Cyclotron Autoresonance Maser Amplifier Research Articles

Mode competition in a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier

Physics issues of mode competition in a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier are investigated. It is shown that the use of a coaxial waveguide brings about two peculiarities: possible enhancement of the transverse dimensional size and the rarefied spectrum and that the applications of a large-orbit electron beam and CARM mechanism can efficiently exclude the parasitic effect of the cyclotron harmonics and spurious oscillations of undesired modes. The dispersion diagram and numerical simulation demonstrate that mode competition and spurious gyrotron-like or CARM-like oscillations can be suppressed when a large-orbit coaxial-waveguide CARM amplifier operates in a higher-order mode.

Nonlinear analysis of a large-orbit coaxial-waveguide cyclotron autoresonance maser amplifier

Nonlinear simulations are presented to analyze the influences of the electron beam and the magnetic field parameters on the output power of a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier. It is found that the guiding-center spread of the relativistic electrons has negligible impact on the output power due to the small field change felt by the large-orbit electrons. The electron-beam velocity spread and energy spread substantially decrease the output power, because these spreads directly affect the beam-wave interaction through the Doppler term and the relativistic cyclotron frequency term in the cyclotron resonance condition. However, this adverse effect may be offset by properly tapering the operating magnetic field. The output power is sensitive to both the slope and the amplitude of the tapered magnetic field. Nonlinear simulation demonstrates the feasibility that a large-orbit coaxial-waveguide CARM amplifier can be expected to provide output power with several megawatts, ultrahigh gain, and good bandwidth in the millimeter and submillimeter wavelength ranges.

Linear Study of a Large-orbit Coaxial-waveguide Cyclotron Autoresonance Maser Amplifier

A linear theory based on Maxwell-Vlasov equations and the Laplace transform is presented to describe the large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier. A 35-GHz, TE5,1-mode large-orbit coaxial-waveguide CARM amplifier is specifically analyzed. Good agreement between the linear theory and the nonlinear simulations is found in the exponentially growing region of the wave. Results show that the operating magnetic field and the electron-beam energy and average radius substantially affect the power, and especially, a small beam current may be unsuitable to a large-orbit CARM amplifier operation.

Nonlinear characteristics of a coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier

A detailed model of nonlinear self-consistent simulation for a coaxial-waveguide cyclotron autoresonance maser amplifier is presented to analyse the nonlinear characteristics of the device. Mode spectrums in cylindrical-waveguide and coaxial-waveguide are analysed. The effects of the electron-beam velocity spread and the misalignment between the electron-beam axis and the waveguide axis on the characteristics of the device are investigated and found to be detrimental to the power and efficiency. Specific simulations show that the output power and the efficiency drop from 493.4 kW and 10.97% to 382.33 kW and 8.5% when the axial velocity spread is 1% and decrease to 3.33% efficiency at 0.1 cm misalignment. However, the use of a tapering guide magnetic field with a slope ε = −0.001 cm−1 and the starting position z = 25 cm may enhance the power and efficiency to 1.84 MW and 40.82%, respectively.

Nonlinear theory of a cyclotron autoresonance maser (CARM) amplifier with outer-slotted-coaxial waveguide

A self-consistent nonlinear theory for the outer-slotted-coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier is presented, which includes the characteristic equation of the wave, coupling equation of the wave with the relativistic electron beam and the simulation model. The influences of the magnetic field, the slot depth and width are investigated. The interesting characteristic of the device is that the mode competition can be efficiently suppressed by slotting the outer wall of the coaxial waveguide. A typical example is given by the theoretical design of a 137 GHz outer-slotted-coaxial-waveguide CARM amplifier by utilizing an electron beam with a voltage of 90 kV, current of 50 A, velocity pitch angle of 0.85 and a magnetic field of 43.0 kG. The nonlinear simulation predicts a power of 467.9 kW, an electronic efficiency of 10.4% and a saturated gain of 46.7 dB, if the electron beam has no velocity spread. However, the axial velocity spread deteriorates the device; for example, if the axial velocity spread is 2%, the peak power decreases to 332.4 kW with an efficiency of 7.4% and a saturated gain of 45.22 dB. Simulation shows that the efficiency of the outer-slotted-coaxial-waveguide CARM amplifier may be increased from 10.4% to 29.6% by tapering the magnetic field.

Influences of the initial parameters on the performance of a coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier

Nonlinear simulations demonstrate the possibility for a coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier to operate at a frequency of 140 GHz emitting hundreds of watts with an ultrahigh gain of 68 dB. Careful design of the operating magnetic field is of importance to the power of the device, although the other initial parameters such as electron-beam radius and voltage also affect the power. Like a cylindrical-waveguide CARM, a coaxial-waveguide CARM amplifier is sensitive to the velocity spread of the electron beam.

