357 research outputs found
Corrugated structure insertion for extending the SASE bandwidth up to 3% at the European XFEL
The usage of x-ray free electron laser (XFEL) in femtosecond
nanocrystallography involves sequential illumination of many small crystals of
arbitrary orientation. Hence a wide radiation bandwidth will be useful in order
to obtain and to index a larger number of Bragg peaks used for determination of
the crystal orientation. Considering the baseline configuration of the European
XFEL in Hamburg, and based on beam dynamics simulations, we demonstrate here
that the usage of corrugated structures allows for a considerable increase in
radiation bandwidth. Data collection with a 3% bandwidth, a few microjoule
radiation pulse energy, a few femtosecond pulse duration, and a photon energy
of 5.4 keV is possible. For this study we have developed an analytical modal
representation of the short-range wake function of the flat corrugated
structures for arbitrary offsets of the source and the witness particles.Comment: 29 pages, 17 figure
Numerical Calculation of Coherent Synchrotron Radiation Effects Using TraFiC4
Coherent synchrotron radiation (CSR) occurs when short bunches travel on
strongly bent trajectories. Its effects on high-quality beams can be severe and
are well understood qualitatively. For quantitative results, however, one has
to rely on numerical methods. There exist several simulation codes utilizing
different approaches. We describe in some detail the code TraFiC4 developed at
DESY for design and analysis purposes, which approaches the problem from first
principles and solves the equations of motion either perturbatively or
self-consistently. We present some calculational results and comparison with
experimental data. Also, we give examples of how the code can be used to design
beamlines with minimal emittance growth due to CSR
Integrated modeling of the TESLA X-ray FEL
The TESLA linear collider incorporates an X-ray SASE FEL which demands challenging electron beam parameters (typically transverse emittance of 1.6 mm-mrd and peak current of 5 kA). For a realistic electron beam distribution at the entrance of the undulator, tracking has to be done from the cathode through the whole accelerator. Non-Gaussian beam profiles have to be taken into account as well as nonlinear effects such as space-charge, coherent synchrotron radiation field and wake fields. We have done this with several codes: Astra, for the low energy part (<100 MeV), TraFiC4 for the bunch compressor chicanes where CSR influences the particle trajectories and the code Elegant for the 6D tracking with wake fields in the linacs and transport lines. The so-generated electron phase space density at the undulator entrance can then passed to SASEFEL simulation codes. Results of this integrated modeling is discussed
Longitudinal phase space disruption in magnetic bunch compressors
It is now well-established [2, 3] that high-charge ultra-short bunches can radiate coherently on curved trajectories (coherent synchrotron radiation). The two main consequences of such an effect are (1) an energy redistribution within the bunch, (2) a potential transverse emittance dilution in the bending plane. This effect is especially important in the foreseen next generation of free-electron laser driver linacs and linear colliders. In this paper after briefly discussing the general aspects of coherent synchrotron radiation (CSR), we report on recent experimental results obtained at the Tesla Test Facility I and compare them with numerical simulations. Schemes for reducing the impact of CSR on the beam dynamics are also discussed in the frame of the TESLA X-ray FEL project
Observation of Target Electron Momentum Effects in Single-Arm M\o ller Polarimetry
In 1992, L.G. Levchuk noted that the asymmetries measured in M\o ller
scattering polarimeters could be significantly affected by the intrinsic
momenta of the target electrons. This effect is largest in devices with very
small acceptance or very high resolution in laboratory scattering angle. We use
a high resolution polarimeter in the linac of the polarized SLAC Linear
Collider to study this effect. We observe that the inclusion of the effect
alters the measured beam polarization by -14% of itself and produces a result
that is consistent with measurements from a Compton polarimeter. Additionally,
the inclusion of the effect is necessary to correctly simulate the observed
shape of the two-body elastic scattering peak.Comment: 29 pages, uuencoded gzip-compressed postscript (351 kb). Uncompressed
postscript file (898 kb) available to DECNET users as
SLC::USER_DISK_SLC1:[MORRIS]levpre.p
Coherent Synchrotron Radiation Measurements in the CLIC Test Facility (CTF II)
Bunches of high charge (up to 10 nC) are compressed in length in the CTF II
magnetic chicane to less than 0.2 mm rms. The short bunches radiate coherently
in the chicane magnetic field, and the horizontal and longitudinal phase space
density distributions are affected. This paper reports the results of beam
emittance and momentum measurements. Horizontal and vertical emittances and
momentum spectra were measured for different bunch compression factors and
bunch charges. In particular, for 10 nC bunches, the mean beam momentum
decreased by about 5% while the FWHM momentum spread increased from 5% to 19%.
The experimental results are compared with simulations made with the code
TraFiC4.Comment: 5 pages, 6 figures, submitted to the LINAC 2000 Conferenc
Impact of Optics on CSR-Related Emittance Growth in Bunch Compressor Chicanes
Abstract The dependence of emittance growth due to Coherent Synchrotron Radiation (CSR) in bunch compressor chicanes on optics has been noticed and empirically studied in the past. We revisit the subject, suggesting a model to explain slice emittance growth dependence on chicane optics. A simplified model to calculate projected emittance growth when it is mainly caused by transverse slice centroid offsets is presented. It is then used to find optimal compensation of centroid kicks in the single chicanes of a two-stage compression system by adjusting the phase advance of the transport in between and the ration of the compression factors
Spectroscopic Properties of a Biologically Relevant [Fe2(Ό-O)2] Diamond Core Motif with a Short Iron-Iron Distance
Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since FeâFe distances of 2.5â3.4â
Ă
were attributed to âopenâ or âclosedâ cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a FeIII2(ÎŒ-O)2 complex (2) with tetrahedral (4C) centres and short FeâFe distance (2.52â
Ă
), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD FeIII centres due to the short FeâÎŒâO bonds. A â2.5â
Ă
FeâFe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2(ÎŒ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short FeâFe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology
Using a fast-gated camera for measurements of transverse beam distributions and damping times
Emittance Growth and Energy Loss due to Coherent Synchrotron Radiation in a bunch compressor
Bunches of high charge (10 nC) are compressed in length in the CTF II bunch compressor from 1.2 mm rms to less than 0.4 mm. The short bunches start to radiate coherently, thus affecting the horizontal and longitudinal phase spaces of the beam. This paper reports the results of measurements and simulations concerning the increase of the beam emittance and the impact on the energy distribution. Beam emittances were measured for different bunch compression factors and bunch charges. For each compressor setting, the energy spectrum of the beam was recorded in order to measure the energy loss due to coherent synchrotron radiation. For bunch charges of 10 nC a maximum increase of the horizontal emittance of 50% was observed at full compression, while the mean beam energy decreased by 5% from 39 MeV to 37 MeV. Both effects are correlated with an increase of the energy spread from 2.3% to 8.5% rms. The experimental results are compared with simulations
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