Photon Pulse Filtering and Modulation Based on the Extreme Temporal Compression and Correlated Energy Spread of the Electron Bunches in the SLAC Linac Coherent Light Source (LCLS)*

smaller than those listed in the table. The LCLS photon pulses are expected to attain unprecedented levels of brightness and brevity in the 300400eV range. Nominally, the photon pulse length will be dominated by the electron bunch length, while the performance of conventional x-ray reflecting and bandshaping optics will be limited by : 1) peak power damage, and 2) transform-limited monochromatization. In this paper we describe how: 1) the correlated energy spread in the electron bunch can be used to selectably compress the LCLS photon pulses to below their nominal length; 2) gas optics can be used to mitigate peak damage problems; 3) the LCLS pulse structure can, in principle, accommodate schemes based on “disposable” optics; and 4) pulse lengthening schemes can be u-sed to extend the attainable degree of monochromatization. An important property of the electron bunch accelerated through the linac is the development of an energy gradient (or “correlated energy spread”) in the forward direction due to electrons in the front of the bunch loading the accelerating fields addressed by the trailing particles [4]. This energy spread is superimposed on the “uncorrelated” energy spread of the bunch characterized by its (stochastic) emittance parameters. For our purposes, we note that these energy spreads show up as, respectively, “inhomogeneous” and “homogene 2 us” line broadenings in the FEL photon pulse. Due to the y (or E2) dependence of the FEL’s output photon energy, these broadenings can be shown to appear with roughly twice the relative size in the photon spectrum as in the electron bunch energy.