to drive a gyrotron traveling wave amplifier into saturation.

Helically corrugated waveguide gyrotron traveling wave amplifier using a thermionic cathode electron gun

Experimental operation of a gyrotron traveling wave amplifier with a helically corrugated waveguide using a thermionic cathode electron gun is presented. The coupling between the second harmonic cyclotron mode of the gyrating electron beam and the radiation occurred in the region of near infinite phase velocity over a broad frequency band. With an axis-encircling electron beam of pitch factor of ∼1.2, energy of 185keV, and current of 6.0A, the amplifier achieved an output power of 220kW, saturated gain of 24dB, saturated bandwidth of 8.4to10.4GHz, and an interaction efficiency of 20%.

Development of high-frequency gyrotrons using high harmonics and a high magnetic field

Our gyrotrons developed at the Research Centre for the Development of the Far Infrared Region, Fukui University (FU), are high-frequency medium-power gyrotrons used as millimetre- to submillimetre-wave radiation sources for application to new far-infrared technologies. We have already developed a gyrotron FU series which consists of eight gyrotrons. The gyrotron series has achieved frequency tuneability in a wide range (from 38 to 889 GHz). The highest frequency corresponds to a wavelength of 337 μm. This is a current record for high-frequency operation of gyrotrons. Recently, we have developed a high-harmonic gyrotron with an axis-encircling electron beam and a terahertz gyrotron with a pulse magnet. In this paper the present status of the gyrotron FU series is described.

Gain-frequency response of nearby waveguide modes in vane-loaded gyro-TWT

In this paper, the behavior of a gyro-traveling wave tube (gyro-TWT) amplifier, in which an axis-encircling electron beam synchronously interacts with the azimuthal modes of the vane-loaded cylindrical waveguide interaction structure, has been analyzed. The dispersion characteristics and gain-frequency response of the nearby waveguide modes (TE/sub 01/, TE/sub 21/, and TE/sub 41/) for the vane-loaded gyro-TWT amplifier have been presented. The gain-frequency response of the device has been studied for both the small-orbit and large-orbit configurations. It has been observed that the TE/sub 41/ mode of the device is well separated on the frequency scale for both these configurations. In addition, it has higher device gain in the large-orbit configuration when compared with the same obtained for the TE/sub 01/ and TE/sub 21/ modes.

Third-Harmonic Frequency Multiplication of a Two-Stage Tapered Gyrotron TWT Amplifier

The frequency multiplication of a two-stage tapered gyrotron traveling wave tube amplifier is experimentally verified in low-voltage and low-current operation. By modulating an axis-encircling electron beam at the fundamental harmonic cyclotron frequency in the input stage, a frequency-tripled signal induced by the third-harmonic component of the modulated beam current is chosen to be extracted from the output stage. Both interaction stages are linearly tapered to improve stability and bandwidth. The third-harmonic frequency multiplication is predicted theoretically and investigated using a self-consistent large-signal theory and a particle-in-cell code simulation, which estimate a pure third-harmonic generation and a power-scaling law. In the experiment, X-band drive signals from 10.6-12 GHz are multiplied by a factor of three to produce Ka-band output frequencies from 31.8-36 GHz, showing reasonable agreement with theoretical predictions for a 30-kV, 160-mA axis-encircling electron beam.

Development of a high harmonic gyrotron with an axis-encircling electron beam and a permanent magnet

A gyrotron with an axis-encircling electron beam is useful for high frequency operation, because the high beam efficiency is kept even at high harmonic of electron–cyclotron resonance. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already succeeded in operation at the third harmonic and the fourth harmonic resonances. The operation frequencies are 89.3 and 112.7GHz, respectively. Operation cavity modes are TE311 and TE411. The permanent magnet system consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in cavity and electron gun regions. The magnetic field at the cavity region can be varied from 0.97 to 1.18T. At the optimum condition of the magnetic field intensity, the output power at the third harmonic operation is 2.5kW. The operation is pulsed, the pulse length is 1 ms and the repetition frequency is 1Hz. The beam energy and current are 40kV and 1.2 or 1.3A. Starting current, beam efficiency and emission pattern also have been measured. In this paper, the operation results of the gyrotron and comparison with the computer simulation results are described.

A High Harmonic Gyrotron With an Axis-Encircling Electron Beam and a Permanent Magnet

A gyrotron with an axis-encircling electron beam is capable of high-frequency operation, because the high-beam efficiency is kept even at high harmonics of the electron cyclotron frequency. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already operated successfully at the third, fourth, and fifth harmonics. The frequencies are 89.3, 112.7, and 138 GHz, respectively, and the corresponding cavity modes are TE/sub 311/, TE/sub 411/, and TE/sub 511/. The permanent magnet system is quite novel and consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in the cavity and electron gun regions. The magnetic field in the cavity region can be varied from 0.97 to 1.18 T. At the magnetic field intensities, the output powers at the third and the fourth harmonics are 1.7 and 0.5 kW, respectively. The gyrotron is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy is 40 kV and the beam current is 1.2-1.3 A. Beam efficiencies and emission patterns have also been measured. In this paper, the experimental results of the gyrotron are described and compared with computer simulations.

Studies on a third-harmonic large-orbit gyro-traveling-wave-tube amplifier

A third-harmonic gyro-traveling-wave-tube amplifier with a smooth cylindrical waveguide using an axis-encircling electron beam is studied. By the use of a novel way of magnetic field tapering, the output efficiency increases from 25% to 39% and the bandwidth increases from 3% to 7.5% compared with the use of a constant magnetic field. Numerical calculations also find that the electron beam reaches a strong bunching state two times before saturation of the radiation field due to the magnetic field tapering.

Experimental verification of low-velocity spread axis-encircling electron beam

We have experimentally demonstrated a low-velocity spread, axis-encircling electron beam using a comparatively simple Pierce-type electron gun and single cusp magnetic field based on a recent theory [Jeon et al., Appl. Phys. Lett. 80, 3703 (2002)] developed with the assumption of small orbit gyration before the magnetic cusp, in which every physical property has been analyzed and the possibility of zero percent axial velocity spread was concluded. The velocity ratio and the axial velocity spread were measured to compare the theory with varied operation conditions of the electron gun. These results agree well with our hypothesis.

Analysis of Axis-Encircling Electron Beam Using a Single-Cusp Magnetic Field

The characteristics of an axis-encircling electron beam using an annular Pierce-type electron gun are calculated analytically. The analytical calculation is carried out by considering the initial canonical angular momentum spreads at the cathode and the crossing of the electron beam with the magnetic flux line before the magnetic cusp. The analytically calculated characteristics of an axis-encircling electron beam such as the axial velocity spread, the velocity ratio, the Larmor radius, and the guiding center radius are in good agreement with the simulated result obtained using an electron trajectory program. In this comparison, a 30 kV, 1.0 A electron beam is used with a peak magnetic flux density of 4.38 kG using the magnetic field with a single cusp. The result obtained by this analysis also shows that the electron beam with a fairly low axial velocity spread of less than 1% can be realized by employing a suitable distance between the cathode and magnetic cusp.

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.