SLAC, Menlo Park, California
DESY, Hamburg
For a SASE-FEL, the FEL pulse energy fluctuates from shot to shot, because the lasing process starts up from shot noise. When operating in the exponential growth regime, the radiation exhibits the properties of completely chaotic polarized light. Hence, the probability distribution of the FEL pulse energy follows a gamma distribution. Based on the measurement of such a distribution function, one can calculate the average number of ‘degrees of freedom’ or ‘modes’ in the radiation pulse. Thus, one can measure the FEL pulse temporal duration. In this paper, we report experimental results at LCLS. Measurements are conducted for both nominal charge (250 pC) and low charge (20 pC) cases. For both cases, results are obtained for different undulator lengths and various electron peak current settings.
DESY, Hamburg
The nonlinearities of the RF fields and the dispersion sections can be corrected with a higher harmonic RF module. In this paper we present an analytical solution for nonlinearity correction up to the third order in a multistage bunch compression and acceleration system without collective effects. A more general solution for a system with collective effects (space charge, wakefields, CSR effects) is found by iterative tracking procedure based on this analytical result. We apply the developed formalism to study two stage bunch compression in FLASH and three stage bunch compression in the European XFEL. Different charges are considered. Analytical estimations of RF tolerances are given.
LMU, Garching
University of Oxford, Clarendon Laboratory, Oxford
MPQ, Garching, Munich
LBNL, Berkeley, California
FZD, Dresden
Latest developments in the field of laser-wakefield acceleration (LWFA) have led to relatively stable electron beams in terms of peak energy, charge, pointing and divergence [13]. Electron beams with energies of up to 1 GeV have been produced from only few-centimeters long acceleration distances [4]. Driving undulators with these electron beams holds promise for producing brilliant X-ray sources on the university-laboratory scale. In this talk, we will present an experimental breakthrough on this path: our laser-driven soft-X-ray undulator source [5]. In the second part of the talk, we will discuss the physics behind the unique characteristics of laser-wakefield accelerated electron beams such as the intrinsic ltrashort pulse duration (expected to be about 10 fs) and the low normalized transverse emittances (expected to be < pi mm mrad). The properties of state-of-the-art wakefield accelerators as well as their limits will be discussed. Finally new schemes to overcome those limits and further improve the beam quality will be presented.
[1] Mangles, S. P. D. et al. in Nature 431, 535538 (2004).
[2] Geddes, C. G. R. et al. in Nature 431, 538541 (2004).
[3] Faure, J. et al. in Nature 431, 541544 (2004).
ELETTRA, Basovizza
University of Nova Gorica, Nova Gorica
LBNL, Berkeley, California
Two machine configurations of the electron beam dynamics in the FERMI@elettra linac have been investigated, namely the one-stage and the two-stage electron bunch compression. One of the merits of the one-stage compression is that of minimizing the impact of the microbunching instability on the slice energy spread and peak current fluctuations at the end of the linac. Special attention is given to the manipulation of the longitudinal phase space, which is strongly influenced by the linac structural wake fields. The electron bunch with a ramping peak current is used in order to obtain, at the end of the linac, an electron bunch characterized by a flat peak current profile and a flat energy distribution. Effects of various jitters on electron bunch energy, arrival time and peak current are compared and relevant tolerances are obtained.
JASRI/SPring-8, Hyogo-ken
RIKEN/SPring-8, Hyogo
An interlock sensor is indispensable to protect the undulator magnets against radiation damage. The beam halo monitor using diamond detectors, which are operated in photoconductive mode, has been developed for the X-ray free electron laser facility at SPring-8 (XFEL/SPring-8). Pulse-by-pulse measurements are adopted to suppress the background noise efficiently, and to improve the detective sensitivity. The feasibility tests of this monitor have been demonstrated at the SPring-8 compact SASE source (SCSS) test accelerator for SPring-8 XFEL. As the next step, we are trying to improve the high-frequency properties: (a) dimension of diamond detectors was newly designed to optimize the beam halo monitor for SPring-8 XFEL, (b) the microstripline structure is applied in the vacuum chamber to improve the high-frequency property, (c) RF fingers are also applied to suppress the effect of the wake field from intense electron beam. Details of these devices and experimental results are presented.