P. Pierini

Large harmonic bunching in a high-gain free-electron laser

Abstract Properly taking into account the coupling between different harmonics in a high-gain free-electron laser (FEL), we demonstrate that the exponential gain of the fundamental radiation wavelength drives the third-harmonic bunching at a rate which is three times that of the fundamental. This leads to almost equal saturation bunching parameters on the fundamental and the harmonic wavelength. This is the underlying mechanism of the resonant-frequency tripling method in a two-wiggler FEL.

The superradiant regime of a FEL: Analytical and numerical results

Abstract We present here an analytical and numerical study of the propagation equations of a high-gain, single-pass FEL amplifier. The analytically derived behaviour of the system is confirmed by numerical integration of the partial differential problem. The occurence of a superradiant trailing-edge spiking behaviour is shown, both in the analytical and numerical case.

TDA3D: Updates and improvements to the widely used three-dimensional free electron laser simulation

Abstract TDA3D is a widely distributed and often used Free Electron Laser (FEL) simulation code. While a number of versions of TDA exist, this paper describes the official version which is well tested and supported. We describe the capabilities of the code emphasizing recent improvements and revisions. TDA3D is a steady-state (time-independent) amplifier code. The code self-consistently solves, after averaging over a wiggler period, the paraxial wave equation for the radiation field and the Lorentz equations of motion for the electrons. The paraxial wave equation includes diffraction and optical guiding. The calculation of the electron beam motion takes into account longitudinal bunching and transverse betatron oscillations, so that emittance, energy spread, and external focusing can be properly modeled. Recent additions to the simulation include the ability to model natural wiggler focusing in one or both planes, alternating gradient quadrupoles or sextupoles, and ion channels. The initial loading of the electron distribution can be controlled to allow for matching into focusing channels, improved quiet starts (non-correlated phase-space distributions), and arbitrary energy spread.

Prospects for high power linac coherent light source (LCLS) development in the 1000 Å−1 Å wavelength range

Electron bunch requirements for single-pass saturation of a Free-Electron Laser (FEL) operating at full transverse coherence in the Self-Amplified Spontaneous Emission (SASE) mode include: a high peak current, a sufficiently low relative energy spread, and a transverse emittance {epsilon}[r-m] satisfying the condition {epsilon}{le}{lambda}/4{pi}, where {lambda}[m] is the output wavelength of the FEL. In the insertion device that induces the coherent amplification, the prepared electron bunch must be kept on a trajector sufficiently collinear with the amplified photons without significant dilution of its transverse density. In this paper we discuss a Linac Coherent Light Source (LCLS) based on a high energy accelerator such as, e.g., the 3 km S-band structure at the Stanford Linear Accelerator Center (SLAC), followed by a long high-precision undulator with superimposed quadrupole (FODO) focusing, to fulfill the given requirements for SASE operation in the 1000 A{minus}1 A range. The electron source for the linac, an RF gun with a laser-excited photocathode featuring a normalized emittance in the 1--3 mm-mrad range, a longitudinal bunch duration of the order of 3 ps, and approximately 10{sup {minus}9} C/bunch, is a primary determinant of the required low transverse and longitudinal emittances. Acceleration of the injected bunch to energies in the 5--25morexa0» GeV range is used to reduce the relative longitudinal energy spread in the bunch, as well as to reduce the transverse emittance to values consistent with the cited wavelength regime. Two longitudinal compression stages are employed to increase the peak bunch current to the 2--5 kA levels required for sufficiently rapid saturation. The output radiation is delivered, via a grazing-incidence mirror bank, to optical instrumentation and a multi-user beam line system. Technological requirements for LCLS operation at 40 A, 4.5 A, and 1.5 A are examined.«xa0less

TAPERING AND SELF-TAPERING IN A FREE ELECTRON LASER AMPLIFIER

We consider an FEL amplifier with variable wiggler field, described by a fully Hamiltonian model for operation in the steady-state regime and by a dissipative model for operation in the superradiant regime. In the steady-state regime we present numerical results for different tapering procedures and derive corresponding scaling laws for optimum tapering and for the growth of intensity and efficiency. The radiated power is shown to scale with respect to electron density as n 5/3 , that is half-way between the untapered high-gain steady-state scaling, n 4/3 , and the superradiant n 2 scaling. In the superradiant regime the tapering is shown to have little relevance, contrary to the steady-state case. However, self-tapering with continuous extraction of electron energy occurs; during the growth of the superradiant pulse the resonance is maintained by a shift in the phase of the radiation field which compensates for the decrease of the electron energy.