Showing papers on "Relativistic plasma published in 1988"

Radio emission from conical jets associated with X-ray binaries

Abstract: Kinematic and physical models for the evolution of synchrotron radio emission in conical twin jets are derived and are shown to have most of the characteristics needed to explain the wide variety of radio properties in X-ray binaries; the spatial distribution of SS 433's radio jets; periodic emission from LSI + 61 deg 303 and quiescent Cyg X-3; and the stable quiescent emission from Cyg X-1, Cyg X-2, and Sco X-1. These models assume adiabatic expansion and are distinguished from other jet models by the dominance of lateral motions due to free or slowed expansion of a collimated flow of a hot, X-ray emitting gas. A laterally expanding environment, behaving like a supernova remnant expanding in a cylindrical geometry, is the location of steady production of relativistic plasma, and, when the relativistic plasma pressure is dominant, this geometry can provide the spherically expanding plasmoids causing radio flares. 16 references.

Wave-breaking amplitude of relativistic oscillations in a thermal plasma.

Abstract: The maximum amplitude of relativistic plasma oscillations (${\ensuremath{\upsilon}}_{\mathrm{ph}}$ near $c$) is obtained with a combined one-dimensional waterbag and warm-fluid model. The waterbag description is used to obtain expressions for the pressure and internal energy as functions of the proper density. A relativistic Euler's equation that is valid for arbitrarily large amplitudes and an analytic expression for the wave-breaking amplitude in the limit ${\ensuremath{\upsilon}}_{\mathrm{ph}}\ensuremath{\cong}c$ are obtained. Even a small amount of thermal energy can significantly reduce the maximum plasma-wave amplitude relative to the cold wave-breaking value. The significance of the results for recent accelerator schemes is discussed.

Evolution of self-focusing of intense electromagnetic waves in plasma

Abstract: The self-focusing of intense electromagnetic waves in a very underdense plasma is studied in computer simulations. Relativistic self-focusing occurs initially. This is followed by ''ponderomotive blowout'' and filamentation at the edge of the channel walls. The self-focusing is more intense for resonant double-frequency than for single-frequency illumination.

Global existence for the relativistic Vlasov-Maxwell system with nearly neutral initial data

Abstract: Global classical solutions to the initial value problem for the relativistic Vlasov-Maxwell equations are obtained in three space dimensions. The initial distribution of the various species may be large, provided that the total positive charge nearly cancels the total negative charge.

Theory of the radio emission of pulsars

Abstract: A consistent theory of excitation, stabilization, and propagation of electromagnetic oscillations in a relativistic one-dimensional electron-positron plasma flowing along curved magnetic field lines is presented. It is shown that in such a medium which is typical of the magnetosphere of a neutron star there exist unstable natural modes of oscillations. Nonlinear saturation of the instability leads to an effective energy conversion into transverse oscillations capable of leaving the magnetosphere of a pulsar. The polarization spectrum and the directivity pattern of generated radiation are determined. A comparison with observations has shown that the theory makes it possible to explain practically all the basic characteristics of observed pulsar radio emission.

Relativistic magnetosonic solitons with reflected particles in electron–positron plasmas

Abstract: The presence of magnetically reflected particles is shown to allow the existence of large amplitude magnetosonic solitary waves in relativistic electron–positron plasmas. If the flow is assumed to contain a single loop of gyrating particles, self‐consistent structures are found with peak field amplitudes (B/B∞)max<(11)1/2, where B∞ is the magnitude of the upstream magnetic field. In contrast, without reflected particles, the amplitude of a relativistic magnetosonic soliton is restricted to (B/B∞) −1<2/γ∞, where γ∞ is the upstream Lorentz factor. Therefore, if γ∞≫1, reflected particles greatly increase the allowable amplitudes of these nonlinear waves. It is also shown that when γ∞≫1, the wave properties are independent of γ∞, and are completely parametrized by the ratio of the Poynting flux to the kinetic energy flux in the upstream flow. Some new features of solitary waves without reflected particles are also derived, and a heuristic model is presented which gives a simple physical interpretation of many...

