Gyrotron Backward-wave Oscillator Research Articles

Experimental investigation of a 140 GHz gyrotron-backward wave oscillator

We report the experimental operation of a voltage tunable gyrotron backward wave oscillator (gyro-BWO) in the frequency range near 140 GHz. Voltage tunability is an important feature of the gyro-BWO for application as a fast tuning source for driving high power free electron lasers or cyclotron autoresonance maser amplifiers. The gyro-BWO operated in an overmoded cylindrical waveguide structure in the TE1,2 mode. The electron beam source was a Pierce-wiggler gun producing an 80 kV, 6.2 A beam. Frequency tuning with voltage between 134 and 147 GHz was achieved in the TE1,2 mode with constant magnetic field. However, this tuning was found to be discontinuous. Output powers of up to 2 kW and 2% efficiency were found, significantly below theoretical predictions for a cold beam. The theoretical beam velocity spread was modeled by a 3D beam transport code. The code results show that space charge forces, coupled with the wiggler-induced helical motion and the short cyclotron wavelength of the beam, produce large increases in velocity spread in the magnetic compression region. A beam with smaller velocity spread would be needed to make the gyro-BWO operate at the desired efficiency.

Starting oscillation conditions for gyrotron backward wave oscillators

A generalized linear theory that allows for mismatched boundary conditions was used in determining the starting oscillation conditions for gyrotron backward wave oscillators (gyro-BWO). The results reveal that the gyro-BWO interaction is normally caused by the interference between a constant-amplitude backward wave and a decaying backward wave. The latter becomes a forward-growing wave if the operating frequency is close to the waveguide cutoff frequency. The starting oscillation length for a reflection-type gyro-BWO is shorter than that of a matched-type gyro-BWO. The mismatch has more of an effect upon the onset of a gyro-BWO, which is operated close to the cutoff frequency.

Linear analysis of backward wave oscillations in azimuthally varying transverse electric (TE) modes

A linear analysis for a gyrotron backward wave oscillator operating at a general transverse electric (TEln) mode of a cylindrical waveguide (slotted or uniform cross section) has been developed using a Maxwell–Vlasov equation under the tenuous beam approximation. The equation is solved by Laplace transformation to allow easy insertion of the boundary values. The start-oscillations conditions are determined from an interference of the waveguide and beam modes with appropriate amplitudes and phases. The theory is used to determine the design parameters for stable operation of a second cyclotron harmonic gyropeniotron amplifier at 35 GHz in the π mode of a six vane magnetron-type circuit with a 70 kV, 3 A beam.

Experimental study of an injection-locked gyrotron backward-wave oscillator.

A Ka-band gyrotron backward-wave oscillator (gyro-BWO) experiment has been performed to investigate the phase control, fast tunability, and power capability. Efficient injection locking was accomplished by novel circuit arrangements. Maximum power of 113 kW at -19% efficiency was achieved, showing favorable scaling to still higher powers. Stable voltage tuning bandwidth of 5% at 67 kW peak power was also observed. The measured performance has demonstrated a strong potential of the gyro-BWO for new applications requiring high-power tunable millimeter-wave sources

Mechanisms of efficiency enhancement in gyrotron backward-wave oscillators with tapered magnetic fields.

Efficiency enhancement in a gyrotron backward-wave oscillator through magnetic field tapering is investigated using computer simulations. The results reveal that the magnetic field tapering with a positive gradient tends to increase the initial frequency mismatch and thus lengthen the confinement of most of the electrons in the energy losing phase, therefore resulting in higher net energy transfer to the wave

High-power harmonic gyro-TWT's. I. Linear theory and oscillation study

A linear theory using Laplace transforms which is applicable to both gyrotron traveling wave amplifiers (gyro-TWTs) and gyrotron backward-wave oscillators (gyro-BWOs) is presented. The validity of the linear theory is verified by comparing it with an existing nonlinear self-consistent theory based on a different approach. In conjunction with a time-dependent multimode particle simulation code, the linear theory is applied to study the stability of harmonic gyro-TWTs. It is shown that a harmonic gyro-TWT can be made stable to all forms of spontaneous oscillations by employing a multistage interaction structure and that it can generate power levels far in excess of those possible for a fundamental gyro-TWT. The linear bandwidth of a second-harmonic gyro-TWT amplifier is also calculated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental study of a Ka-band gyrotron backward-wave oscillator

A gyrotron backward-wave oscillator (gyro-BWO) has been operated and magnetically tuned over the frequency range 27-32 GHz. Tuning by varying the electron beam voltage was effective over a smaller frequency range ( Delta f approximately 1 GHz). Output power was as large as 7 kW, corresponding to a device efficiency of 19%. This large efficiency value was unexpected, and related analysis indicates it may be associated with the nonuniform magnetic field profile in the interaction region. >

Floquet's theorem in three dimensions and its application to the dispersion characteristics of circular waveguides with screw periodicity for use with gyrotron backward-wave oscillators

Floquet's theorem for three dimensions is proved for each of the two mathematical conditions of periodicity: the differential equation and the boundary condition. The theorem applied to circular waveguides with screw periodicity provides the dispersion characteristics of the uncoupled modes. These characteristics are compared to those of a normal circular waveguide to show how screw periodic RF structures shift in phase the waveguide cutoff point and how such a shift is advantageous to gyrotron backward-wave oscillators.

Non-linear analysis of the gyro-BWO in three dimensions

The non-linear evolution of the gyrotron backward wave oscillator (gyro-BWO) is investigated numerically in the presence of a weakly non-uniform axial guide magnetic field. The set of coupled non-linear differential equations which govern the self-consistent evolution of the TE modes and the trajectories of an ensemble of electrons in a gyrotron are solved subject to boundary conditions for both the usual BWO (power extracted at the input end) and the reflection-type BWO (power extracted in the forward direction) configurations. Space charge effects are neglected in the analysis. Numerical simulations are carried out to model gyro-BWOs operating in the Ka -band with rectangular TE10 modes and near 104GHz with TE11 cylindrical modes. For a cold beam, calculations show that the efficiency of the device has a modest value of 10–15% in a uniform magnetic field but can be significantly enhanced to 25–30% by tapering the magnetic field. The frequency of the oscillator can be magnetically tuned over a 15% bandwi...

Optimization of the efficiency in gyrotron backward-wave oscillator via a tapered axial magnetic field

The efficiency of a gyrotron backward-wave oscillator (gyro-BWO) in the presence of a tapered axial guide magnetic field is calculated from a three-dimensional self-consistent theory. The saturation efficiency has a low value in the range 10–15% when the external magnetic field is uniform. However, the efficiency can be enhanced to over 30% by a small taper in the magnetic field. Numerical results are shown for operation in the TE11 mode of a cylindrical waveguide.

A linear theory and design study for a gyrotron backward-wave oscillator†

Abstract A linear theory for a gyrotron backward-wave oscillator (gyro-BWO) is developed. The theory solves a reduced one-dimensional Maxwell-Vlasov equation in the form of a linear integro-differential equation using the Laplace transformation. The relative amplitudes among the waveguide modes and beam modes are completely determined and enable one to calculate gyro-BWO start-oscillation conditions. Using this analysis and including velocity spread effects, a design of a millimetre-wave gyro-BWO has been carried out based on the operating parameters of an existing electron gun. Tunability over a range of 86 GHz to 103 GHz is predicted with output power estimated to be ∼ 1 kW.