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Gyromonotrons are typically believed to rely on the convective interaction between the co-propagating beam and wave, with the extended energy-exchanging process stemming from the external feedback. However, numerous studies focusing on both transverse electric (TE) mode and transverse magnetic (TM) mode gyrotrons have consistently shown that beam–wave interactions in weak-feedback systems or even in uniform tubes without any structural feedback can yield a theoretical beam efficiency of more than 30% with major forward-wave output during near-cutoff operation, which is the typical operating condition for gyromonotrons. These intriguing findings raise questions about the actual feedback mechanism of gyromonotrons. In this article, comparative studies on the linear and nonlinear behaviors of uniform-tube gyrotron are investigated. The forward and backward waves are observed to co-generate and exhibit similar characteristics of ultra-slow group velocity under near-cutoff operation. This situation allows the as-generated forward wave to modulate the fresh beam, establishing a new backward-wave-like internal feedback loop. Additionally, the quasi-degenerate nature of the bi-directional propagating waves ensures their intrinsic in-phase relationship. The consequent constructive interference enables the uniform tube to function as a high-Q resonator. These findings are found to be independent of the choices of TE or TM modes, providing valuable insights into the underlying interaction mechanism of gyrotron devices. [ABSTRACT FROM AUTHOR]

Plasma photonic crystals (PPCs) have the potential to significantly expand the capabilities of current millimeter wave technologies by providing high speed (microsecond time scale) control of energy transmission characteristics in the GHz through low THz range. Furthermore, plasma-based devices can be used in higher power applications than their solid-state counterparts without experiencing significant changes in function or incurring damage. Plasmas with periodic variations in density can be created externally, or result naturally from instabilities or self-organization. Due to plasma's diffuse nature, PPCs cannot support rapid changes in density. Despite this fact, most theoretical work in PPCs is based on solid-state photonic crystal methods and assumes constant material properties with abrupt changes at material interfaces. In this work, a linear model is derived for a one-dimensional cold-plasma photonic crystal with an arbitrary density profile. The model is validated against a discontinuous Galerkin method numerical solution of the same device configuration. Bandgap maps are then created from derived group velocity data to elucidate the operating regime of a theoretical PPC device. The bandgap maps are compared for one-dimensional PPCs with both smooth and discontinuous density profiles. This study finds that bandgap behavior is strongly correlated with the density profile Fourier content and that density profile shapes can be engineered to produce specific transmission characteristics. [ABSTRACT FROM AUTHOR]

A spontaneously excited longitudinal backward dust wave was experimentally examined. Dust particles were confined in a vertical glass cylinder. The spontaneously excited wave propagated downward. A dispersion relation was obtained by measuring the wavenumber and phase velocity. The dispersion relation showed that the wave consisted of two different modes, including a backward wave. The Trivelpiece–Gould (TG) mode was invoked as a dispersion relation to explain the backward wave, although the cyclotron angular frequency, ω c , appearing in the TG mode was replaced with ω a ∼ ω p d , where ω p d is the dust angular frequency. The experimental results were compared with the calculated dispersion relation, including the TG mode. The group velocity of the spontaneously excited backward wave is larger than the value predicted with the TG mode. It is necessary to explain the backward wave to modify the TG mode model or to construct a new model. [ABSTRACT FROM AUTHOR]

An axiparabola-based flying focus laser possesses a long focal depth, a small focal spot, and a controllable group velocity. It has been proposed for wide applications, such as phase-locked laser wakefield acceleration and photon acceleration. We numerically study the propagation of axiparabola-focused laser pulses in plasmas and find that such lasers can propagate stably over long distances in plasmas at low intensity. When the laser intensity increases to the relativistic intensity, they no longer propagate stably. Pulse front deformation and fracture appear due to the formation of plasma density modulations. We propose three schemes to mitigate the unstable propagation of axiparabola-focused lasers: (i) adding a radially dependent pulse front delay, (ii) placing the plasma away from the beginning of the focal line, and (iii) using an axiparabola mirror with a negative focal line. All these methods are relatively easy to implement. Our studies can provide guidance for applications of axiparabola-focused lasers. [ABSTRACT FROM AUTHOR]