Linear and nonlinear investigation of a coaxial-waveguide cyclotron autoresonance maser amplifier

A different concept of gyrodevices in the form of a coaxial-waveguide cyclotron autoresonance maser (CARM) is proposed. Embodying the salient features of large cross-sectional area, low guiding magnetic field, and good mode selectivity as compared to the gyrotron and to the conventional CARM, the proposed device is well suited for operation at both high power and ultrahigh gain in the millimeter wave and far infrared regions. Gyrokinetic analysis and nonlinear simulation are given in detail for a coaxial configuration—with outer and inner radii of 1.83 and 0.50 cm—designed to operate in the TE28,16 mode at 335 GHz, yielding a gain of 64 dB on using a 90 keV and 50 A electron beam confined by a 9.35 T magnetic field. Nonlinear simulation demonstrates the possibility of power enhancement via a tapered magnetic field. The competition of the degenerate mode and the excitation of the absolute instability are discussed.

Simulation of high power broadband cyclotron autoresonance maser amplifier and electron beam experiments

The design, simulation, and preliminary experimental implementation of an efficient, broadband cyclotron autoresonance maser (CARM) amplifier operating over the 9–13 GHz frequency band is presented. The amplifier is based on a high current accelerator capable of generating a ∼35 A pencil electron beam at an accelerating voltage of ∼450 kV. A full three-dimensional numerical model of the CARM amplifier has been constructed within the particle-in-cell code KARAT taking into account electron beam parameters derived from simulation and experiment. An electron beam current of 32A at an accelerating voltage of 400 kV was measured. Numerical simulations demonstrate the possibility of obtaining 37 dB gain and an interaction efficiency of 17%. In addition a viable amplification bandwidth of 9–13 GHz is apparent, with a minimum gain and efficiency of 25 dB and 10%, respectively, at the boundaries of the amplification band. The peak modeled efficiency and gain (17%, 37 dB) were obtained at a frequency of 12 GHz. Computational simulations have also revealed correlation between the instantaneous amplification bandwidth and the spectral width of cyclotron superradiant emission within the system.

Nonlinear Investigation of a Coaxial-Waveguide Cyclotron Autoresonance Maser Amplifier

A nonlinear approach is presented to simulate the coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier. Simulation shows an interesting result that the efficiency of a coaxial-waveguide CARM amplifier may be increased from 7.5% to 33.9% by tapering the magnetic field.

Induced inhibition of electron acceleration in wiggler enhanced cyclotron autoresonance maser amplifiers

The performance of cyclotron autoresonance maser amplifiers can be substantially improved by using a properly spatially profiled helical wiggler. The wiggler behaves as a phase filter and tends to reduce the number of accelerating electrons. The proposed configuration is able to alleviate the severe problem of performance degradation by beam velocity spread. Simulation results show that more than 20% output efficiency can still be attained at 94 GHz, even with 5% beam velocity spread. The inclusion of the ac space charge field was observed to only slightly deteriorate the amplifier performance.

Modified self-consistent nonlinear simulation of cyclotron autoresonance maser amplifiers

Since the electron cyclotron auto-resonance maser (CARM) is operated with the wave refractive index being very close to unity, a little variation of the refractive index in the CARM calculations might be crucial. Therefore, a modified self-consistent one-dimensional nonlinear model was developed by exactly setting the evolving wave refractive index and keeping all second-order terms to examine the inadequacy induced by the simplification of the earlier one-dimensional models. Comparisons of the numerical results obtained by the earlier models and the modified model for a CARM amplifier reveal that the inadequate relaxation of the limited azimuthal and axial bunching region for electrons to lose energy in the earlier models was the main reason for the overestimate of the linear gain and the saturated efficiency for the fast (the slow) wave branch with initial refractive index n0<1 (n0>1). However, the earlier model with fixed n0=1 is an excellent approximation whatever linear or saturated state is considered. The efficiency dependence of a CARM amplifier on some basic operating parameters was reexamined by the modified formulations.

Wiggler enhanced cyclotron autoresonance maser amplifiers

The performance of cyclotron autoresonance maser amplifiers can be significantly improved by using a properly spatially profiled helical wiggler. The proposed configuration is able to alleviate the severe problem of performance degradation by beam velocity spread. Simulation results show that more than 20% output efficiency can still be attained at 94 GHz, even with 5% beam velocity spread.

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.

Effective bandwidth and the Kompfner dip for cyclotron autoresonance maser amplifiers.

