• X.J. Wang, Y. Hidaka, J.B. Murphy, B. Podobedov, H.J. Qian, S. Seletskiy, Y. Shen, X. Yang
  BNL, Upton, Long Island, New York
• C.-X. Tang
  TUB, Beijing

There is a growing interest in optimizing the electron beam for an X-ray Free Electron Laser (FEL) in the low charge (10 to 200 pC) and femto-seconds regimes. We have experimentally demonstrated sub-picosecond high-brightness electron beam for a 40 pC charge with ballistic bunch compression and a reduced laser spot size*. Simulation studies showed the feasibility of generating 10 femto-seconds kilo-ampere electron beam with a 20 pC charge**. This paper reports the progress of experimental demonstration of a femto-seconds kilo-ampere electron beam at the NSLS Source Development Lab (SDL). The femto-seconds kilo-ampere electron beam will be used to drive a self-amplified spontaneous emission (SASE) FEL, and SASE FEL spectra and pulse length will be used to measure the electron beam bunch length. The transverse properties of the electron beam will also be experimentally characterized.

*X.J. Wang, et al, Phys. Rev. E , 54, No.4, R3121 -3124 (1996).
**X.J. Wang and X.Y. Chang, Nuclear Instruments and Methods in Physics Research A 507, 310–313 (2003)

• R.K. Li, H. Chen, Q. Du, T. Du, Y.-C. Du, W.-H. Huang, J. Shi, C.-X. Tang, L.X. Yan
  TUB, Beijing
• X.J. Wang
  BNL, Upton, Long Island, New York

Funding: Supported by National Natural Science Foundation of China (No.10735050, No.10875070) and National Basic Research Program of China (973 Program)(No.2007CB815102)

Time-resolved MeV ultra-fast electron diffraction (UED) is a promising tool for studying of structural dynamics on the fundamental temporal and spatial scales of atomic motion. To reach the desired temporal and spatial resolutions, precise control and measurement of ultra-short, low emittance electron pulses are required. A MeV UED system based on an S-band photocathode RF gun is built and optimized at Tsinghua University. We present the experiment results here.

• X.J. Wang, Y. Hidaka, J.B. Murphy, B. Podobedov, H.J. Qian, S. Seletskiy, Y. Shen, X. Yang
  BNL, Upton, Long Island, New York
• B. Hafizi
  Icarus Research, Inc., Bethesda, Maryland
• J. Penano, P. Sprangle
  NRL, Washington, DC

The NSLS Source Development Laboratory (SDL) has been a world leader in the development of laser seeded free electron lasers (FEL). Recently we initiated an experimental program to investigate a Self-Amplified Spontaneous Emission (SASE) FEL in both the exponential gain and the saturation regimes. We have experimentally demonstrated the saturation of a SASE FEL in the visible to near IR. The experimental characterization of the transverse and spectral properties of the SASE FEL along the undulator for a uniformed and tapered undulator will be presented. In addition, an efficiency enhancement concept for a SASE FEL, which involves a step wiggler taper in the exponential gain regime prior to trapping, will be presented. Simulations of the SASE FEL processes will employ the GENESIS FEL code.

• X.J. Wang, D.A. Harder, J.B. Murphy, H.J. Qian, Y. Shen, X. Yang
  BNL, Upton, Long Island, New York
• H. Freund, W.H. Miner
  SAIC, McLean

Funding: This work is supported in part by the Office of Naval Research (ONR), the Joint Technology Office, and U.S. Department of Energy (DOE) under contract No. DE-AC02-98CH1-886.

We report the first experimental characterization of efficiency and spectrum enhancement in a laser-seeded free-electron laser (FEL) using a tapered undulator. Output and spectra in the fundamental and 3rd harmonic were measured versus distance for uniform and tapered undulators. With a 4% field taper over 3 m, a 300% (50%) increase in the fundamental (3rd harmonic) output was observed. A significant improvement in the spectra with the elimination of side-bands was observed for the first time using a tapered undulator. The experiment is in good agreement with predictions using the MEDUSA simulation code.

