Electromagnetic Wiggler Research Articles

Higher Harmonic in Free Electron Laser with Combined Electromagnetic Wiggler and Axial Guide Magnetic Fields

Following a single-particle approach, a detailed analysis of the higher harmonic in free electron laser (FEL) with combined electromagnetic wiggler and axial guide magnetic fields is derived. The spontaneous emission spectrum and the gain in the low-gain Compton regime are also calculated in detail. This analysis results in a clear resonance condition and shows that this kind of FEL can generate significant coherent harmonic emission of radiation with axial guide magnetic field of suitable strength.

Studies of insertion devices for producing circularly polarized radiation with variable helicity in ELETTRA

Insertion device sources of circularly polarized radiation with the capability of switching rapidly between right- and left-handed polarization are considered and two schemes are put forward: a ‘‘modulated asymmetric wiggler’’ and an electromagnetic elliptical wiggler. Recent studies of these devices and of the alternative crossed-undulator scheme are summarized, and their expected performance compared in ELETTRA. The optimum choice for a broadband, high flux source is an electromagnetic elliptical wiggler. Design parameters for a device with the potential of reaching a switching rate of 100 Hz are presented.

The new CHESS wiggler

A 25-pole permanent magnet hybrid wiggler has been built at CHESS and installed on the CESR (Cornell Electron Storage Ring). This device has a magnetic period of 19.6 cm, a peak on-axis field of 1.2 T at the nominal operating gap of 4.0 cm, and a K parameter of 22. The wiggler has been designed to provide radiation for two new experimental stations with approximately four times the flux available from the present CHESS six-pole electromagnet wiggler. Under normal running conditions at 100 mA currents, the total power radiated should exceed 6 kW making this one of the highest flux x-ray sources below 1 Å critical wavelength. In this paper an overview of the development of the wiggler is given, including the unique features in its design and construction as well as results of measurements obtained on its magnetic and spectral properties.

Influence of Beam Equilibrium Self-fields on Linear Gain for a Free Electron Laser

The small-signal linear gain for a combined electromagnetic wiggler and an axial-guide magnetic field free electron laser is derived within the presence of beam equilibrium self-fields. The numerical results show that the gain is reduced as the self-fields increase.

The nonlinear evolution of a free electron laser with electromagnetic wigglers

The nonlinear evolution of a free electron laser (FEL) with an electromagnetic wiggler in the high gain regime is investigated. Two opposite configurations for the wiggler field propagation direction are considered: the wiggler field propagating counter to the electron beam, which is subject to a depletion in the nonlinear stage, and the one propagating parallel to the electron beam, which is subject to an amplification. Especially for the latter case, the possibility of an explosive instability has been fully investigated. A self-consistent set of equations is derived and solved numerically. In the Compton regime the radiation field saturates at a peak amplitude a2=0.6(ωpb/ω1)4/3a1/3w, where a(aw) is the usual normalized vector potential of the radiation (wiggler) field and ωpb(ω1) is the beam plasma (radiation) frequency in the ponderomotive wave frame. In the Raman regime the FEL instability saturates either by particle trapping or by detuning from the resonance due to the nonlinear frequency of the beam mode, depending on the initial wiggler strength. For the copropagating wiggler field system the explosive behavior can be seen only in an impractically small wiggler strength region. In a parameter regime of practical interest the behavior of the radiation field around the saturation is not affected much by the pump depletion or amplification.

Nonlinear interaction of a relativistic electron beam with electromagnetic waves

The self-consistent nonlinear interaction of an electron beam with electromagnetic waves is described with a Lagrangian action, specialized for a free-electron laser with an electromagnetic wiggler. The approximations and the averaging over the wiggler period are made on the action, thereby preserving the Hamiltonian structure of the equations.

LINEAR AND NONLINEAR CALCULATIONS OF THE 3-DIMENSIONAL MOTION OF RELATIVISTIC ELECTRONS IN THE ELECTROMAGNETIC WIGGLER AND AXIAL GUIDE FIELD

The 3-D motion of the relativistic electrons is analysed under the combination of a cylindrical waveguide TE mode (or TM mode) and an axial guide field. Both linear and nonlinear calculations are carried out. Generally speaking, the projection of the electron orbits onto the transverse plane is an annulus, the thickness of which is dependent on the amplitude of the electromagnetic wiggler. However, the electron orbits may diverge when ω+kzvzo+ ( m±1 )ωc≈0 Discussion is also given to the special case that the electromagnetic wiggler is a slow wave propagating parallel to the electron motion.