This paper investigates the inadequacy of using the growth rate from the dispersion relation to represent the bandwidth of a cyclotron autoresonance maser (CARM) amplifier. It is shown that the dispersion relation can accurately predict the amplifier`s bandwidth, but at least three waves that the electron beam couples to must be retained in the calculation. Interference between these waves must be included. The bandwidth of the coupling coefficient for the forward growing wave can be much narrower than the bandwidth of this wave`s growth rate. In addition, the input signal has been observed to be almost totally absorbed by the electron beam at a specific frequency which is a function of the operating conditions. This phenomenon is caused by the beating between the forward constant amplitude wave and the forward growing wave. It is analogous to the Kompfner dip in a conventional linear beam traveling wave tube and can, therefore, become a useful diagnostic for determining the experimental parameters of a CARM amplifier.

Autophase cyclotron autoresonance maser amplifiers

The phase stability of the output power from high-power microwave tubes is a critical issue in applications such as electron acceleration and radar. The radio frequency (rf) phase stability of cyclotron autoresonance maser (CARM) amplifiers is investigated, and the phase stability of a CARM amplifier with a suitable correlation between perpendicular momentum and beam energy is shown to result in an order-of-magnitude improvement in phase stability over other rf sources. A technique for producing such a correlation, termed autophasing, is identified.

Linear and nonlinear theory of cyclotron autoresonance masers with multiple waveguide modes

The interaction of multiple waveguide modes with a relativistic electron beam in an overmoded, single-frequency, cyclotron autoresonance maser amplifier is analyzed using a nonlinear self-consistent model and kinetic theory. It is shown analytically, and confirmed by simulation, that all of the coupled waveguide modes grow at the spatial growth rate of the dominant unstable mode, but suffer different launching losses which depend upon detuning. The phases of coupled modes are locked in the exponential gain regime, and remain approximately locked for some finite interaction length beyond saturation. The saturated power in each mode is found to be insensitive to the input modal radio-frequency (rf) power distribution, but sensitive to detuning. Simulations indicate that the saturated fractional rf power in a given mode reaches a maximum at its resonant magnetic field, and then decreases rapidly off resonance. Good agreement is found between the simulations and the kinetic theory in the linear regime.

CARM amplifier theory and simulation

The theory and simulation of cyclotron autoresonance maser (CARM) amplifiers are presented, including studies of amplifier phase stability, multimode phenomena, and susceptibility to absolute instabilities. Recent results include particle-in-cell simulations of the onset of absolute instabilities and numerical modeling of multimode effects. Phase stability studies indicate that the output phase of CARM amplifiers may be relatively insensitive to fluctuations in beam energy, pitch, and current, for optimized designs; simulations show a phase sensitivity of ∼ 2° per percent beam energy variation. An experimental design for a long-pulse 17 GHz CARM amplifier is presented.

Experimental and theoretical studies of a 35 GHz cyclotron autoresonance maser amplifier

Experimental and theoretical studies of a cyclotron autoresonance maser (CARM) amplifier are reported. The measurements are carried out at a frequency of 35 GHz using a mildly relativistic electron beam (1.5 MeV, 130 A, 30 nsec) generated by a field emission electron gun followed by an emittance selector that removes the outer, hot electrons. Perpendicular energy is imparted to the electrons by means of a short bifilar helical wiggler. The entire system is immersed in a uniform axial magnetic field of 6–8 kG. With an input power of 17 kW at 35 GHz from a magnetron driver, the saturated power output is 12 MW in the lowest TE11 mode of a circular waveguide, corresponding to an electronic efficiency of 6.3%. The accompanying linear growth rate is 50 dB/m. When the system operates in the superradiant mode (in the absence of the magnetron driver) excitation of multiple waveguide modes is observed. A three-dimensional simulation code that has been developed to investigate the self-consistent interaction of the copropagating electromagnetic waveguide mode and the relativistic electron beam is in good agreement with the experimental observations.

CARM amplifier designs for high power

The cyclotron autoresonance maser (CARM) is shown to be capable of extremely high power efficient operation with relatively weak magnetic field requirements. A general description of the CARM traveling wave amplifier is presented. The linear theory is applied to study the stability of the device and a self-consistent numerical simulation code is used to predict the performance. It is found that grazing intersection often gives superior performance, especially for high current electron beams. The designs of two CARM amplifiers for the applications of a high gradient rf linac and electron cyclotron resonance plasma heating as well as a proof of principle experiment are presented.

Multimode interactions in cyclotron autoresonance maser amplifiers.

The interaction of transverse eigenmodes with a relativistic electron beam is analyzed in an overmoded cyclotron autoresonance maser amplifier, using a nonlinear self-consistent model and kinetic theory. It is shown that all of the coupled modes grow with the dominant unstable mode at the same growth rate, but suffer different launching losses. The phases of coupled modes are locked in the linear and nonlinear regimes. Simulations indicate that the rf power distribution among the interacting modes at saturation is {ital insensitive} to input power distribution but {ital sensitive} to detuning.