|Progress using an FEL Oscillator for Inverse Compton Scattering
Funding: Work supported by U.S. Grant: DE-FG02-97ER41033.
Since mid 1990s, oscillator FELs have been used to produce x-rays and gamma-rays via Compton scattering. These FELs are based on several accelerator technologies, including room-temperature and superconducting linacs and storage rings. The most successful Compton sources are operated in the gamma-ray region, beyond the reach of synchrotron radiation sources and x-ray FELs. With almost two decades of continued effort, High Intensity Gamma-ray Source (HIGS) at Duke University has been developed into a world-leading Compton gamma-ray facility for frontier research in nuclear physics and astrophysics, and applied research in national security and industry. The two outstanding features of the HIGS are (1) a wide energy range of operation from 1 to 100 MeV; and (2) an exceptionally high flux in the few MeV to 10 MeV region. At the HIGS, the further development of the oscillator FEL will lead to new gamma-ray beam capabilities. Research to develop a VUV FEL at 170 or 150 nm will allow the production of gamma-rays up to ~160 MeV. Research on the FEL beam polarization will lead to the production of gamma-ray beams with switchable helicity and rotatable linear polarization.
|Slides WEA01 [23.535 MB]
|Free Electron Laser Oscillator: Short Pulses, Mode Locking, Harmonic Generation and Tapering
|In Free Electron Laser oscillators the growth of the intracavity laser power determines the most interesting aspects of the system dynamics. In the case of short pulses operation the system undergoes genuine mode-locking mechanisms, which provide a wealth of interesting phenomena associated with the possibility of generating very short pulses. We explore the mechanisms of superradiance in FEL operating in the over-saturated regime and analyze the emerging short pulse structures and the relevant physical meaning. We also explore the pulse shape of the higher order harmonics generated in this regime and the possibility of modelling the pulse width and power by suitable combination of cavity length control and of undulator tapering.
|Slides WEA02 [13.588 MB]
|Higher Harmonic XFELO with the Planned 4 GeV LCLS II SCRF Linac
Funding: This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under contract No. DE-AC02-06CH11357 (ANL) and DE-AC02-76SF00515 (SLAC)
An x-ray FEL oscillator (XFELO) will produce hard x-ray pulses of ultra-fine spectral resolution (~ meV) that combines FEL brightness with storage ring stability . Thus, for example, the long-standing problem of high-TC superconductivity could be solved by inelastic x-ray scattering. In addition, an x-ray spectral comb can in principle be generated, vastly expanding the reach of experimental x-ray quantum optics. The accelerator for an XFELO should optimally be of the CW superconducting type. The linac for the European XFEL can be operated in CW mode without adding more cooling capacity if the energy is lowered from 14 to 7 GeV . It is also possible to drive a hard x-ray XFELO at lower than 7 GeV, if a higher harmonic is chosen as the operating wavelength . We have studied XFELO for 1 Å x-rays operating at the third or fifth harmonic using the 4 GeV SCRF linac planned for LCLS-II. Assuming bunch charge=50 pC, normalized rms emittance=0.2 mm-mrad, rms energy spread=500 keV, rms bunch length=190 fs, and undulator period length=2.6 cm, the gain at 1 Å as a 5th harmonic is found to be about 40%, sufficient for lasing allowing for the various losses.
 K.-J. Kim, Y. Shvyd’ko, and S. Reiche, Phys. Rev. Lett. 100,244802 (2008)
 J.K. Sekutowicz, et al., 2013 FEL Conf.(2013)
 J. Dai, H. Deng, and Z. Dai, Phys. Rev. Lett. 108,034802(2012)
|Slides WEA03 [2.703 MB]
|First Lasing from a High Power Cylindrical Grating Smith-Purcell Device
Funding: Work supported by ONR under Contract No. N00014-10-C-0191 and N62909-13-1-N62.
Many applications of THz radiation remain impractical or impossible due to an absence of compact sources with sufficient power. A source where the interaction occurs between an annular electron beam and a cylindrical grating is capable of generating high THz power in a very compact package. The strong beam bunching generates significant power at the fundamental frequency and harmonics. A collaboration between Advanced Energy Systems and CEA/CESTA has been ongoing in performing proof-of-principle tests on cylindrical grating configurations producing millimeter wave radiation. First lasing was achieved in such a device. Further experiments performed with a 6 mm period grating produced fundamental power at 15 GHz, second harmonic power at 30 GHz and although not measured, simulations show meaningful third harmonic power at 45 GHz. Comparison with simulations shows very good agreement and high conversion efficiency. Planned experiments will increase the frequency of operation to 100 GHz and beyond. Ongoing simulations indicate excellent performance for a device operating at a fundamental frequency of 220 GHz with realistic beam parameters at 10 kV and simple extraction of the mode.
|Slides WEA04 [2.344 MB]