An experimental study of a shielded pulsed electromagnet wiggler concept

A new shielded pulsed electromagnet (SPEM) wiggler concept is introduced to increase the gap field strength. The advantages of a SPEM have been demonstrated with a wiggler period λw of 12 cm, a working gap 2δ of 4.4 cm, a pulse duration of 2 ms and maximum ampere-turns of 12000 A t. It has been found that a SPEM has excellent capability of reducing the saturation in the core resulting in a 38% increase in field strength.

Design of a high-field taperable helical wiggler

The advantages of a helical wiggler have long been recognized, but earlier concepts do not meet the taper and field-control requirements of a modern high-gain free electron laser. This work presents a design of an electromagnetic helical wiggler where the field and steering-free taper are completely adjustable. The unique coil and core geometry makes it possible to produce fields over 5 kG in a bore of 2.5 cm. The paper discusses the concept, magnetic-field analysis, electron-beam trajectories and practical considerations of the wiggler.

Three-wave interaction in a free-electron laser with a circularly polarised electromagnetic wave wiggler and an axial guide field

The interaction of the pump wave, the scattered wave and the space-charge wave in a free-electron laser with an electromagnetic wave wiggler and an axial guide field is analysed, where both the scattered wave and pump wave are circularly polarised. The linear dispersion and non-linear growth of these three waves are derived in the laboratory frame. The possibility of explosive instabilities for a fast electromagnetic wave pump propagating counter to the electron beam is discussed and its critical condition is ( omega p+kp nu z0)- omega b/ gamma zo< omega c<(kp nu z0), where omega p and kp are the pump wave frequency and wave number, omega b, nu zO and gamma z0 are the plasma frequency, initial axial velocity and Lorentz factor of the relativistic electron beam, respectively, and omega c is the electron cyclotron frequency in the axial guide field. Group II orbits ( omega c> omega p+kp nu z0) may no longer give positive gain in the process of three-wave interaction.

Design and optimization of short-period wigglers for free-electron lasers

In this paper, simple expressions for optimized dimensions of the electromagnet wiggler are derived and experimentally verified.

Coaxially fed folded foil electromagnet wiggler

A simple electromagnet wiggler, constructed of copper foil windings, is presented. The design is based upon a concept that combines two conducting foils in back and forth directions, thus obtaining coaxial current feeding that cancels asymmetrical fringe fields at both ends of the wiggler. Analytical expressions for the off-axis magnetic fields are given and compared to exact numerical results and to experimental measurements. Both comparisons show high accuracy of the analytical expressions in the entire wiggler volume. Electron trajectories in the wiggler and optimal injection conditions are described and considered in this paper. A wiggler construction of λ W = 2 cm that produces a 345 G peak on-axis at 1 kA feeding current is presented. The feasibility of a miniature wiggler based on this concept is discussed, and a scheme of λ W = 5 mm, B W = 0.3 T ( a W = 0.15), fed by a current of I = 1.5 kA, is proposed.

Analysis of free-electron lasing stimulated by a counterpropagating plasma wave

A relativistic fluid theory is constructed to analyze the inverse Compton scattering of an unmagnetized counterpropagating plasma wave by a cold relativistic electron beam. A Langmuir wave and electromagnetic plasma wave wigglers are considered, and the properties of the two types of wigglers are comparatively discussed. The growth rates are comparable to those of the ac free-electron laser and the free-electron lasers with magnetostatic wigglers in the Raman regime.

3-D magnetic field calculations for wigglers using magnus-3D

The recent but steady trend towards increased magnetic and geometric complexity in the design of wigglers and undulators, of which tapered wigglers, hybrid structures, laced electromagnetic wigglers, magnetic cladding, twisters and magic structures are examples, has caused a need for reliable 3-D computer models and a better understanding of the behavior of magnetic systems in three dimensions. The capabilities of the MAGNUS-3D group of programs are ideally suited to solve this class of problems and provide insight into 3-D effects. MAGNUS-3D can solve any problem of magnetostatics involving permanent magnets, linear or nonlinear ferromagnetic materials and electric conductors of any shape in space. The magnetic properties of permanent magnets are described by the complete nonlinear demagnetization curve as provided by the manufacturer, or, at the user's choice, by a simpler approximation involving the coercive force, the residual induction and the direction of magnetization. The ferromagnetic materials are described by a magnetization table and an accurate interpolation relation. An internal library with properties of common industrial steels is available. The conductors are independent of the mesh and are described in terms of conductor elements from an internal library. MAGNUS-3D uses the finite element method and the two-scalar-potentials formulation of Maxwell's equations to obtain the solution, which can then be interactively used to obtain all kinds of tables and plots and to calculate quantities needed in magnetic engineering. This includes tables of values of the field components at specified points or lines, plots of field lines in 3-D or of the magnetic bodies seen from any point of view in space, function graphs representing a field component plotted against a coordinate along any line in space (such as the beam line), and views of the conductors and the mesh. All views feature color, perspective, and partial or full hidden surface removal. The magnetic quantities that can be calculated include the force or torque on conductors or magnetic bodies, the energy, the flux through a specified surface, line integrals of any field component along any line in space, and the average field or potential harmonic coefficients. This paper describes the programs with emphasis placed on their use for wiggler design, and an example of mesh generation and calculations for an advanced undulator concept.