Operating modes of relativistic rising-sun and A6 magnetrons

Abstract: The operating characteristics of a relativistic 16-vane rising-sun magnetron were investigated with particular emphasis on determining the operating regimes of different modes. The magnetron performance was studied as a function of voltage, magnetic field, cathode geometry, axial boundary conditions, and output coupling. Operation was observed in the 3 pi /8 mode at 3.3 GHz, in the pi /2 or 3 pi /8 mode at 3.5 GHz, and in the pi or 7 pi /8 mode at 4.6 GHz. A maximum power of 80 MW was emitted in the 3 pi /8 mode with an efficiency of 4.5%. Typical pulse lengths were 40-50 ns. Cold tests were performed to measure the resonant frequencies and azimuthal electric fields in the interaction space which agreed within 1-4% of theoretical calculations. The operating modes were inferred from close agreement between hot-test frequencies and cold-test results and because high-power RF emission occurred at, or just above, the Buneman-Hartree threshold calculated for these modes. The characteristics of a six-vane A6-magnetron operating in the pi and 2 pi modes were also studied. A unique transmitting-receiving system, which was used as a microwave diagnostic, is described. >

Experimental confirmation of the reditron concept

Abstract: A description is given of experiments demonstrating a method for producing high-power microwave emission. The unstable oscillations of a virtual cathode, which forms when a magnetized relativistic electron beam is injected into a circular waveguide, generates the microwave radiation. In contrast to other virtual-cathode microwave-generation techniques, electrons in the waveguide are prevented from reflexing back into the diode region by use of a slotted range-thick anode. Electrons injected into the waveguide are guided through the slot by an applied magnetic field, while reflected electrons, under the proper conditions, are intercepted by the anode. Several advantages of this approach are described, and experimental confirmation of this mode of high-power microwave generation is demonstrated. Data showing frequency scaling with beam parameters and magnetic field are also presented. Using this technique, 1.4 GW was produced at 3.9 GHz with several hundred megawatts radiated in harmonic radiation. >

Particle simulations of plasma and dielectric Čerenkov masers

Abstract: A new particle simulation model has been developed to investigate Cerenkov masers. The novel aspects of the code are briefly described, and results of simulations of two types of Cerenkov masers are presented. The first device uses a conventional dielectric lining as its slow‐wave structure. The second is a new type of Cerenkov maser in which a circular waveguide is partially filled with a dense annular plasma instead of a dielectric layer. Both simulations agree well with experimental results and linear theory calculations. Saturation of the instability is shown to be due to trapping of the beam electrons. The relative merits of each system are discussed.

Propagation of solitary waves in relativistic plasmas

Abstract: By applying a reductive perturbation technique to the basic system of equations governing the plasma dynamics, a modified Korteweg-de Vries (K-dV) equation has been derived in relativistic plasma that includes cold ions and warm nonisothermal electrons. By reducing the effect of nonisothermality, the authors demonstrate the modification of the K-dV equation into different forms which show how to link the behavior of ion-acoustic waves in nonisothermal plasmas with that in isothermal plasmas. >

Weakly relativistic dispersion of Bernstein waves

Abstract: Weakly relativistic effects on the dispersion of Bernstein waves are investigated for waves propagating nearly perpendicular to a uniform magnetic field in a Maxwellian plasma. Attention is focused on those large‐wave‐vector branches that are either weakly damped or join continuously onto weakly damped branches since these are the modes of most interest in applications. The transition between dispersion at perpendicular and oblique propagation is examined and major weakly relativistic effects on dispersion at perpendicular propagation are found to persist at other angles; these effects can dominate even in low‐temperature plasmas. A number of simple analytic criteria are obtained which delimit the ranges of harmonic number and propagation angle within which various types of weakly damped Bernstein modes can exist.

