We present the first observation of self-amplified spontaneous emission (SASE) in a free-electron laser (FEL) in the vacuum ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approximately 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width, and intensity fluctuations, are all consistent with the present models for SASE FELs.
The accelerating gradient performance of superconducting niobium cavities is rapidly progressing as a result of reduced field emission due to improvement in surface preparation techniques. Beyond the field emission limitation, there exists a fundamental limit imposed by the critical magnetic field of niobium. One way to tackle this limit is to reduce the ratio of the peak magnetic field to the accelerating gradient so that a higher accelerating gradient is possible while the cavity is still superconducting. New cavity shapes of reentrant type have been proposed and optimized [1]. A single cell 1300MHz cavity of this new class of shapes has been fabricated. Because of the reentrant geometry, the fabrication and surface cleaning of the cavity becomes challenging. In this paper, we present some calculation prerequisites and the fabrication and preparation results for this new cavity.
We report measurements of the temperature dependence of the surface resistivity of highquality YBa2Cu307 crystals at 5.95 GHz. A narrow transition with 89-K onset is observed.Resistivity in the normal state is found to be R", 126&10 Q. %'e establish an upper limit for the resistivity at 77 K of 1.0&10 0 and at 2 K of 1.5&10 A. The dissipation in four crystals with a total area of about 20 mm2 is measured to be nearly independent of peak microwave magnetic field over the range of 0.5 to 93 G in sharp contrast to the strong field dependence observed in bulk sintered samples. Dissipation of microwaves in superconductors is due to the interaction of the oscillating fields with the unpaired
A parallel coupled RF cavity structure which provides favorable solutions to all of the require-. merits for use in an e e-storage ring is descnbed. Properties of this structure have been determined mathematically and through measurements on S-band models. An L-band prototype is being constructed and will be tested at high power.
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