IMP, a free electron laser amplifier for plasma heating in the Microwave Tokamak Experiment

The Intense Microwave Prototype (IMP) is an induction-linac based free electron laser (IFEL) amplifier system that is presently under construction at the Lawrence Livermore National Laboratory (LLNL). It will produce up to 2 MW of average power at 250 GHz for electron cyclotron resonance heating experiments in the Microwave Tokamak Experiment (MTX). The Experimental Test Accelerator-II (ETA-II) will provide the electron beam. ETA-II is designed to produce an electron beam with a current of 3 kA at an energy of 10 MeV and a brightness of over 10 8 A/(m rad) 2. In addition, it is designed to produce 70-ns-FWHM pulses at a repetition rate of 5 kHz. The high magnetic field and wide tunability capabilities required for the FEL will be provided by a permanent magnet-laced electromagnetic wiggler with a 10-cm period and an overall length of 5.5 m. We present the physics design and expected performance of the FEL, along with a description of the experiment and of the phased development to high average power.

A gyrotron-powered standing-wave electromagnetic wiggler experiment

Experimental results on a gyrotron-powered standing-wave electromagnetic wiggler are presented. The gyrotron interaction cavity and the standing-wave storage cavity are designed as a single unit. The gyrotron was operated at a high field intensity and short interaction length regime. The operating mode is the TE 13 mode at 129.5 GHz. A value of 0.0057 is obtained for the normalized vector potential of the wiggler field. The experimental results indicate that higher wiggler field strengths should be feasible with higher-power electron beams; such an advance would make possible the development of compact, low-voltage, near-IR FELs.

Depletion and diffraction of an electromagnetic wiggler field during the free electron laser interaction

The self-consistent interaction of an electron beam with an electromagnetic wiggler and a radiation field is analyzed. Equations are derived for the depletion and diffraction of the wiggler wave. The formalism yields local conservation laws for the system energy flux and photon flux. Using a one-dimensional approximation, pump depletion is examined for pulsed and steady-state electron beams. A three-dimensional analysis shows that the wiggler diffraction is dominated by the nonresonant interaction of the electrons with the wiggler wave. This is studied in detail for a specific electron pulse shape. It is concluded that wiggler depletion and diffraction should not substantially degrade the free electron laser interaction for experimentally obtainable system parameters.

Sideband excitation in an electromagnetic wiggler pumped free electron laser

A kinetic formalism of sideband excitation at the multiples of bounce and cyclotron frequencies of electrons trapped in the ponderomotive potential well and a guide field is developed. A positive (negative) slope of the distribution function with parallel energy is necessary for the excitation of upper (lower) sidebands of the bounce frequency. The cyclotron harmonic sideband excitation requires large perpendicular energy; the latter, however, tends to lower the growth rate of bounce harmonic sidebands. In either case the growth rate falls off rapidly with the harmonic number.

Experiment and theory on CO2 laser powered wiggler and induction linac FEL

As an electromagnetic wiggler experiment, CO2 laser light (100 J/100 ns) is injected into a high current electron beam (1 kA, 0.5 MeV). Scattered visible light (λ = 8000-5000 Å) is observed. From the spontaneous emission power the gain for the CO2 laser powered FEL is estimated.The induction linac for the FEL which is developed at our institute and the FEL driven by the induction linac are also reported.

Explosive free-electron-laser instability with an electromagnetic wiggler

A new scheme for generating free electron laser via explosive instabilities with electromagnetic wave wiggler is proposed. This is achieved when a slow electromagnetic wiggler ( ω 0 k 0 <ν b) propagating parallel to the electron beam is upconverted into a negative energy beam mode and a laser wave. The explosion occurs in one growth period ∼4 ( γ 0ω 1 ω′ p ) 1 2 kv 0 and may lead to more efficient generation of intense laser in short pulse.