Diagnosing superstrong turbulence in plasma by forbidden line measurements

Abstract: Observations of satellites of forbidden helium lines yield a new diagnostic of superstrong turbulence in a relativistic beam–plasma system. Comparing the satellite‐to‐allowed line intensities allows estimates of the volume fraction of atoms that experience high Langmuir fields, i.e., 〈E2〉≥4πnTe. An independent estimate follows from counting the total number of events, and using the calculated optical efficiency of the system, this ratio is estimated independently—agreement is good. This ratio may be the packing fraction of high field cavitons. Its value (∼0.1) implies that collision processes may be important to caviton dynamics.

Relativistic structure of stochastic wave–particle interaction

Abstract: Stochastic interactions of charged particles with electrostatic waves propagating at arbitrary angles to an external magnetic field are studied based on a relativistic canonical Hamiltonian formalism. The present theory, however, is valid also for electromagnetic waves after a slight modification. The stochasticity threshold is derived utilizing Chirikov’s criterion. It is found that relativistic effects are important for electrons interacting with relatively high phase velocity waves even at nonrelativistic initial energies. In particular, the relativistic generalization of a previous theory [Phys. Rev. Lett. 34, 1613 (1975); Phys. Fluids 21, 2230 (1978)] moves the degeneracy of primary resonances in nearly perpendicular directions to the angles where the parallel phase speed approximately equals the speed of light. It was also demonstrated for the first time that initially low energy electrons can gain relativistic energies (γ≫1) by means of the stochastic interaction with an electrostatic wave, where γ...

Higher-order corrections to the ion-acoustic waves in a relativistic plasma (isothermal case)

Abstract: As a continuation of our earlier work, we have analysed the higher-order perturbative corrections to the formation of (ion-acoustic) solitary waves in a relativistic plasma. It is found that the relativistic considerations affect the amplitude and width variation - as conjectured in our previous paper. Our analysis employs a higher-order singular perturbation technique, with the elimination of secular terms in stages. In this way we arrive at an inhomogeneous KdV-type equation, which is then solved exactly. At this point a new phenomena arises at a critical value of the phase velocity at which the coefficient of the nonlinear term in the KdV equation vanishes. A new set of stretched co-ordinate is then used to derive a modified KdV equation. In both cases we have numerically computed the specific physical profile of the new solitary wave and its width.

Diocotron instability for intense relativistic non‐neutral electron flow in planar diode geometry

Abstract: The extraordinary‐mode eigenvalue equation is used to investigate detailed properties of the diocotron instability for sheared, relativistic electron flow in a planar diode. The theoretical model is based on the cold‐fluid‐Maxwell equations assuming low‐frequency flute perturbations about a tenuous electron layer satisfying ω2pb(x)≪ω2c and ‖ω−kVy(x)‖2≪ω2c. The cathode is located at x=0; the anode is located at x=d; the outer boundary of the electron layer is located at x=x+b

Numerical simulations of the reditron

Abstract: The reflected-electrons discrimination microwave generator (reditron) is a high-power, narrow-band, and single-mode microwave generator that makes exclusive use of the oscillatory character of the virtual-cathode of a relativistic electron beam. The complex, nonlinear character of the virtual-cathode device necessitates particle-in-cell plasma simulation techniques. Investigations indicate two sources of the radiation: (1) the trapped electrons reflexing between the real and virtual cathodes, and (2) the oscillation of the virtual cathode. In the conventional design, the two mechanisms coexist and interfere with each other destructively, causing degradation of the efficiency of microwave generation. The authors have investigated a configuration with a slotted, thick anode and an external magnetic field, which effectively eliminates the reflexing electrons. Two-dimensional particle-in-cell simulations showed that such a configuration exploits the oscillation of the virtual cathode exclusively, and it generates single-mode, narrow bandwidth, and high-power microwave radiation with a potential efficiency over 10%. It was found that further optimization could be achieved by the use of a density (current) modulated electron beam at appropriate frequencies. >

