Mode competition and control in higher-power gyrotron oscillators

One of the most important problems in the design of high-power millimeter-wave sources such as gyrotron oscillators is insuring that the device operates in the desired mode. For high-power and short-wavelength devices the effective mode density is high, in that the current is above threshold for many modes. One then is led to ask whether operation in a single mode is possible and what steps must be taken to maximize the electronic efficiency of the device while ensuring single-mode operation. The answer to the first question has been determined to be yes. Provided that certain conditions are met, single-mode operation is stable. The present results emphasize time-dependent multimode simulations showing how these stable states can be accessed. In particular, the accessibility to the stable single mode with maximum efficiency is studied. Regions of parameter space for which stable single-mode operation is possible are plotted for an annular beam for a closed-cavity gyrotron operating at a high-order whispering-gallery mode (TE_80.4). These results also apply to the quasioptical gyrotron with a pencil electron beam     

Mode competition, suppression, and efficiency enhancement in overmoded gyrotron oscillators

Gyrotron oscillators are of great interest as sources of high power mm wave radiation for electron cyclotron resonance heating and current drive in magnetic fusion research devices. Gyrotrons capable of efficiently generating cw power 1 megawatt will be required in future magnetic fusion studies. However, as gyrotron power approaches the megawatt level, a very large, overmoded cavity must be employed in order to keep ohmic power losses in the cavity at an acceptable level, and the problem of mode overpopulation becomes severe. Also, it becomes increasingly important to optimize gyrotron efficiency for a number of important reasons including minimizing the problem of collecting the electron beam energy. In the present paper, a detailed experimental and theoretical study of mode competition and mode locking in an overmoded gyrotron is presented. Efficiency enhancement (to 60%) and high peak power (430 kW) were achieved in the TE_2,4,1 mode using magnetostatic profiling in the cavity. With selective mode suppression, peak power of 475 kW was generated in the TE_0,4,1 mode.This work was performed at the Naval Research Laboratory. Some of the authors have affiliations with other institutions, as indicated:     

Pattern formation and competition in photorefractive oscillators

We introduce a general model of pattern formation in optical systems made of a cavity with an active medium as a photorefractive crystal fed by a pump. The model is based on the interplay of a diffractive equation for the optical field and a diffusive equation for the medium refractivity. The aim of the model is to describe a series of experiments which have shown mode competition (periodic or chaotic alternation) for low Fresnel numbers (F) and mode coexistence, leading to short range space correlations, for high F. For low F, a linear stability analysis provides the set of modes above threshold as a function of the transverse wave number. Due to the interplay of the optical and the diffusive interactions, different behaviors result depending on the thickness of the medium as compared to the optical absorption length and diffusion length. Including the leading nonlinearities compatible with the symmetry constraints, we introduce normal form equations which describe the time-dependent mode competition. In the case of a large number of modes (high F), nonlinear mode-mode interaction is equivalent to a self-induced noise. In this limit, the relevant feature to be compared with the experiment is the power spectrum.     

模式竞争与压缩现象

双模激发将使场趋近初始相干态,此时场的某一正交分量的压缩交浅.讨论了加深压缩的一种可能性并举了级联单模情况为例.     

Mode expansions and bifurcations in nonlinear single-well oscillators

Nonlinear single-well oscillators are studied under the influence of a periodic driving force and damping. The first bifurcation from the symmetric solution is investigated using different mode expansions. The results are compared with a numerical solution of a Duffing oscillator.     

Mode competition in the orotron

A nonlinear, self-consistent and multimode analysis of the orotron is presented. The field in the cavity is expanded into the Hermite-Gaussian modes with time-dependent amplitudes, for which a set of ordinary differential equations is obtained from Maxwell's equations. The equations for the amplitudes are coupled to the equations of motion for the electrons. To yield a self-consistent solution, this set of coupled equations is solved simultaneously. The calculations yield transient and steady state behaviour, saturated efficiency, mode competition and multi-frequency behaviour.     

Mode competition and frequency locking

A theoretical study is presented of frequency locking to be observed in lasers with broad homogeneous emission lines, when the cavity losses are reduced, at a certain frequency ω₀, by placing inside the cavity a suitable element (e.g., a selectively reflecting “mirror”). Two kinds of locking effects are analyzed: i) When tuning the center frequency of the laser output ω_f towards ω₀, by means of a filter or a grating, the laser frequency becomes locked to ω₀, when |ω_f ? ω₀| reaches a critical value. ii) In the absence of frequency-selective elements, the laser oscillates only in a narrow spectral range centered at ω₀. Special emphasis is given to the role played by mode competition in these locking phenomena.     