• Y. Hidaka, C.C. Kao, J.B. Murphy, S. Pjerov, B. Podobedov, H.J. Qian, S. Seletskiy, Y. Shen, X.J. Wang, X. Yang
  BNL, Upton, Long Island, New York

Ultrafast electron diffraction (UED) is a promising technique that allows us to observe a molecular structure transition on a time scale on the order of femtoseconds. The UED has several advantages over the competing technology, X-Ray Free Electron Laser (XFEL) in terms of its compactness, 6 orders of magnitude larger cross section, and less damaging ability to the samples being probed. Present state-of-the-art UED systems utilize subrelativistic electron bunches as the probing beam. With such low energy, however, the number of electrons in the bunch must be significantly decreased for a short bunch length (~100 fs) due to space charge effects. This limits the detection capability of such keV UED devices. To overcome this issue, a UED system using an MeV electron beam has been proposed, and designed at Source Development Laboratory (SDL) in National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). A detailed performance analysis of this system using the particle tracking code, GPT, from the photoinjector cathode to the detector, will be presented, as well as the status of the commissioning of our UED system.

• H.J. Qian, C.-X. Tang
  TUB, Beijing
• Y. Hidaka, J.B. Murphy, B. Podobedov, H.J. Qian, S. Seletskiy, Y. Shen, X.J. Wang, X. Yang
  BNL, Upton, Long Island, New York

Synchronization between a laser system and an electron beam plays a critical role in photoinjector operation, pump-probe experiments and many other applications. Here we report two novel experimental techniques for measuring the laser to RF timing jitter in a photoinjector, and e-beam arrival timing jitter after a magnetic chicane bunch compressor. The laser to RF timing jitter was characterized by observing the electron beam charge fluctuation through the Schottky effect. This technique was used to characterize the SDL photoinjector laser to RF timing jitter as a function of the temperature fluctuation in the laser room, and we have shown the resolution of this technique is ~100 fs. A stripline beam position monitor (BPM) located down stream of the compressor will be used to investigate the e-beam arrival timing jitter after a magnetic chicane bunch compressor; the outputs of the stripline BPM can be used to measure the arrival timing jitter by mixing them with a RF reference signal. The effect of the chicane on the arrival time jitters will be studied for the first time using this technique.

• S. Seletskiy, Y. Hidaka, J.B. Murphy, B. Podobedov, H.J. Qian, Y. Shen, X.J. Wang, X. Yang
  BNL, Upton, Long Island, New York

In this paper we report the current status of the studies of a phenomenon of microbunching at NSLS Source Development Laboratory (SDL). We observed the microbunching inside 70MeV electron bunches even for subpicosecond beams of 10pC charge. Additional microbunching is formed when the beam is compressed in the bunch compressor utilizing the 4-magnet chicane. We study the mechanisms of microbunching in an electron beam generated by a 100fs laser pulse. It allows reducing the possibility of having beam structures induced by photo-injector laser, eliminating effects of RF curvature, and enhancing the longitudinal space charge (LSC) and the coherent synchrotron radiation (CSR) effects.

• S. Seletskiy, Y. Hidaka, J.B. Murphy, B. Podobedov, H.J. Qian, Y. Shen, X.J. Wang, X. Yang
  BNL, Upton, Long Island, New York

At BNL NSLS Source Development Laboratory (SDL) 70MeV electron bunches are compressed by the bunch compressor (BC) consisting of a linac section followed by a 4-magnet chicane. The achievable beam compression is limited by nonlinear beam dynamics in the BC and by coherent synchrotron radiation (CSR) effect. In this report we present a novel beam-based technique of chicane calibration, describe the measurements of CSR effect on the beam in the chicane, and discuss the possible scenarios of the BC optimization.