An exact solution to the relativistic equation of motion of a charged particle driven by a linearly polarized electromagnetic wave

Abstract: An exact analytic solution is found for a basic electromagnetic wave-charged particle interaction by solving the nonlinear equations of motion. The particle position, velocity, and corresponding time are found to be explicit functions of the total phase of the wave. Particle position and velocity are thus implicit functions of time. Applications include describing the motion of a free electron driven by an intense laser beam. >

Direct experimental observation of pi -mode oscillation in a relativistic magnetron

Abstract: The authors report the direct measurement of the operating mode of a relativistic magnetron. The S-band 2.65-GHz magnetron used has a washer cathode and is driven by a 1-MV 2.8- Omega 65-ns pulser. Power and pulse shape are measured simultaneously by calibrated couplers, cables, and crystal diodes. The frequency is measured as the heterodyne or difference frequency between a local oscillator and the extracted signal. This difference frequency is several-hundred megahertz. By directly measuring the phase relationship, any questions concerning perturbations of the resonance by the large current of relativistic electrons are avoided. >

Cubic dispersion relation for a relativistic backward-wave oscillator

Abstract: The cubic approximation to the dispersion relation for a relativistic backward-wave oscillator is obtained, and the utility and limits of the approximation are presented. The approximation is obtained by Taylor series expansion of the wave admittance in the dispersion relation for the transverse-magnetic and free-streaming modes of a relativistic, thin, hollow, cylindrical electron beam moving along the axis of a disc-loaded waveguide in a strong axial magnetic field. The resulting cubic dispersion relation yields instability growth rates and frequencies which fall off beyond their maximum more sharply with increasing wavenumber than for the complete dispersion relation. The approximation is found to be quite good near the operating points of contemporary high-power relativistic backward-wave oscillators, namely, for relatively long wavelength and small ratio of Budker's parameter to the relativistic gamma factor of the beam. >

Method of accelerating photons by a relativistic plasma wave

Abstract: Photons of a laser pulse have their group velocity accelerated in a plasma as they are placed on a downward density gradient of a plasma wave of which the phase velocity nearly matches the group velocity of the photons This acceleration results in a frequency upshift If the unperturbed plasma has a slight density gradient in the direction of propagation, the photon frequencies can be continuously upshifted to significantly greater values

Dynamics of nonlinearly excited plasma waves

Abstract: In an underdense plasma a large‐amplitude plasma oscillation may be produced by the beating of two electromagnetic waves with a frequency difference approximately equal to the plasma frequency. In the spatially one‐dimensional, cold, and collisionless plasma the large‐amplitude plasma oscillation is limited by the nonlinearity caused by relativistic effects. In this paper a simple nonlinear equation, resembling the original equation of Rosenbluth and Liu [Phys. Rev. Lett. 29, 701 (1972)], is derived in the weak beat power limit from the fully relativistic fluid model proposed by Sprangle, Sudan, and Tang [Appl. Phys. Lett. 45, 375 (1984); Phys. Fluids 28, 1974 (1985)]. This equation also contains the effects of the relativistic transverse motion. Its analytical solution, describing the plasma oscillation dynamics, is given in a closed form by using Jacobian elliptic functions. The analytical computations are compared with numerical computations. Finally the fully relativistic equation, describing free pla...

Wave–plasma interaction near the second electron cyclotron harmonic

Abstract: The linear wave propagation and spatial damping near the second electron cyclotron harmonic in a weakly relativistic Maxwellian plasma for an arbitrary angle of wave incidence are investigated. It is found that mode interaction with linear conversion of the extraordinary mode into a quasilongitudinal mode occurs over a relatively large range of plasma parameters. The absorption properties of the extraordinary and ordinary modes and their scaling laws are examined. The noninductive current drive by electron cyclotron waves is also discussed. The results obtained indicate quite favorable features of the wave–plasma interaction near the second electron cyclotron harmonic for plasma heating, current drive, and diagnostics applications.

