for VUV FELs estimate the cost to be $30M to $50M, depending on whether the system is a demonstration model or a full user facility. A few years of research and development in key areas could improve the reliability and possibly lower the cost of a VUV FEL. Areas for further study should include (1) the RF photocathode electron gun, to produce short pulses with high beam quality, (2) beam transport, in order to maintain the necessary beam quality through the accelerator and to the FEL undulator, (3) the undulator design, (4) mirror technology, particularly for wavelength extension down to 10 nm, and (5) system optimization. The use of the gain in higher-frequency harmonics, operating simultaneously on multiple colors, and developing two-stage FELs also deserve additional research. These design options present more risk now, but with research and development they have the potential to allow short-wavelength operation at reduced size and cost.
There are areas of chemistry, physics, biology, and materials science that would benefit from improved photon sources in the VUV region. Third-generation synchrotron sources are likely to be a proving ground for many of these ideas. The committee focuses here on four areas in which a VUV FEL would provide important advances.
Specific quantum-state analysis for chemical studies using pump-probe techniques has been made possible through the development of lasers. Applied to photochemistry, this technique uses an initial laser pulse to excite a molecule to a specific state; the molecule then dissociates, producing a set of fragments that a second laser probes to determine their internal state or velocity distribution. These studies require lasers because of the need for high pulse energy (especially in the pump), tunability (to allow one to pump and probe a well-defined state), and accurate timing (to allow for synchronization of the pump and probe pulses). Because of the current wavelength limitation of lasers, the initial photolysis step, which requires the highest intensity, accesses only the lowest valence levels of most molecules. The photochemistry that occurs after absorption to higher valence levels or to Rydberg levels is inaccessible to analysis, even for such basic molecules as CH4, H2O, and CO2. The availability of a tunable laser in the VUV with the requisite 1015 photons/pulse, in a bandwidth comparable to Rydberg absorption levels (ca. 1 cm−1), and with subpicosecond timing would open up this area of photochemistry for study.