We theoretically study the formation of periodical trains of microwave self-induced-transparency solitons (M\W-SIT soliton combs), which arise under cyclotron resonant interaction of an initially rectilinear electron beam with a steady-state electromagnetic wave counter-propagating in a cylindrical waveguide. Depending on the contained energy, solitons can either propagate toward the electron beam (i.e., in the direction of the unperturbed group velocity) or be entrained by the beam in the direction of its translational motion. As a result, a kind of feedback arises, leading to the appearance of soliton combs emitted from both the left and right boundaries of the system. This process can be described by the non-stationary self-consistent model, which is based on the parabolic equation for the field evolution taking into account the waveguide dispersion. Within the framework of the developed model, it is shown that waveguide dispersion does not prevent the formation of ultrashort solitons with duration of about several dozens wave periods. Moreover, the peak power of the entrained solitons can anomalously increase as the frequency of the incident wave approaches the cutoff frequency of the operating mode. [ABSTRACT FROM AUTHOR]

We show the possibility of increasing the amplitude of ultrashort microwave pulses in a passive mode-locked oscillator in a circuit containing a chain of traveling wave tubes (TWTs) one of which operates in the amplification regime and the other two in the nonlinear Kompfner suppression regime. An increase in the number of nonlinear filtering stages makes it possible to enhance the filtering threshold level, below which low background noise is suppressed, while an almost complete transparency is maintained for peak amplitudes of the pulse signal. As a result, with an increase in the length of the amplifier, a linear growth in the peak amplitude of microwave pulses takes place because of the commutative extraction of energy from different electron fractions, along which the pulse is shifted due to the difference in its group velocity from the translational velocity of the particles. With an optimal selection of the parameters of the absorbers, the peak amplitude of the generated periodic sequence of ultrashort pulses can exceed the amplitude of stationary generation by more than an order of magnitude for given parameters of the TWT like current, voltage, and coupling impedance. [ABSTRACT FROM AUTHOR]

We study spontaneous coherent super-radiative undulator emission in the terahertz frequency range from a short (as compared to the wavelength of the radiated wave), dense electron bunch. Since the group velocity of the wave is close to the bunch velocity, this is a process of spontaneous radiation followed by amplification of a single wave cycle. Despite the Coulomb repulsion of electrons inside the bunch, its compactness, which is necessary to ensure the spontaneous coherent character of the radiation process, is provided by the compression of the bunch under the action of its own radiation fields. As a result, formation of an ultra-short (several cycles long) powerful wave packet occurs when the bunch moves through several undulator periods with high (∼20% in optimized profiled systems) efficiency of extraction of the electron energy and high intensity (∼100 MV/m) of the peak wave field. [ABSTRACT FROM AUTHOR]

We observe complete absorption of an ∼1.2 GW, 0.5 ns, 25.6 GHz high power microwave pulse propagating in a plasma-filled waveguide when the plasma density dependent waveguide cutoff frequency is close to the pulse frequency. Some of the plasma electrons are ejected to the walls, leaving in the waveguide an uncompensated ion charge which forms a potential well where the remaining electrons oscillate in the pulse field. Due to the decreased group velocity of the wave, these trapped electrons have sufficient time to collide with ions, while their regular oscillatory motion becomes chaotic and thermal. Almost all the energy of the electromagnetic pulse is transferred to the kinetic energy of the electrons. This mechanism of absorption is absent when the pulse power is low, and a potential well does not form in the waveguide. [ABSTRACT FROM AUTHOR]