Double-layer disk and magnetic field dynamics in the crab nebula

Abstract: The mechanism of the large-scale magnetic field generation in the Crab Nebula is proposed. The basis for the considered ‘fast’ mechanism is the model of the central region of Crab Nebula amorphous part having the form of slightly divergent double-layer disk consisting of the relativistic electron-positron plasma.

High-frequency emissions during the propagation of an electron beam in high-density plasma

Abstract: A relativistic annular electron beam passing through a high-density plasma excites Langmuir waves via Cerenkov interaction. The Langmuir waves are backscattered off ions via nonlinear ion Landau damping. At moderately high amplitudes these waves are parametrically up-converted by the beam into high-frequency electromagnetic radiation, as observed in some recent experiments. A nonlocal theory of this process is developed in a cylindrical geometry. It is seen that the growth rate of the Langmuir wave scales as one-third the power of beam density. The growth rate of parametric instability scales as one-fourth the power of beam density and the square root of beam thickness. >

Quasiboson excitation spectrum of the relativistic quantum scalar plasma in the Hartree-Vlasov approximation

Abstract: Using plasma-physics techniques (e.g., covariant Wigner functions) the excitation spectrum of the quasibosons within the so-called relativistic quantum scalar plasma (i.e., a system of spin-(1/2 fermions interacting with self-coupled scalar bosons via a Yukawa-type interaction) is derived both in the linearized Hartree-Vlasov approximation (random-phase approximation) and in perturbation at order 2. These spectra are compared and discussed in two cases: (i) in the ''semiclassical'' cases where vacuum effects are discarded and (ii) in the renormalized cases. Except at low densities where both perturbative and nonperturbative results agree, in general, strongly different behaviors occur.

Particle simulations of merging ion rings in resistive plasmas

Abstract: A numerical study, employing the particle code ringa [Plasma Phys. 23, 521 (1981)], of two ion rings that merge with each other due to their mutual magnetic attraction is presented. It is shown that an ion ring with closed magnetic field lines (field reversed) can be formed by merging two weaker non‐field‐reversed rings. A rough calculation shows that there is an optimal plasma conductivity σ≊c2/4πaRΩζ1/2 (a and R are the respective axial width and major radius of the ring, Ω is the ion cyclotron frequency in the external magnetic field, and ζ is the field reversal factor of the weaker ring) for which most of the energy of mutual magnetic attraction between the rings is dissipated during the process of merging.

Relativistic theory of Alfvén solitons

Abstract: It is shown that in a collisionless magnetoactive electron-ion plasma the nonlinear stage of propagation of a large-amplitude Alfven wave, in the field of which the oscillation velocities of the plasma components may approach the velocity of light (a realistic situation for the plasma in a number of cosmic objects), is described by a Schroedinger equation with nonlinearity of the derivative and with nonzero boundary conditions at infinity Some simple soliton structures (and also solutions of the type of simple waves) for this equation are considered, and it is shown that relativistic effects strongly influence the formation of nonlinear waves in the plasma

Amplification of whistler waves propagating through inhomogeneous, anisotropic, mirror‐confined hot‐electron plasmas

Abstract: A fully relativistic local dispersion relation for whistler waves has been solved at closely spaced points along the magnetic field lines of a 2:1 magnetic mirror in which a highly anisotropic, spatially inhomogeneous, hot‐electron plasma is confined. The limiting plasma parameters for convective (spatial) growth have been determined numerically and used to identify plasma conditions leading to maximum amplification of input microwave signals introduced in the form of whistler waves. The maximum gain has been evaluated numerically for a range of values of the hot‐electron plasma within which all major stability criteria are satisfied. Very high gains (∼40 dB) are indicated over the entire range of beta investigated.