RESUMEN
The bunch length in a linac driven Free Electron Laser (FEL) is a major parameter to be characterized to optimize the final accelerator performance. In linear machines, this observable is typically determined from the beam imaged on a screen located downstream of a Transverse Deflecting Structure (TDS) used to impinge a time dependent kick along the longitudinal coordinate of the beam. This measurement is typically performed during the machine setup and only sporadically to check the beam duration, but it cannot be continuously repeated because it is time consuming and invasive. A non-invasive method to determine the electron bunch length has already been presented in the past. This method is based on the analysis of the synchrotron radiation light spot emitted by the bunch passing through a magnetic chicane, provided that the energy chirp impinged on the bunch by the upstream radio frequency structures is known. In order to overcome a systematic discrepancy affecting the synchrotron radiation monitor based results compared to the absolute TDS based ones, we implemented and optimized a machine learning approach to predict the bunch length downstream of the two SwissFEL compression stages-from about 10 fs up to about 2 ps-as well as the beam longitudinal profile at the first one.
RESUMEN
The transverse emittance of the electron beam is a fundamental parameter in linac-based x-ray free-electron lasers (FELs). We present results of emittance measurements carried out at SwissFEL, a compact x-ray FEL facility at the Paul Scherrer Institute in Switzerland, including a description of the novel high-resolution measurement techniques and the optimization procedure. We obtained slice emittance values at the undulator entrance down to 200 nm for an electron beam with a charge of 200 pC and an rms duration of 30-40 fs. Furthermore, we achieved slice emittances as low as 100 nm for 10 pC beams with few fs duration. These values set new standards for electron linear accelerators. The quality, verification, and control of our electron beams allowed us to generate high-power FEL radiation for a wavelength as short as 0.1 nm using an electron beam with an energy of only 6 GeV. The emittance values demonstrated at SwissFEL would allow producing hard x-ray FEL pulses with even lower-energy beams, thus paving the way for even more compact and cost-effective FEL facilities.
RESUMEN
We evidence numerically and experimentally that advection can induce spectrotemporal defects in a system presenting a localized structure. Those defects in the spectrum are associated with the breakings induced by the drift of the localized solution. The results are based on simulations and experiments performed on the super-ACO free-electron laser. However, we show that this instability can be generalized using a real Ginzburg-Landau equation with (i) advection and (ii) a finite-size supercritical region.
RESUMEN
We show that the pulsed regimes observed in free-electron lasers (FELs) can be suppressed using feedback control. By applying tiny parameter perturbations, the feedback allows to keep the systems onto a stationary state that is naturally existing in phase space, but is usually inaccessible because of its unstable nature. We test this method numerically on a master equation derived from the classical iterative model. Then we present the experimental results obtained on the super-ACO FEL. This method is in principle directly applicable to the other free-electron lasers, whose instabilities have a dynamical (deterministic) origin.