Smith-Purcell (SP) radiation is emitted when an electron passes close to the surface of a metallic grating. The radiation becomes coherent (fluence proportional to the square of the number of electrons) when the electrons are in bunches whose dimensions are smaller than the wavelength of the radiation. This has been observed in experiments in which the electrons are prebunched by an rf linac. The enhancement of the spectral intensity is accompanied by large changes in the angular and spectral distribution of the radiation when the electrons appear in periodic bunches. This is called superradiance. Recently, superradiant SP radiation has been observed from a so-called Smith-Purcell free-electron laser (SP-FEL) in which the electrons are bunched by the lasing process. As in other slow-wave structures, the electron beam in a SP-FEL interacts with an evanescent wave for which the phase velocity matches the electron velocity and amplifies it. The frequency of this wave lies below the range of SP radiation and the wave is not radiated except from the ends of the grating. However, the bunching of the electrons by the interaction with the evanescent wave enhances the ordinary Smith-Purcell radiation and changes the angular and spectral distribution due to superradiant effects. In this article, we introduce a new method for computing the SP radiation in three dimensions, including the effects of finite grating length and superradiance due to periodic electron bunching at an arbitrary frequency. We show that the SP radiation develops spectrally and angularly narrow peaks at the harmonics of the bunching frequency. In rf linacs, where the bunches are widely spaced, several closely spaced harmonics lie under the spectral envelope of the emission from a single electron. In a SP-FEL the harmonics are widely spaced and the SP radiation appears in narrow cones at the SP angles corresponding to the harmonics of the bunching frequency. Finally, we calculate the angular spectral fluence radiated by an electron passing over a lamellar grating of finite length, examine its coherent enhancement in SP-FELs and rf linacs, and compare the results with numerical simulations and available experimental data.

The photoinjector test facility at DESY, Zeuthen site (PITZ), was built to develop and optimize photoelectron sources for superconducting linacs for high-brilliance, short-wavelength free-electron laser (FEL) applications like the free-electron laser in Hamburg (FLASH) and the European x-ray free-electron laser (XFEL). In this paper, the detailed characterization of two laser-driven rf guns with different operating conditions is described. One experimental optimization of the beam parameters was performed at an accelerating gradient of about 43 MV=m at the photocathode and the other at about 60 MV=m. In both cases, electron beams with very high phase-space density have been demonstrated at a bunch charge of 1 nC and are compared with corresponding simulations. The rf gun optimized for the lower gradient has surpassed all the FLASH requirements on beam quality and rf parameters (gradient, rf pulse length, repetition rate) and serves as a spare gun for this facility. The rf gun studied with increased accelerating gradient at the cathode produced beams with even higher brightness, yielding the first demonstration of the beam quality required for driving the European XFEL: The geometric mean of the normalized projected rms emittance in the two transverse directions was measured to be 1:26 ` 0:13 mm mrad for a 1-nC electron bunch. When a 10% charge cut is applied excluding electrons from those phase-space regions where the measured phase-space density is below a certain level and which are not expected to contribute to the lasing process, the normalized projected rms emittance is about 0.9 mm mrad

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It has previously been shown that the electron beam in a Smith-Purcell free-electron laser interacts with a synchronous evanescent wave. At high electron energy, the group velocity of this wave is positive and the device operates on a convective instability, in the manner of a traveling-wave tube. For operation as an oscillator, the gain must exceed the losses in the external feedback system. At low electron energy, the group velocity of the synchronous evanescent wave is negative and the device operates on an absolute instability, like a backward-wave oscillator, and no external feedback is required. For oscillation to occur, the current must exceed the so-called start current. At an intermediate energy, called the Bragg condition, the group velocity v g of the evanescent wave vanishes and both the gain and the attenuation due to resistive losses in the grating diverge. It is shown that near the Bragg condition the gain depends on v ÿ1=3 g , while the attenuation depends on v ÿ1 g . Since the attenuation increases faster than the gain near the Bragg condition, the Smith-Purcell free-electron laser cannot operate at the point of maximum gain. The effects of resistive losses become increasingly important as Smith-Purcell free-electron lasers move to shorter wavelengths.

Superconducting radio-frequency electron guns are viewed by many as the preferred technology for generating the high-quality, high-current beams needed for future high power free-electron lasers and energy recovery linacs. All previous guns of this type have employed elliptical cavities, but there are potential advantages associated with other geometries. Here we describe the design, commissioning, and initial results from a superconducting radio-frequency electron gun employing a quarter-wave resonator configuration, the first such device to be built and tested. In initial operation, the gun has generated beams with bunch charge in excess of 78 pC, energy of 469 keV, and normalized rms emittances of about 4:9 m. Currently, bunch charge is limited by the available drive laser energy, and beam energy is limited by x-ray production and the available rf power. No fundamental limits on beam charge or energy have been encountered, and no high-field quenching events have been observed.