We study the generation of short (sub 10 fs) pulses in the X-ray spectral region using an energy chirped electron beam in a Self Amplified Spontaneous Emission Free Electron Laser (SASE FEL) and a self-seeding monochromator [1-4]. The monochromator filters a small bandwidth, short duration pulse from the frequency chirped SASE spectrum. This pulse is used to seed a small fraction of the long chirped beam, hence a short pulse with narrow bandwidth is amplified in the following undulators. We present start-to-end simulation results for LCLS operating in the soft X-ray self-seeded mode with an energy chirp of 1% over 30 fs and a bunch charge of 150 pC. We show the possibility to generate
5 fs pulses with a bandwidth 0.3 eV. We also assess the possibility of further shortening the pulse by utilizing one more chicane after the self-seeding stage and shifting the radiation pulse to a “fresh” part of the electron beam. Experimental study on this short pulse seeding mode has been planned at the LCLS.
We propose a significant enhancement of the electron
peak current entering a SASE undulator by inducing an en-
ergy modulation in an upstream wiggler magnet via res-
onant interaction with an optical laser, followed by mi-
crobunching of the energy-modulated electrons at the ac-
celerator exit. This current enhancement allows a reduc-
tion of the FEL gain length. The x-ray output consists of
a series of uniformly spaced spikes, each spike being tem-
porally coherent. The duration of this series is controlled
by the laser pulse and in principle can be narrowed down
to just a single, -attosecond spike. Given potentially
absolute temporal synchronization of the x-ray spikes to
the energy-modulating laser pulse, this scheme naturally
makes pump-probe experiments available to SASE FEL�s.
We also study various detrimental effects related to the high
electron peak current .
Electron beams are strongly microbunched near the high-gain free-electron laser
(FEL) saturation with a rich harmonic content in the beam current. While
the coherent harmonic emission is possible in a planar undulator, the third-
harmonic radiation typically dominates with about 1% of the fundamental power
at saturation. In this paper, we discuss the second-harmonic radiation in the
Linac Coherent Light Source. We show that by a suitable design of an second-
stage undulator with its fundamental frequency tuned to the second harmonic of
the first undulator, coherent second-harmonic radiation much more intense than
the third-harmonic is emitted. Numerical simulations predict that GW-level, sub-
Angstrom x-ray pulses can be generated in a relatively short second-harmonic
Today, many bright photon beams in the ultraviolet and x-ray wavelength range are produced by insertion devices installed in specially designed third-generation storage rings. There is the possibility of producing photon beams that are orders of magnitude brighter than presently achieved at synchrotron sources, by using self-amplified spontaneous emission (SASE). At the Advanced Photon Source (APS), the low-energy undulator test line (LEUTL) free-electron laser (FEL) project was built to explore the SASE process in the visible through vacuum ultraviolet wavelength range. While the understanding gained in these experiments will guide future work to extend SASE FELs to shorter wavelengths, the APS FEL itself will become a continuously tunable, bright light source. Measurements of the SASE process to saturation have been made at 530 and 385 nm. A number of quantities were measured to confirm our understanding of the SASE process and to verify that saturation was reached. The intensity of the FEL light was measured versus distance along the FEL, and was found to flatten out at saturation. The statistical variation of the light intensity was found to be wide in the exponential gain region where the intensity is expected to be noisy, and narrower once saturation was reached. Absolute power measurements compare well with GINGER simulations. The FEL light spectrum at different distances along the undulator line was measured with a high-resolution spectrometer, and the many sharp spectral spikes at the beginning of the SASE process coalesce into a single peak at saturation. The energy spread in the electron beam widens markedly after saturation due to the number of electrons that transfer a significant amount of energy to the photon beam. Coherent transition radiation measurements of the electron beam as it strikes a foil provide additional confirmation of the microbunching of the electron beam. The quantities measured confirm that saturation was indeed reached. Details are given in Milton et al., Science 292, 2037 (2001) (also online at www.sciencexpress.org as 10.1126/science. 1059955, 17 May 2001), and Lewellen et al., "Present Status and Recent Results from the APS SASE FEL," to be published in the Proceedings of the 23rd International Free-Electron Laser Conference, Darmstadt, Germany, 20?24 August 2001.
Self-amplified spontaneous emission in a free-electron laser has been proposed for the generation of very high brightness coherent X-rays. This process involves passing a high-energy, high-charge, short-pulse, low-energy-spread, and low-emittance electron beam through the periodic magnetic field of a long series of high-quality undulator magnets. The radiation produced grows exponentially in intensity until it reaches a saturation point. We report on the demonstration of self-amplified spontaneous emission gain, exponential growth, and saturation at visible (530 nanometers) and ultraviolet (385 nanometers) wavelengths. Good agreement between theory and simulation indicates that scaling to much shorter wavelengths may be possible. These results confirm the physics behind the self-amplified spontaneous emission process and forward the development of an operational X-ray free-electron laser. (30 References).
Exponential growth of self-amplified spontaneous emission at 530 nm was first experimentally observed at the Advanced Photon Source low-energy undulator test line in December 1999. Since then, further detailed measurements and analysis of the results have been made. Here, we present the measurements and compare these with calculations based on measured electron beam properties and theoretical expectations. (31 References).
Experimental evidence for self-amplified spontaneous emission (SASE) at 530 nm is reported. The measurements were made at the low-energy undulator test line facility at the Advanced Photon Source, Argonne National Laboratory. The experimental setup and details of the experimental results are presented, as well as preliminary analysis. This experiment extends to shorter wavelengths the operational knowledge of a linac-based SASE free-electron laser and explicitly shows the predicted exponential growth in intensity of the optical pulse as a function of length along the undulator. (20 References).