Search Results (13 total)

We report experiments at Jefferson National Accelerator Facility (Jlab) and computer simulations performed at the Naval Postgraduate School (NPS) designed to probe the small Rayleigh length regime. We compare the gain, power, and sensitivity to mirror and electron beam misalignments as a function of decreasing Rayleigh length. The agreement is quite good, with experiments and simulations showing comparable trends as the Rayleigh length is decreased. In particular, we find that the gain and power do not decrease substantially at short Rayleigh length, contrary to a common Gaussian-mode filling factor argument. Within currently achievable alignment tolerances, the gain and power are still acceptable for FEL operation.

Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third to one half of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. We model this interaction using a coordinate system that expands with the rapidly diffracting optical mode from the ends of the undulator to the mirrors. Simulations show that the interaction of the strongly focused optical mode with a narrow electron beam inside the undulator distorts the optical wave front so it is no longer in the fundamental Gaussian mode. The simulations are used to study how mode distortion affects the single-pass gain in weak fields, and the steady-state extraction in strong fields.

Motivated by the prospect of constructing a short Rayleigh length free-electron laser in a high-vibration environment, we demonstrate the use of a collection of rays to study the effect of mirror vibration and distortion on the behavior of the fundamental optical mode of a cold-cavity resonator. We find that the ray collection accurately describes both on-axis and off-axis optical beams. We show that a tilt or transverse shift of a mirror causes the optical mode to rock about the original resonator axis, while a longitudinal mirror shift or a change in the mirror’s radius of curvature causes the beam diameter at a mirror to successively dilate and contract on the mirror. Results are in excellent agreement with analytic calculations and wave front propagation simulations as long as the mirrors remain large with respect to the beam diameter.

We present experiments and simulations showing the behavior of a free-electron laser (FEL) with both positive and negative linear tapers along the wiggler. We show the power desynchronism curve widths, efficiency, exhaust electron energy spread, and wavelength dependence as a function of taper for 3- and 6-μm optical wavelengths and for resonators with 10% and 2% loss/pass. Simulations of the experiments, using a multimode analysis, are seen to be in general agreement with the experimental results, carried out at the IR Demo FEL at Thomas Jefferson National Accelerator Facility. We find that short-pulse effects are more effective than tapers in producing high efficiency with low exhaust energy spread, and the expected performance enhancement of FEL tapering is not achieved.

In a free-electron laser oscillator, a variation in the electron beam energy leads to a change in the resonant optical frequency. Simulations are used to study the optical response to an electron beam energy change. A step change in the electron beam energy is used to define a characteristic response time for changing the optical frequency.

The free-electron laser can be described by solving the Lorentz-Maxwell equations self-consistently in weak optical fields. The field evolution is determined by an integral equation that allows the inclusion of an arbitrary electron distribution function in a simple way. Contour maps are used to show the gain degradation due to an electron-beam energy spread and an electron-beam angular spread. In the limit of low gain, the gain spectrum is related to the spontaneous emission line shape through successively higher derivatives. In the limit of high gain, it is shown that the growth rate becomes less susceptible to degradation from the electron-beam quality.

The gain of the klystron free-electron laser is calculated with use of the coupled, self-consistent Lorentz-Maxwell equations. For high gain, the objective of the klystron configuration, the gain spectrum is found to be modified from the previously known low-gain result. This is caused by the shifting of the optical phase during the gain process. The klystron saturation in strong nonlinear optical fields is also discussed.

The theory of free-electron lasers is extended to include the new coupling between an electron beam and optical wave propagating at an angle ϑ in an arbitrary harmonic. The coupling allows the laser to be tuned to a wider range of wavelengths and to include the effects of emittance in the electron beam. The formulation of the results in terms of coupling constants means that the existing knowledge of high gain, low gain, weak optical fields, strong optical fields, and short pulses in free-electron lasers can be immediately generalized to off-axis propagation in an arbitrary harmonic.

The spectrum, angular distribution, polarization, and coherence properties of the radiation emitted by relativistic electrons undulating through a quasiperiodic tapered magnetic field are studied. Tapering the wavelength and/or field strength along the undulator's axis has the effect of spreading the spectral line to higher frequencies; interference over this broader spectral range results in a more complex line shape. The angular dependence, on the other hand, is not affected by the amount of taper. The polarization of the radiation in the forward direction is determined by the transverse polarization of the undulator, but the polarization changes off axis. The radiation patterns predicted here are distinct from those of untapered undulators and their detection is now feasible. They will provide useful diagnostics of electron trajectories and threshold behavior in free-electron-laser oscillators using tapered undulators.

The nonlinear self-consistent theory of short-pulse free-electron laser oscillators can now be extended to include transverse diffraction within the optical resonator mode. The theory provides efficient solutions to the three-dimensional parabolic-wave equation coupled to the Lorentz force equation. The method is general enough to include arbitrary magnet designs, optical mirror arrangements, and driving currents. New mode structures are predicted which should be observed in future experiments at Stanford University.

In a free-electron laser oscillator, both the amplitude and frequency of the optical field are free to evolve. We use a Lagrangian formalism to describe the dynamics of the interacting electron beam and optical wave in a single pass. The character of the evolution over many passes is controlled by the design of the undulating magnetic field in the free-electron laser interaction region. Several laser magnet designs are presented under the same formalism and compared: the undulator, the tapered undulator, the optical klystron, and transverse gradient ("gain-expansion") undulator. For each of these designs we numerically calculate the gain as a function of optical amplitude and frequency. These "gain surfaces" are used to infer a variety of properties of oscillator evolution and clearly demonstrate the relative merits of each magnet design.

The nonlinear wave equation and self-consistent pendulum equation are used to generalize free-electron-laser operation to higher harmonics; this can significantly extend their tunable range to shorter wavelengths.

We have calculated the gain of a free-electron laser in the small-gain-per-pass limit by using the single-particle model. The electron equations of motion reduce to that of a simple pendulum. As operating levels increase, the theory predicts that by varying the amount of detuning, gain enhancement should occur. In addition, we calculate the electron energy and phase distributions at the output of the amplifier assuming the electrons entering are monoenergetic and have a uniform phase distribution. Above a certain operating level, the theory predicts and explains for the first time the occurrence of discontinuities in both distribution functions. The effect of these discontinuities in storage-ring high-power applications is not yet known.