Search Results (27 total)

A high-gain free-electron laser is modeled using an expansion of the radiation field in terms of guided Laguerre-Gaussian modes of a virtual dielectric waveguide [E. Hemsing, A. Gover, and J. Rosenzweig, preceding paper, Phys Rev. A 77, 063830 (2008)]. The radiation profile evolution, power gain, and detuning efficiency characteristics are investigated for seeding with fundamental Gaussian and higher-order Laguerre-Gaussian input modes on a Gaussian e-beam in the collective regime. The full wave evolution solution at different seed radiation injection conditions results in determination of the optimal waist size and waist position of the seed radiation beam for maximum power coupling efficiency. Results for guided mode evolution and power gain are shown to be consistent with simulations performed with the code GENESIS 1.3. The amplification and spontaneous generation of FEL radiation with orbital angular momentum is also considered.

A set of mode-coupled excitation equations for the slowly growing amplitudes of dielectric waveguide eigenmodes is derived as a description of the electromagnetic signal field of a high-gain free-electron laser (FEL), including the effects of longitudinal space charge. This approach of describing the field basis set has notable advantages for FEL analysis in providing an efficient characterization of eigenmodes, and in allowing a clear connection to free-space propagation of the input (seeding) and output radiation. The formulation describes the entire evolution of the radiation wave through the linear gain regime, prior to the onset of saturation, with arbitrary initial conditions. By virtue of the flexibility in the expansion basis, this technique can be used to find the direct coupling and amplification of a particular mode. A simple transformation converts the derived coupled differential excitation equations into a set of coupled algebraic equations and yields a matrix determinant equation for the FEL eigenmodes. A quadratic index medium is used as a model dielectric waveguide to obtain an expression for the predicted spot size of the dominant system eigenmode, in the approximation that it is a single Gaussian mode.

Gravitational interactions of higher spin fields are generically plagued by inconsistencies. There exists however, a simple framework that couples higher spins to a broad class of gravitational backgrounds (including Ricci flat and Einstein) consistently at the classical level. The model is the simplest example of a Yang-Mills detour complex and has broad mathematical applications, especially to conformal geometry. Even the simplest version of the theory, which couples gravitons, vectors and scalar fields in a flat background is rather rich, providing an explicit setting for detailed analysis of ghost excitations. Its asymptotic scattering states consist of a physical massless graviton, scalar, and massive vector along with a degenerate pair of zero norm photon excitations. Coherent states of the unstable sector do have positive norms, but their evolution is no longer unitary and amplitudes grow with time. The class of models proposed is extremely general and of considerable interest for ghost condensation and invariant theory.

The problems of spin-polarized free-electron beam interaction with electromagnetic wave at electron-spin resonance conditions in a magnetic field and of superradiant spin-flip radiative emission are analyzed in the framework of a comprehensive classical model. The spontaneous emission of spin-flip radiation from electron beams is very weak. We show that the detectivity of electron spin resonant spin-flip and combined spin-flip/cyclotron-resonance-emission radiation can be substantially enhanced by operating with ultrashort spin-polarized electron beam bunches under conditions of superradiant (coherent) emission. The proposed radiative spin-state modulation and the spin-flip radiative emission schemes can be used for control and noninvasive diagnostics of polarized electron/positron beams. Such schemes are of relevance in important scattering experiments off nucleons in nuclear physics and off magnetic targets in condensed matter physics.

Radiative emission from the magnetic moments of the spins of an electron beam has never been observed directly, because it is fundamentally much weaker than the electric charge emission. We show that the detectivity of spin-flip and combined spin-flip–cyclotron-resonance-emission radiation can be substantially enhanced by operating with ultrashort spin-polarized electron beam bunches under conditions of superradiant (coherent) emission. The proposed superradiant spin-flip radiative emission scheme can be used for noninvasive diagnostics of polarized electron or positron beams. Such beams are of relevance in important scattering experiments off nucleons in nuclear physics and off magnetic targets in condensed matter physics.