Mode competition in multi-core fiber amplifier

3D beam propagation numerical code and a newly developed optical mode solver were implemented for studies on the recently experimentally realized Yb fiber amplifier with 7-core hexagonal structure. It was found that the launched seven single-frequency beams matched with the cores converge to a wave field with permanently diminishing space beating patterns, but the result is sensitive to phases of separate beams. Namely, at a spread of phases of individual beams within a few tenths of radian, the convergence of the wave field to the in-phase mode is observed, while at phases random spread on the order of π the out-of-phase mode is dominating. Such behavior is observed at strong gain saturation in the cores. Both effects take place even if the guiding index of the cores fluctuates within a limited range. The mode solver predicts that the in-phase mode has the minimal small signal gain, and the gain of the out-of-phase mode is maximal one. This fact proves that there is no strict correlation between modal gains and amplification in strong saturated multi-core fiber. The smaller core index step significantly improves the convergence of the amplified radiation to the in-phase mode. An approach to the waveguide structure tailoring is described which assures the single in-phase mode stability.     

Harmonic lasing in a free-electron-laser amplifier

A method is demonstrated that allows a planar wiggler high-gain Free-Electron-Laser (FEL) amplifier to lase so that the interaction with an odd harmonic of the radiation field dominates that of the fundamental. This harmonic lasing of the FEL is achieved by disrupting the electron interaction with the usually dominant fundamental while allowing that of a harmonic interaction to evolve unhindered. The disruption is achieved by a series of relative phase changes between the electrons and the ponderomotive potentials of both the fundamental and harmonic fields. Such phase changes are relatively easy to implement and some current FEL designs would require little or no structural modification to test the scheme.     

1:1 Mode locking and generalized synchronization in mechanical oscillators

We describe the relation between the complete, phase and generalized synchronization of the mechanical oscillators (response system) driven by the chaotic signal generated by the driven system. We identified the close dependence between the changes in the spectrum of Lyapunov exponents and a transition to different types of synchronization. The strict connection between the complete synchronization (imperfect complete synchronization) of response oscillators and their phase or generalized synchronization with the driving system (the (1:1) mode locking) is shown. We argue that the observed phenomena are generic in the parameter space and preserved in the presence of a small parameter mismatch.     

Mode competition effects in CW dye lasers

The output characteristics of a rhodamine 6G jet stream cw dye laser equipped with a mirror grating combination for mode selection, have been studied experimentally. The tuning range proved to be considerably narrower than that to be expected from the gain profile under the (erroneous) assumption of independently oscillating modes. Actually, the reduction of the tuning range is a consequence of mode competition, as pointed out in a previous theoretical paper. Moreover, the intensity of the selected mode has been found to be independent of the wavelength. All experimental findings are in a good agreement with the results of a theoretical analysis.     

Beam-splitting and recombining of free-electron-laser extreme-ultraviolet radiation

The design of systems for the splitting of the radiation from a free-electron-laser (FEL) in the extreme-ultraviolet spectral region is presented. Systems for the beam-splitting of the FEL radiation are important, both for having the light available simultaneously to several experiments, and for the division and recombination of the beam (pump-probe techniques). We present systems with a grazing-incidence plane grating, which is illuminated by the single FEL beam and gives several output beams (the zero order and the diffracted orders). The grating gives intrinsically a time stretching of the diffracted beam, so time-compensated configurations are used. Systems for the splitting of the primary beam in two secondary beams and for the splitting and recombining from two beams with variable delay are presented. PACS 41.60.Cr; 42.79.Dj; 42.79.Fm     

Mode competition in acousto-optically Q-switched planar waveguide lasers

Multi-mode rate equations have been developed to investigate mode competition in high-power acousto-optically Q-switched planar waveguide lasers. The mode competition arises from coupling effects and temporal losses in the transform between guided modes and free-space propagation. Pulse-to-pulse instability and temporal beam distortions are enlarged by mode competition when the laser works in the multi-mode regime. The influence of parasitic oscillation is also discussed. A Nd:YAG planar waveguide laser has been established with a folded hybrid/unstable resonator. A maximum average power of 83 W with a beam propagation factor is obtained. The theoretical simulation agrees well with the experimental observation.     

Mode competition in nonstationary regimes of high-power gyrotrons

We propose a method for analyzing the nonstationary processes resulting from variation of the accelerating voltage. We present a theoretical basis for the experimentally observed generation at parasitic modes in the case of reduced voltage. The results obtained allow us to estimate the possibility of obtaining single-mode oscillations with different frequency in gyrotrons with large-diameter cavities. “Gikom” Joint-Stock Company, Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika Vol. 41, No. 6, pp. 803–812, June, 1998.     

Mode competition in a high harmonic tunable gyrotron

The nature of the competition between a fundamental mode and a second harmonic mode depends on whether their azimuthal mode numbers are the same or different. If they are the same, the second harmonic may aid the excitation of the fundamental and eventually, at high beam currents, be suppressed by the fundamental, if the azimuthal mode numbers are different, the two modes may grow together.