An ordinary high frequency radio wave radiated at the Spitze angle after reflection from the ionosphere does not return to the ground. It is converted into the Z-mode, and in the vicinity of the critical height, this mode becomes quasi-electrostatic and it is absorbed by electrons. This well-known result obtained in the cold plasma approximation is revised in our paper. It is shown that the Z-mode twice changes the direction of propagation in the vicinity of the critical height. When the Z-mode becomes quasi-electrostatic due to the increase in the vertical wave number, thermal corrections come into play. It is argued that this wave changes the direction of its vertical component of the group velocity at a particular altitude slightly smaller than the critical height. Various nonlinear effects (modulation instability, formation of solitons, and acceleration of electrons) possibly take place in the localized region near the reflection height. [ABSTRACT FROM AUTHOR]

Azimuthal surface waves are well known to be eigenwaves of cylindrical waveguide structures—plasma–dielectric–metal—placed into an external axial static magnetic field. The angular velocity of energy transfer along with the angular group velocity is applied to analyze electromagnetic energy rotation in these structures. Optimal magnitudes of effective azimuthal wavenumbers are determined for which the angular velocity reaches its maximum. The angular velocity is shown to decrease with the increasing absolute value of the azimuthal wavenumber and with the decreasing strength of the external axial static magnetic field. [ABSTRACT FROM AUTHOR]

We examine with particle-in-cell simulations how a parallel shock in pair plasma reacts to upstream waves, which are driven by escaping downstream particles. Initially, the shock is sustained in the two-dimensional simulation by a magnetic filamentation (beam-Weibel) instability. Escaping particles drive an electrostatic beam instability upstream. Modifications of the upstream plasma by these waves hardly affect the shock. In time, a decreasing density and an increasing temperature of the escaping particles quench the beam instability. A larger thermal energy along than perpendicular to the magnetic field destabilizes the pair-Alfvén mode. In the rest frame of the upstream plasma, the group velocity of the growing pair-Alfvén waves is below that of the shock and the latter catches up with the waves. Accumulating pair-Alfvén waves gradually change the shock in the two-dimensional simulation from a Weibel-type shock into an Alfvénic shock with a Mach number that is about 6 for our initial conditions. [ABSTRACT FROM AUTHOR]

The principle of chromatic matching, i.e., using two laser modes of different geometric mode numbers and different colors so that they copropagate at the same group velocity, is proposed in order to realize the plasma undulator concept. Chromatic matching allows one to overcome group velocity slippage, whereby lower-order modes outrun higher-order ones, allowing for extended interaction lengths. The dephasing limit can be mitigated by using a special tapering of the plasma channel such that the electron bunch propagates in phase with the laser mode in addition to maintaining a constant undulator frequency. Furthermore, controlled dephasing is proposed as a means to induce chirp in the generated X-ray spectrum. [ABSTRACT FROM AUTHOR]

Medium-frequency waves in the ion-electron two-fluid plasma with frequencies between the electron gyrofrequency and the ion gyrofrequency are studied. The dispersion relations and the Friedrichs diagrams of the group velocities are plotted with color-coded electric-field information. The group-velocity distributions and the properties of the electric fields can help us explain the observed dynamic spectra of waves when the waves are generated by a localized broadband disturbance. Possible sources of such disturbances are explored. One of them is the magnetic trapped energetic electrons with pitch angles greater than 70°. The dynamic spectra of the waves generated by these energetic electrons are ray-angle dependent. For the ray angle greater than 2°, the dynamic spectra of the rising-tone quasielectromagnetic chorus waves are characterized by a frequency gap near half of the electron gyrofrequency. The dynamic spectra of the rising tone chorus waves without the frequency gap can only be found when either the ray angle is less than 2° or when the lower-band chorus waves are quasielectrostatic. Lightning from the neutral atmosphere provides another type of localized disturbance. Depending on the magnetic latitude, lightning can result in two types of falling-tone electromagnetic whistler waves. One of them is the low- k ⊥ whistler waves with highly field-aligned phase velocity and group velocity. The other is the high- k ⊥ whistler waves with highly field-aligned group velocity but with phase velocity almost perpendicular to the background magnetic field. The results of this study will provide a road map for future studies of the waves in this frequency range. [ABSTRACT FROM AUTHOR]

