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September 25, 2002 Nonlinear Harmonic Radiation from a Visible Self-Amplified Spontaneous Emission Free Electron LaserA. Tremaine1, A. Murokh1, C. Pellegrini1, S. Reiche1, J. Rosenzweig1, M. Babzien2, I. Ben-Zvi2, R. Malone2, G. Rakowsky2, J.Skaritka2, X. Wang2, V. Yakamenko2, R. Carr3, M. Cornacchia3, H.-D. Nuhn3, R. Ruland3, L. Bertolini4, M. Libkind4, A. Toor4, and K. Van Bibber4 Scientists from the University of California, Los Angeles, Brookhaven National Laboratory, Stanford Linear Accelerator Center and Lawrence Livermore National Laboratory have demonstrated for the first time the feasibility of using nonlinear harmonic self-amplified spontaneous emission (SASE) free electron laser (FEL) radiation to produce coherent, femtosecond x-rays. Nonlinear harmonic radiation (NHR) was observed using the visible-to-infrared SASE amplifier (VISA) FEL at saturation. The scientists characterized experimentally the second and third NHR modes and measured a power of several megawatts and sharp spectra for these modes.
We report here the results from a light source called visible-to-infrared SASE amplifier (VISA) FEL, which served as research and development for the linear accelerator (linac) coherent light source (LCLS), a 1-angstrom (Ĺ) FEL to be built at the Stanford Linear Accelerator Center (SLAC) in California. VISA exemplifies that future facilities can use SASE nonlinear harmonic radiation (NHR) to produce narrower bandwidth and harder x-rays compared to the fundamental radiation.
NHR accompanies the fundamental radiation only in the SASE high gain regime. Figure 1 shows the superimposed VISA spectrum at the end of the undulator for the three lowest FEL modes. The fundamental spectrum is centered at 845 nanometers (nm), and, as expected, the second and third nonlinear harmonics are centered at 422 and 280 nm, respectively.
By the undulator exit, the energies of the second and third NHRs are two percent and one percent of the fundamental energy, respectively, confirming theoretical predictions. Our results show that high-gain SASE FELs generate substantial power and narrow spectra for the NHR. We measured about five megawatts of 280-nm (third harmonic) NHR, an impressive power considering our relatively small system. Extending these results, the third NHR for the LCLS will be peaked narrowly around 0.33 Ĺ with power several orders of magnitude larger than current third-generation synchrotron light sources. FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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The growth of synchrotron light source applications in the last
few decades is due largely to the improved brightness of the x-ray
beams they produce. During this same period, high power, femtosecond-laser
technologies have opened new frontiers in spectroscopy and dynamics
system studies. The free electron laser (FEL), which produces
coherent, femtosecond, x-rays, can bridge these technologies. Due to
the lack of x-ray seeds and mirrors, all proposed x-ray FELs are
based on self-amplified spontaneous emission (SASE), which is a
high-gain, single-pass FEL amplifier. 
Gain lengths for each mode are calculated from the data shown in
the log-linear plot of Figure 2. A fundamental gain length of
centimeters (cm) is measured, and NHR gain lengths of 9.8 cm and 6
cm are obtained for the second and third NHR, respectively, using
only the data in the nonlinear regime. The NHR grows faster than the
fundamental by , verifying theoretical predictions.