We report experimental studies of the spectral linewidth and chirp characteristics of the mm-wave rf radiation of the Israeli Electrostatic-Accelerator free electron laser (EA-FEL), along with theory and numerical simulations. The simulations, matching the experimental data, were carried out using a space-frequency-domain model. EA-FELs have the capacity to generate long pulses of tens microseconds and more, that in principle can be elongated indefinitely (cw operation). Since a cold beam FEL is by nature a “homogeneously broadened laser,” EA-FEL can operate, unlike other kinds of FELs, at a single longitudinal mode (single frequency). This allows the generation of very coherent radiation. The current status of the Israeli Tandem Electrostatic-Accelerator FEL, which is based on an electrostatic Van de Graaff accelerator, allows the generation of pulses of tens microseconds duration. It has been operated recently past saturation, and produced single-mode coherent radiation of record narrow inherent relative linewidth ∼Δf/f=10^-6 at frequencies near 100 GHz. A frequency chirp was observed during the pulses of tens of microseconds (0.3–0.5  MHz/ms). This is essentially a drifting “frequency-pulling effect,” associated with the accelerator voltage drop during the pulse. Additionally, damped relaxation of the FEL oscillator was experimentally measured at the beginning and the end of the lasing pulse, in good correspondence to our theory and numerical simulations. We propose using the chirped signal of the pulsed EA-FEL for single pulse sweep spectroscopy of very fine resolution. The characteristics of this application are analyzed based on the experimental data.

Further enhancement of the intense coherent superradiant and stimulated-superradiant emission from prebunched electron beams is possible, in schemes of prebunched beam radiation devices, and particularly free electron laser (FEL). The enhancement of coherent power and spectral power by use of a waveguide, particularly at the zero-slippage condition, is evaluated. A special scheme of a stimulated-superradiance FEL oscillator is analyzed and is shown to feature ultimate radiative energy conversion efficiency (near 100%).

A formulation for the characterization of superradiant and stimulated-superradiant radiative emission from bunched electron beams is presented. The radiation is characterized in terms of power and spectral power per radiation mode, which provide a measure of the useful spatially coherent radiation power and spectral power emitted by a radiation source. When the bunched electron beam emits superradiantly, these parameters scale like the square of the number of electrons, orders of magnitude more than spontaneous emission. The formulation applies to emission from single bunches, a finite number of bunches in a macropulse, or periodic bunching. It can be employed on any kind of e-beam radiation scheme. Specific analytic expressions are derived for coherent synchrotron radiation and prebunched free-electron laser, providing a basis for comparing and understanding their connection.

An electron beam, prebunched at the synchronous free-electron laser frequency and passing through a magnetic undulator, emits coherent (superradiant) synchrotron undulator radiation at the bunching frequency. If an external electromagnetic wave is introduced into the interaction region, at the same frequency and at a proper phase, the radiation process will be stimulated (stimulated prebunched beam radiation). We report first experimental measurements of stimulated superradiant emission in a prebunched free-electron maser. Measurements are in good agreement with theory.

Single mode operation was exhibited on the Israeli tandem free-electron laser (FEL). This enabled judicious measurements of narrow spectral linewidth, frequency chirp, and relaxation-oscillation effects. Exact 3D simulations of the FEL oscillator showed good agreement with the measurements, and permitted an estimation of the fundamental Schawlow-Towns linewidth limit of the FEL (including “ α effect”) as well as technical noise linewidth limits. We estimate that with voltage-controlled stabilization high-power (10 kW) tunable (over 60% bandwidth) quasi-cw coherent [(Δν/ν)_rms≈10^-10] mm-FIR (far infrared) radiation is attainable in the tandem FEL.

We derive the conditions for establishment of a transverse supermode in a free-electron laser (FEL) oscillator, and demonstrate the evolution of a supermode by means of a three-dimensional nonlinear code. Both the analytical formulation and the numerical code are based on coupled-mode theory. The oscillator supermode is a combination of transverse modes that keeps its field profile at any point along the oscillator intact after each round trip, and therefore it is the steady-state result of the oscillation buildup process. In the FEL, as in any laser, the oscillator supermode is identical with the amplifier supermode only if the feedback process is entirely nondispersive. If this is not the case, the steady-state supermode field profile varies along the oscillator axis. The simulations demonstrate that the transverse supermode evolution process is primarily a linear regime process and can be proceeded or even completed before saturation.