Microwave sources transform the kinetic energy of an electron beam into microwaves through the interaction of electrons with a periodic slow wave structure (SWS). A metamaterial (MTM) waveguide is proposed for use in a microwave oscillator instead of a conventional periodic SWS that has been used for a long time to generate high power microwave radiation. MTMs have interesting properties such as the negative refractive index, low group velocity, and below cutoff propagation, among others. In this work, we study the interaction of a multibeam cathode with a set of MTM structures inside a cylindrical waveguide. We developed a structure comprising a number of MTM metallic plates that have periodicity in the axial direction and are repeated in the azimuthal coordinate. Using eigenmode simulations, we obtained negative dispersion around the operating frequency where the group velocity is negative and extremely small. The fully electromagnetic, relativistic particle-in-cell codes MAGIC and CST Particle Studio and the fully electromagnetic software tool CST Microwave Studio were used to obtain the results in this study. [ABSTRACT FROM AUTHOR]

This work continues a series of papers where we propose an algorithm for quasi-optical modeling of electromagnetic beams with and without mode conversion. The general theory was reported in the first paper of this series, where a parabolic partial differential equation was derived for the field envelope that may contain one or multiple modes with close group velocities. In the second paper, we presented a corresponding code PARADE (PAraxial RAy DEscription) and its test applications to single-mode beams. Here, we report quasi-optical simulations of mode-converting beams for the first time. We also demonstrate that PARADE can model splitting of two-mode beams. The numerical results produced by PARADE show good agreement with those of one-dimensional full-wave simulations and also with conventional ray tracing (to the extent that one-dimensional and ray-tracing simulations are applicable). [ABSTRACT FROM AUTHOR]

This work opens a series of papers where we develop a general quasi-optical theory for mode-converting electromagnetic beams in plasma and implement it in a numerical algorithm. Here, the basic theory is introduced. We consider a general quasimonochromatic multicomponent wave in a weakly inhomogeneous linear medium with no sources. For any given dispersion operator that governs the wave field, we explicitly calculate the approximate operator that governs the wave envelope ψ to the second order in the geometrical-optics parameter. Then, we further simplify this envelope operator by assuming that the gradient of ψ transverse to the local group velocity is much larger than the corresponding parallel gradient. This leads to a parabolic differential equation for ψ ("quasioptical equation") on the basis of the geometrical-optics polarization vectors. Scalar and mode-converting vector beams are described on the same footing. We also explain how to apply this model to electromagnetic waves in general. In the next papers of this series, we report successful quasioptical modeling of radio frequency wave beams in magnetized plasma based on this theory. [ABSTRACT FROM AUTHOR]

In this paper, we derive the quantum Vlasov equation as a differential equation of the Wigner function directly from the electromagnetic Schrödinger equation and apply it to the plasma waves propagating in the direction parallel to the ambient magnetic field. The upper branches of the L and R waves in the plot of (ω, k) space have dispersion relations similar to those of their respective classical waves, with only minor corrections. The lower R-wave branch also has a dispersion relation similar to that of the classical whistler wave for a small wavenumber k. However, the dispersion curve encounters a region of anomalous dispersion, exhibiting a negative group velocity, as k increases. Furthermore, the branch becomes a damping wave as k increases above a certain critical value and eventually the wave becomes ill-defined for larger k values. [ABSTRACT FROM AUTHOR]

An advanced focusing scheme, called a "flying focus," uses a chromatic focusing system combined with a broadband laser pulse with its colors arranged in time to propagate a high intensity focus over a distance that can be much greater than its Rayleigh length while decoupling the speed at which the peak intensity propagates from its group velocity. The flying focus generates a short effective pulse duration with a small diameter focal spot that co- or counter-propagates along the optical axis at any velocity. Experiments validating the concept measured subluminal (−0.09c) to superluminal (39c) focal spot velocities with a nearly constant peak intensity over 4.5 mm. Experiments that increased the peak intensity above the ionization threshold for gas demonstrated ionization waves propagating at the velocity of the flying focus. These ionization waves of any velocity overcome several laser-plasma propagation issues, including ionization-induced refraction. The flying focus presents opportunities to overcome current fundamental limitations in laser-plasma amplifiers, laser wakefield accelerators, photon accelerators, and high-order frequency conversion. [ABSTRACT FROM AUTHOR]

