Since demonstrating High Gain Harmonic Generation (HGHG) in the deep ultraviolet 1½ years ago, the Brookhaven Source Development Laboratory (SDL) has pursued an extensive program of developing new methods, diagnostics, applications and sources. In this talk, I will give an overview of those efforts, and focus on a few of our recent results. In HGHG, a coherent optical seed pulse at a subharmonic of the resonant FEL output wavelength is used to impose a coherent microbunching of the electron bunch before it enters the radiator. This permits rapid saturation of the harmonic output over short distances, and the possibility of reaching X-ray wavelengths by cascading the process through a relatively small number of stages. An important advantage of this scheme is the longitudinal coherence of the harmonic output. Longitudinal coherence in itself is important for experimental applications of short-pulse X-rays, but it also presents the possibility of achieving femtosecond pulses through chirped pulse amplification (CPA), and shaping short-wavelength pulses on ultrafast time scales. Such pulse shaping in the visible and infrared has been used to demonstrate quantum control in numerous atomic, molecular, and solid-state systems. In principle, it can be extended all the way down to X-ray wavelengths. The question then becomes what role noise or distortion introduced by HGHG will have on the shaping capabilities. For this reason, it is useful to consider the full “HGHG transfer function”, and measure both the amplitude and phase of the output pulse as a function of the modulation of the seed pulse. This should also have an added benefit as an FEL diagnostic, since modulations introduced by HGHG are likely to be sensitive functions of electron bunch characteristics and radiator dynamics. We are pursuing this by measuring the electric field of the ultraviolet HGHG output pulse, using a SPIDER technique, and are studying the fidelity with which phase modulations in the seed are transferred to the FEL output. Another important focus of the facility has been the development of accelerator based THz radiation sources. The subpicosecond electron bunch required for FEL operation is also used to generate coherent radiation in the THz regime. I will discuss our efforts at manipulating this through shaping of the electron bunch, our recent results on generating single cycle pulses with peak electric field intensities in the MV/cm range and peak magnetic fields in the kilogauss range, and the potentials for this new source.


Talk Slides: (slides)