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Notes: A tapered undulator, in which the entrance and exit magnetic gaps are different, will show an increase in the energy width of the harmonics compared to the nontapered case. This increase can occur without substantial broadening in the collimation of the undulator radiation. We present the spectral properties measured for a tapered 3.3-cm period prototype undulator of the Advanced Photon Source at the Cornell High Energy Synchrotron Source. The width of the on-axis first harmonic at 6.9 keV increased from 0.32 to 0.66 keV for a gap tapering of 12%. Minimal angular broadening in the first harmonic x rays was detected and measurements of off-axis radiation were also performed. The results are found to be in good agreement with model calculations.

Notes: The first undulator radiation has been extracted from the Advanced Photon Source (APS). The results from the characterization of this radiation are very satisfactory. With the undulator set at a gap of 15.8 mm (K=1.61), harmonics as high as the 17th were observed using a crystal spectrometer. The angular distribution of the third-harmonic radiation was measured, and the source was imaged using a zone plate to determine the particle beam emittance. The horizontal beam emittance was found to be 6.9±1.0 nm-rad, and the vertical emittance coupling was found to be less than 3%. The absolute spectral flux was measured over a wide range of photon energies, and it agrees remarkably well with the theoretical calculations based on the measured undulator magnetic field profile and the measured beam emittance. These results indicate that both the emittance of the electron beam and the undulator magnetic field quality exceed the original specifications. © 1996 American Institute of Physics.

Notes: Since high-energy photons ((approximately-greater-than)50 keV) are well suited for certain types of x-ray scattering experiments, we present calculated results for the Advanced Photon Source (APS) Undulator A and APS Wiggler A at high energies. The undulator calculations include the effect of magnetic field errors, which is to smear the high-order spectral harmonics. At their anticipated initial minimum gap settings, Undulator A should perform better than Wiggler A from the point of view of most high energy experiments up to at least ∼280 keV. A comparison of APS insertion devices to high energy insertion devices in other synchrotron radiation laboratories is also provided. © 1996 American Institute of Physics.

Notes: 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. © 2002 American Institute of Physics.