The effects of electron-beam prebunching on the radiation buildup process in a free-electron maser oscillator operating in the frequency range of 4–5 GHz were studied. An electron beam of 10 keV, 1 A was prebunched by means of a microwave tube section, accelerated to 70 keV, transported through a drift region by a solenoidal magnetic field and injected into a linearly polarized wiggler in which a free-electron laser interaction takes place. Single-frequency selection and locking due to electron-beam prebunching was demonstrated; we show that by tuning the electron-beam prebunching frequency we can set the oscillator frequency at any of the resonant frequencies of the cavity within the net gain bandwidth. An appreciable speedup of the oscillation buildup due to prebunching was also observed; the buildup time of radiation in the presence of prebunching is shortened compared to the case of the free-running oscillator. ©1996 The American Physical Society.

We report a first demonstration of single-mode selection in a free-electron maser (FEM) using electron-beam prebunching at or near the natural oscillation frequencies of the resonator. The FEM oscillation frequency can be selectively locked to each eigenfrequency of the resonant waveguide cavity within the frequency band of the FEM net gain. When the electron beam is prebunched at a frequency close to an eigenfrequency of the cavity, the oscillation buildup process is sped up and the radiation buildup time is shortened significantly. Measurements are in good agreement with collective (Raman) free-electron laser theory.

An analytical three-dimensional model is presented for free-electron lasers (FELs) operating in the small-signal linear regime. The excitation of radiation and space-charge waves is found by expanding the total electromagnetic field in terms of transverse eigenmodes in a waveguide of arbitrary cross section and solving the evolution of their amplitudes from a set of coupled excitation equations. Coupled-mode theory is employed to derive dispersion relations for the space-charge waves and for the gain. The eigenmodes of the FELs (‘‘supermodes’’) and the gain for each of them are derived after diagonalization of the coupled-mode system. It is found that for the case of degenerate coupled modes (equal axial wave numbers), the normal modes satisfy the well known FEL gain dispersion equation with a modified gain parameter. The gain of the supermode, calculated according to the presented coupled-mode theory, is higher than the gain of the individual modes if calculated on the basis of a single-mode model. We demonstrate the formalism by finding the gain of the TE₀₁, and the coupled TE₂₁ and TM₂₁ modes excited simultaneously in a rectangular waveguide.

An alternative mechanism for Smith-Purcell radiation is proposed. This mechanism may have relevance to recent reports of higher radiation power. The electron beam excites resonant transitions of atomic quantum levels in the optical grating material by the fields of the traversing electrons. The dipole moments of all the atoms which are excited by the same electron radiate in phase with each other and produce ‘‘super-radiant radiation.’’ To calculate the radiant intensity due to this process we first calculate the dipole moments of the atoms excited by the classical electrical field of the traversing electron. Assuming that the dipole oscillations are dominated by a collision time T₂ we calculate the classical radiant intensity from the optical gratings due to this process. Sample numerical calculations based on a ruby grating result in substantial radiation levels.

The coherent synchrotron radiation process in a waveguide is theoretically investigated. A single, short bunch propagating through a wiggler is considered. In a waveguide, two very distinct regimes are possible. At grazing, where the beam velocity matches the wave group velocity, the bunch emits a single, ultrashort chirped pulse whose duration is determined by the interaction bandwidth and the waveguide dispersion. Away from grazing, where slippage dominates, two distinct pulses are radiated at the Doppler upshifted and downshifted frequencies. Both the time and frequency domain expressions for the radiation characteristics are derived.

An alternative approach for the analysis of the electromagnetic field and plasma-wave propagation in a waveguide filled with an electron (e) beam is presented. The analysis is based on a formal exact expansion of the total electromagnetic field in terms of waveguide modes. We subsequently use linear fluid plasma equations and electromagnetic coupled-mode theory to find the dispersion relation for the eigenmodes of the beam (plasma) loaded waveguide. The proposed method enables one to solve for the Langmuir space-charge waves in an e beam with an arbitrary transverse geometry and density distribution, moving along any uniform-cross-section waveguide at constant average velocity. The use of the method is demonstrated by presenting a calculation of the dispersion curve and the plasma frequency reduction factor of plasma modes in a practical case of a circular beam drifting along a rectangular waveguide.

A Comment on the Letter by D. B. Chang and J. C. McDaniel, Phys. Rev. Lett. 63, 1066 (1989).

A comparative analysis of wave profile modification effects in Raman free-electron lasers is presented. The analysis is based on a 3D theoretical model that is valid in both Raman and Compton regimes. We study two companion effects, the optical guiding and the excitation of space-charge waves with transverse field components. Both effects are compared through exemplary parameters based on previous free-electron laser experiments. We conclude that transverse field profile modification due to space-charge waves may be significant in comparison to the optical guiding effect.

