DESY, Hamburg
With the present undulator (planar, period 2.73 cm, peak field 0.486 T) the minimum wavelength of 4.5 nm at FLASH is determined by the maximum electron beam energy of approximately 1.2 GeV. On the other hand, many perspective user applications require shorter wavelength radiation and circular polarization. In this paper we perform analysis of a helical afterburner for generation of short wavelength, helically polarized radiation. We consider two options, operation of the afterburner at the second (frequency doubler), and the fourth (frequency quadrupler) harmonics. Since even harmonic of the SASE FEL radiation are suppressed, there is no linearly polarized background radiation from the main undulator. Our simulations show that relatively high level of the radiation power can be achieved in the afterburner, about 60 MW in the frequency doubler, and about 5 MW in the frequency quadrupler.
DESY, Hamburg
This report deals with the analysis of the parameter space of the European XFEL. An impact of two potential changes is analyzed: consequences of the operation with low-emittance beams, and decrease of the driving energy of the accelerator from 17.5 to 14 GeV.
DESY, Hamburg
Wavelength range of high scientific interest refers to K- and L- absorption edges of magnetic elements which spans from 2.5 nm to 1.4 nm (500 - 900 eV). This wavelength range can be partially covered by SASE3 at the European XFEL, from 1.6 nm and down when operating at the nominal energy of 17.5 GeV. Operation at the reduced energy would allow to cover complete wavelength range of interest. Currently SASE3 is a planar device producing linearly polarized radiation. On the other hand, it is important to have circular polarization for experiments with magnetic samples. Solution of the problem of polarization is installation of an afterburner generating circularly polarized radiation. This can be helical afterburner or crossed-planar afterburner operating at the fundamental or double frequency. Here we present the results for a helical afterburner operating at the double frequency.
DESY, Hamburg
With bright electron beams the full length of gap-tunable X-ray FEL undulators can be efficiently used to generate multiple x-ray beams with different independent wavelengths for simultaneous multi-user operation. We propose a betatron switcher and show that one only needs to install a compact fast kicker in front of an undulator without any modifications of the undulator itself. Different groups of bunches get different angular kicks, and for every group a kick is compensated statically (by corrections coils or moving quadrupoles) in a part of the undulator, tuned to the wavelength designated to the given group. As a generalization of the method of the betatron switcher, we briefly describe a scheme for pump-probe experiments.
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
Start-up of the amplification process in x-ray FELs from the shot noise in the electron beam defines a specific behavior of longitudinal and transverse coherence properties of the radiation. Particularly important is the case of an x-ray FEL optimized for maximum gain of the fundamental radiation mode. Applying similarity techniques to the results of numerical simulations allowed us to find universal scaling relations for the main characteristics of an optimized X-ray FEL operating in the saturation regime: efficiency, coherence time and degree of transverse coherence. We find that with an appropriate normalization of these quantities, they are functions of only the ratio of the geometrical emittance of the electron beam to the radiation wavelength. Statistical and coherence properties of the higher harmonics of the radiation are highlighted as well.
DESY, Hamburg
Longitudinal space charge (LSC) driven microbunching instability in electron beam formation systems of X-ray FELs is a recently discovered effect hampering beam instrumentation and FEL operation. The instability was observed in different facilities in infrared and visible wavelength ranges. In this paper we propose to use such an instability for generation of VUV and X-ray radiation. A typical longitudinal space charge amplifier (LSCA) consists of few amplification cascades (drift space plus chicane) with a short undulator behind the last cascade. A wavelength compression could be an attractive option for LSCA since the process is broadband, and a high compression stability is not required. LSCA can be used as a cheap addition to the existing or planned short-wavelength FELs. In particular, it can produce the second color for a pump-probe experiment. It is also possible to generate attosecond pulses in the VUV and X-ray regimes. Finally, since the amplification mechanism is broadband and robust, LSCA can be an interesting alternative to self-amplified spontaneous emission free electron laser (SASE FEL) in the case of using laser-plasma accelerators as drivers of light sources.
Report DESY 10-048, March 2010