This paper presents a detailed study on the controlled experimental observation of drift wave instabilities in an inhomogeneous six pole cusp magnetic field generated by an in-house developed multi-pole line cusp magnetic field device [Patel et al., Rev. Sci. Instrum. 89, 043510 (2018)]. The device is composed of six axially symmetric cusps and non-cusp (in between two consecutive magnets) regions. The observed instability has been investigated in one of these non-cusp regions by controlling the radial plasma density gradient with changing pole magnetic field which is a unique feature of this device. It has been observed that the frequency of the instability changes explicitly with the density gradient. Moreover, the scale lengths of plasma parameters, frequency spectrum, cross-correlation function, and fluctuation level of plasma densities have been measured in order to identify the instability. The cross field drift velocity due to fluctuation in plasma parameters has been measured from the wave number-frequency S (kz, ω) spectrum and verified with the theoretical values obtained from density scale length formula. Further, from the S (kz, ω) spectrum, it has been found that the drift velocity alternates the sign in the consecutive non-cusp regions. [ABSTRACT FROM AUTHOR]

A self-consistent Hamiltonian model based on equations describing the coupled dynamics of whistler and lower frequency waves in inhomogeneous plasmas is built. On this basis, different aspects of whistler turbulence are studied, concerning mainly the development of modulational instabilities and the dynamics of envelope solitons in irregular plasmas. Numerical simulations based on the model show that modulational instabilities can lead to the generation of a beating of stable nonlinear whistlers propagating with a speed near the group velocity. The whistler envelope soliton is determined analytically, and its propagation in plasmas presenting random density fluctuations and weakly irregular density structures of different scales and amplitudes is studied, showing that the envelope is very weakly affected by these inhomogeneities, whereas the wavelengths and the amplitudes of the phase oscillations strongly vary. Moreover, simulations show for the first time that two whistler solitons moving with different but close velocities and colliding one with the other remain unchanged after this collision, independently of their initial amplitudes and velocities. Finally, we study the dynamics of sonic whistler envelope solitons and show that the propagation of their lower frequency perturbation is governed by a KdV-type equation. [ABSTRACT FROM AUTHOR]

An electron beam in the slow wave structure of a traveling wave tube may be subjected to absolute instability at the lower and upper band edges, where the group velocity is zero. From a careful re-examination of the immediate vicinity of these band edges, we use the Briggs-Bers criterion to show that, contrary to previous findings, an absolute instability may arise at the lower band edge if the beam current is sufficiently high, even if the beam mode intersects with the circuit mode with a positive group velocity. However, the upper band edge was found to be more susceptible to absolute instability than the lower band edge. The threshold condition for the onset of absolute instabilities is derived analytically at both band edges. The Green's function shows possible transient temporal growth with an exponentiation rate proportional to t1/3, whether or not the band edge is subject to an absolute instability. [ABSTRACT FROM AUTHOR]

The combined effects of pulse slippage and the transverse magnetic field are studied on terahertz radiation excitation by nonlinear beating of two cosh-Gaussian (ChG) laser pulses propagating in a corrugated plasma. The beating lasers exert nonlinear ponderomotive force on plasma electrons. The oscillating electrons couple with corrugations present in the plasma and resonantly excite a nonlinear current (at different frequencies) which drive the terahertz wave at proper phase matching conditions. As the group velocity of THz radiation is higher than the group velocity of beating lasers, the THz pulse slips forward the pump lasers, and its saturation takes place. The effects of THz wave frequency, the decentred parameter (of beating lasers), the periodicity of the density structure, and the applied dc magnetic field are studied on THz emission. An efficiency of ~10-4 is achieved for a laser intensity of ~2×1015 W/cm2 (at a laser wavelength of ~10.6 μm for the CO2 laser). [ABSTRACT FROM AUTHOR]

