Vacuum ultraviolet radiation source using Cerenkov radiation

The goal of this research has been to develop a source of vacuum ultraviolet radiation based on the Cerenkov emission of radiation from relativistic electrons traversing a region of space filled with gas. An apparatus has been designed, fabricated, ad installd in a drift section near the end of the Stanford Linear Accelerator Center (SLAC) three-kilometer accelerator. When the apparatus is filled with helium, the SLAC primary beam characterized by its high energy, high current, and small emittance, generates a Cerenkov spectrum which is continuous from the infrared through the visible to 584A. Dispersion of the refractive index of the helium causes radiation at different wavelenghts to be emitted into separate cone angles. The optical system, designed to collect and focus only that radiation emitted into a 7 mard cone angle, allows selection of the wavelength brought to a focus through the variation of helium pressure. Low electron beam emittance insures that the collected radiation has a narrow optical bandwidth. The temporal structure of the collected light is the same as the electron beam that generates it. Variation of helium pressure in the cell from 10 torr to 600 torr resulted in collected radiation which was tuneable from 800A through the visible, a bandwidth of approx. = 4% of 1000A, and a flux of approx. = 1012 photons/mA of beam current. Collected radiation was focused to a 3 mm diameter spot with an angular divergence of 0.14 mrad. Typically, randomly light brought to the focus of the optical system was radomly polarized; collection of only a small azimuthal slice of the entire Cerenkov cone did result in linearly polarized light at the focal plane. The time structure of collected radiation was observed to be identical with that of the SLAC beam