Experimental optimization of the 6-dimensional electron beam emittance generated by a Magnesium (Mg) photocathode RF gun is presented in this report. A new electron beam optimization algorithm for a low charge ( < 100 pC) beam was experimentally demonstrated: where the electron beam velocity bunching inside the RF gun plays a critical role, and the transverse emittance as a function of the laser-RF timing jitter was experimentally characterized for the first time. A 20 pC electron beam was optimized to have a normalized slice emittance of 0.15 mm mrad and a longitudinal projected emittance of 3.9 ps key. Furthermore, the upper limit of the measured thermal emittance-0.5 mm mrad per mm of the rms laser size, is about 50% lower than the theoretical prediction for a Mg cathode (Qian et al., 2010) [1]. (C) 2011 Elsevier B.V. All rights reserved.

We present the first experimental studies of the initial source of electron beam microbunching instability in a free electron laser (FEL) injector. By utilizing for the studies a transform-limited laser pulse at the photocathode, we eliminated laser-induced microbunching at the National Synchrotron Light Source Source Development Laboratory (SDL). The detailed measurements of the resulting electron beam led us to conclude that, at SDL, microbunching arising from shot noise is not amplified to any significant level, thereby allowing us to set an upper limit on the initial modulation depth of microbunching arising from shot noise. Our analysis demonstrated that the only significant source of microbunching instability under normal operational conditions at SDL is the longitudinal modulation of the photocathode laser pulse. Our work shows that assuring a longitudinally smoothed photocathode laser pulse allows mitigating microbunching instability at a typical FEL injector with a moderate microbunching gain.

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.

The NSLS Source Development Laboratory (SDL) has been a world leader in the development of laser seeded free electron lasers (FELs). Recently we initiated an experimental program to investigate a Self-Amplified Spontaneous Emission (SASE) FEL in both the exponential gain and the saturation regimes. To improve the performance of a SASE FEL, we will explore new techniques such as detuning and undulator tapering. We have achieved saturation of a SASE FEL in the visible to near infrared (IR) spectra for the first time at the NSLS SDL. We have observed spectral narrowing in the exponential gain regime and spectral broadening in the saturation regime. The experimental results are compared with simulations employing the GENESIS code [1].

A single-shot, nondestructive, electro-optical, electron bunch length monitor is experimentally verified by encoding the Coulomb field of the bunch profile on the spatial intensity distribution of an unchirped femtosecond laser pulse in an orthogonal geometry, hence a temporal-to-spatial transformation. This electron bunch measurement scheme can simultaneously measure large timing jitter (approximately in picoseconds) with a wide measurement time span covering picosecond to subpicosecond ranges.

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.

We report the experimental characterization of high-brightness electron beam generation from a magnesium (Mg) photocathode. Both the quantum efficiency (QE) and the thermal emittance of an Mg cathode are experimentally investigated. The measured QE∼0.2% is the highest reported for a metal cathode. We observed no change in the Mg cathode thermal emittance as the QE varies from 0.015% to 0.15%. The upper-limit of the thermal emittance, 0.5 mm mrad, is about 50% lower than the theoretical prediction. Our results demonstrated the feasibility of having a high QE and a low thermal emittance simultaneously for a robust metal photocathode.

With the scientific successes of the soft X-ray FLASH facility in Germany and the recent spectacular commissioning of the Linac Coherent Light Source at SLAC, free electron lasers are poised to take center stage as the premier source of tunable, intense, coherent photons of either ultra-short time resolution or ultra-fine spectral resolution, from the far infrared to the hard X-ray regime. This paper examines the state of the art in FEL performance and the underlying enabling technologies. It evaluates the state of readiness of the three basic machine architectures—SASE FELs, seeded FELs, and FEL oscillators—for the major X-ray science user facilities on the 5–10 years time scale and examines the challenges that lie ahead for FELs to achieve their full potential throughout the entire spectral range. In soft and hard X-rays, high longitudinal coherence, in addition to full transverse coherence, will be the key performance upgrade; ideas using laser-based or self-seeding or oscillators can be expected to be qualitatively superior to today's SASE sources. Short pulses, from femtoseconds to attoseconds, can be realistically envisioned. With high repetition rate electron sources coupled to superconducting radiofrequency linear accelerators, unprecedented average beam brightness will be possible and many users would be served simultaneously by a single accelerator complex.

We report the first experimental characterization of efficiency and spectrum enhancement in a laser-seeded free-electron laser using a tapered undulator. Output and spectra in the fundamental and third 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 (third harmonic) output was observed. A significant improvement in the spectra with the elimination of sidebands was observed using a tapered undulator. The experiment is in good agreement with predictions using the MEDUSA simulation code.

Last year saw the successful demonstration of an autonomously controlled two-stage turbo-Brayton cooler. The electronics developed for this demonstration evolved from the seminal work of the NICMOS cryocooler, the first flight-qualified turbo-Brayton cryocooler. Two key features of the electronics are the ability to control temperature without the use of trim heaters and the ability to recover expander power back to the main power bus. The development of the control and power electronics for this two-stage cooler was the first step toward a space qualifiable design. The technical challenges, solu- tions and performance of the electronics for the two-stage turbo-Brayton cooler are presented.

In this Letter we report the first experimental characterization of superradiance in a single-pass high-gain free-electron laser (FEL) seeded by a 150 femtosecond (FWHM) Ti:sapphire laser. The nonlinear energy gain after an exponential gain regime was observed. We also measured the evolution of the longitudinal phase space in both the exponential and superradiant regimes. The output FEL pulse duration was measured to be as short as 81 fs, a roughly 50% reduction compared to the input seed laser. The temporal distribution of the FEL radiation as predicted by a numerical simulation was experimentally verified for the first time.

The experimental observation of FEL efficiency enhancement using a tapered undulator in a single-pass seeded FEL at the NSLS SDL is reported. The last 2.5 m of the 10 m NISUS undulator was linearly tapered so that the magnetic field strength at the end of the undulator was reduced by 5%. The FEL energy gain along the undulator was measured for both the tapered and un-tapered undulators. We observed that the FEL efficiency was more than doubled by applying the taper. The experimental results are compared with the numerical simulation code,GENE- SIS1.3 [1].

We report the experimental characterization of efficiency enhancement in a single-pass seeded free-electron laser (FEL) where the electron energy is detuned from resonance. Experiments show a doubling of the efficiency for beam energies above the resonant energy. Measurements of the FEL spectra versus energy detuning shows that the wavelength is governed by the seed laser. The variation in the gain length with beam energy was also observed. Good agreement is found between the experiment and numerical simulations using the MEDUSA simulation code. (C) 2007 American Institute of Physics.

We have used electro-optic methods to investigate single-cycle THz pulses produced as coherent transition radiation from relativistic electron bunches. The pulses have energies up to 100 muJ such that electric fields approaching 100 MV/m should be attainable. We find that the combination of these large fields and their rapid variation (on a sub-ps time scale) results in significant time-dependent phase modulation of a laser pulse when co-propagated in a non-linear medium such as ZnTe.

A Reply to the Comment by R. Bonifacio et al..Received 1 June 2007DOI:https://doi.org/10.1103/PhysRevLett.99.029502©2007 American Physical Society

th harmonic HGHG and ESASE FELs was observed. We also observed the spectral broadening and instability of the 4 th harmonic HGHG.