The (low frequency) zonal flow-ion temperature gradient (ITG) wave system, constructed on Braginskii's fluid model in tokamak, is shown to be a reaction-diffusion-advection system; it is derived by making use of a multiple spatiotemporal scale technique and two-dimensional (2D) ballooning theory. For real regular group velocities of ITG waves, two distinct temporal processes, sharing a very similar meso-scale radial structure, are identified in the nonlinear self-organized stage. The stationary and quasi-stationary structures reflect a particular feature of the poloidal group velocity. The equation set posed to be an initial value problem is numerically solved for JET low mode parameters; the results are presented in several figures and two movies that show the spatiotemporal evolutions as well as the spectrum analysis--frequency-wave number spectrum, auto power spectrum, and Lissajous diagram. This approach reveals that the zonal flow in tokamak is a local traveling wave. For the quasi-stationary process, the cycle of ITG wave energy is composed of two consecutive phases in distinct spatiotemporal structures: a pair of Cavitons growing and breathing slowly without long range propagation, followed by a sudden decay into many Instantons that carry negative wave energy rapidly into infinity. A spotlight onto the motion of Instantons for a given radial position reproduces a Blob-Hole temporal structure; the occurrence as well as the rapid decay of Caviton into Instantons is triggered by zero-crossing of radial group velocity. A sample of the radial profile of zonal flow contributed from 31 nonlinearly coupled rational surfaces near plasma edge is found to be very similar to that observed in the JET Ohmic phase [J. C. Hillesheim et al., Phys. Rev. Lett. 116, 165002 (2016)]. The theory predicts an interior asymmetric dipole structure associated with the zonal flow that is driven by the gradients of ITG turbulence intensity. [ABSTRACT FROM AUTHOR]

Antenna radiation patterns in the vicinity of a helicon antenna are investigated in hydrogen plasmas produced in the MAGPIE linear plasma device. Using a uniform cold-plasma full-wave code, we model the wave physics in MAGPIE and find good agreement with experimental wave measurements. We show for the first time which antenna elements in a helicon device couple most strongly to the plasma and discuss the physical mechanism that determines this effect. Helicon wavefields in the near field of the antenna are best described in terms of the group velocity and ray direction, while far from the antenna, helicon wavefields behave like plane waves and are best described in terms of eigen-modes. In addition, we present recent 2D axis-symmetric full-wave simulations of the 120kW helicon source in ProtoMPEX [Rapp et al., IEEE Trans. Plasma Sci. 44(12), 3456-3464 (2016); Caughman et al., J. Vac. Sci. Technol. Vac. Surf. Films 35, 03E114 (2017) and Goulding et al., Fusion Sci. Technol. 72(4), 588-594 (2017)] (ne ∼ 5x1019m-3, B0 ∼ 70 mT and f = 13.56 MHz) where the antenna radiation patterns are evident, and we provide an interpretation of the numerical results using the ideas developed in this paper. [ABSTRACT FROM AUTHOR]

The experimental observation of second order ion acoustic Peregrine breathers in multicomponent plasma with negative ions is reported. A long wavelength initial perturbation on a continuous carrier frequency ~0.5 ωpi (where ωpi is the ion plasma frequency) of finite amplitude is found to undergo self-modulation due to the interplay between nonlinear dispersive effect and group velocity dispersion because of modulational instability. Wave energy focusses to a smaller localized and isolated group of waves within the packet with amplitude amplification up to 5 times of the background carrier wave. The experimental results are compared with second order breather solution of nonlinear Schrodinger equation. The wavelet analysis and fast Fourier transform analysis of the experimental time series data indicate strong nonlinear evolution (wave energy focusing and spectral broadening) conforming to the formation of second order Peregrine solitons. [ABSTRACT FROM AUTHOR]