Search Results (8 total)

The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.

In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.

We consider an ultrarelativistic particle traveling on axis in an infinitely long cylindrical metallic beam pipe with azimuthally varying conductivity. For a circular geometry, a semianalytical solution is obtained using the Green functions and applying approximate boundary conditions for conductors. The theory predicts an image current distribution on the pipe walls practically independent of the azimuth, at least in the frequency range relevant for future machines such as the LHC. Numerical electromagnetic simulations and bench measurements confirm the theoretical predictions. Implications for the beam-induced heating in the copper-coated, welded LHC beam screen are also addressed.

Within the framework of the CERN program on electron cloud effects in accelerators, a coaxial multipacting test stand was built. In order to simulate bunched beam, the test stand is subjected to short rf pulses. The field strength in a traveling wave mode is sufficient to trigger multipacting in “as received” surfaces, but not in chambers treated to reduce the secondary emission yield. Thus a number of upgrades in the bench setup have been pursued, mainly in two directions. The first one is a general reduction in mismatching (i.e., electrical losses) amongst the different parts of the setup. Secondly, instead of dumping the pulsed power into a load, it is recirculated by means of a broadband working regime resonant ring. This ring required the design of a directional coupler with up to 1 kV dc isolation, very low transmission losses, and a four octave bandwidth. This paper reports on the steps required to build this traveling wave resonant ring (improvements on the chamber and implementation of the coupler) and includes an appendix on the main properties of the setup that relate to electron multipacting studies.

The problem of the wakefields generated by an ultrarelativistic particle traveling in a long beam tube with a periodic rough surface has been revisited by means of a standard theory based on the hybrid modes excited in a periodically corrugated rectangular waveguide. Slow surface waves synchronous with the particle can be excited in the structure, producing wakefields whose frequency and amplitude depend on the depth of the corrugation. We apply our results to the case of the CERN Large Hadron Collider beam screen and the Linac Coherent Light Source undulator.

In the framework of the modified Bethe's diffraction theory, we study the energy lost by a relativistic particle beam traveling in a coaxial liner with many holes, including the effect of attenuation in the coaxial region. The interference among the holes is the main source of losses and is affected by the attenuation in the coaxial only over sufficiently long distances. We derive analytical formulas for all the interesting quantities and particular attention is given to clarifying the physical meaning of the results; numerical examples are considered using LHC-like parameters.

The longitudinal impedance and loss factor for a long narrow slot in a coaxial pipe are calculated by means of the modified Bethe’s diffraction theory. The effects of the interference between the fields scattered by dipoles along the slot are taken into account, obtaining a final expression valid even for slots longer than the wavelength.

The problem of many holes in a coaxial beam pipe is studied by means of the modified Bethe theory. The electromagnetic fields propagating in the coaxial region couple the equivalent dipole moments of the holes. The effect of the coupling on the longitudinal impedance and on the loss factor is investigated, showing that the interference phenomena are significant for such geometries.