We report the observation and investigation of synchronous energy exchange between nonrelativistic electrons and the ponderomotive (beat) force of two counterpropagating intense pulsed CO₂ laser beams, operating at different frequencies in a stimulated Compton-scattering scheme. The interaction occurred in the nonlinear (trapping) regime, the physics of which is the same as that which occurs in laser accelerators and efficiency-enhanced free-electron lasers (FEL’s) with long wigglers. The experiment is a first demonstration of the principle of inverse FEL acceleration and electromagnetic pump FEL operation in the nonlinear (trapping) regime. It can also be described as a demonstration of a ‘‘traveling beat-wave’’ Kapitza-Dirac effect in the nonlinear regime. Two different mechanisms of enhanced energy transfer were observed—electron trapping and phase-area displacement. Experimental results and computer simulations of both mechanisms are presented.

This article presents a unified formulation and review of an extensive class of radiation effects and devices based on free or quasifree electrons. The effects and devices reviewed include slow-wave radiators [such as Čerenkov, Smith-Purcell, and TWT (traveling-wave tube) effects and devices], periodic bremsstrahlung radiators [such as undulator radiation, magnetic bremsstrahlung FEL's (free-electron lasers), and coherent bremsstrahlung in the crystal lattice], and transverse-binding radiators [such as the CRM (cyclotron resonance maser) and channeling radiation]. Starting from a general quantum-electrodynamic model, both quantum and classical effects and operating regimes of these radiation devices are described. The article provides a unified physical description of the interaction kinematics, and presents equations for the characterization of spontaneous and stimulated radiative emission in these various effects and devices. Universal relations between the spontaneous and stimulated emission parameters are revealed and shown to be related (in the quantum limit) to Einstein relations for atomic radiators and (in the classical limit) to the relations derived by Madey for magnetic bremsstrahlung FEL for on-axis radiative emission. Examples for the application of the formulation are given, estimating the feasibility of channeling radiation x-ray laser and optical regime Smith-Purcell FEL, and deriving the gain equations of magnetic bremsstrahlung FEL and CRM for arbitrary electron propagation direction, structure (wiggler) axis, and radiative emission angle.

We present an analysis of a free-electron laser within a three-dimensional model, in which the finite transverse dimensions of the electron beam are taken into account together with the confinement of the electromagnetic modes by a guiding structure. The analysis is based on the expansion of the beam’s self-potential into an infinite set of modes that interact simultaneously with the wiggler and signal fields. Explicit gain-dispersion equations are developed for several cases including a waveguide tube completely filled by an electron beam of uniform density, and a uniform-density electron beam partially filling a waveguide (or in free space). This is carried out in both the magnetized beam limit and the general case which includes surface currents. The results bear significant effect on free-electron-laser gain operating parameters in the collective regimes and on the threshold of absolute instability oscillation.

We report the observation of synchronous energy exchange between nonrelativistic electrons and the ponderomotive (beat) force of two counterpropagating, intense, pulsed CO₂ laser beams, operating at different frequencies in a stimulated Compton scattering scheme. The interaction takes place in the nonlinear (trapping) regime, and its physics is the same as in laser accelerators and efficiency-enhanced free-electron lasers with long wigglers. Two different mechanisms of enhanced energy transfer were observed—electron trapping and phase-area displacement.

The problem of fundamental laser line broadening due to random spontaneous emission of radiation and amplification of thermal radiation noise is analyzed in terms of a classical fluctuating field phasor model. We derive a general expression for the intrinsic linewidth, given in terms of the spectral power of the radiation noise source, which can be classical or quantum mechanical in nature. In the case of a two-level atomic laser, we recover by the use of Einstein relations, the traditional linewidth formula of the Schalow-Townes form. In the case of the free-electron laser (FEL), using the explicit expression for the spontaneous emission, we present calculation of the laser linewidth by purely classical methods. The result agrees with the one obtained in the framework of a quantum-mechanical model. By using ‘‘extended Einstein relations’’ which are applicable to classical radiators, we show that a Schalow-Townes-type formula can also be obtained for the FEL. The theory predicts extremely narrow intrinsic linewidth (10^-7 Hz) for cw FEL’s with parameters similar to those of the FEL experiment of Elias et al.