49385021
AN OPTICAL CROSS-CORRELATION SCHEME TO SYNCHRONIZE DISTRIBUTED LASER SYSTEMS AT FLASH
The VUVand soft X-ray free-electron laser FLASH and the planned European XFEL generate X-ray light pulses in the femto-second range. For time-resolved pump-probe experiments, future operation modes by means of laser seeding and for special diagnostic measurements it is crucial to synchronize various laser systems to the electron beam with an long-term stability of better than 30 fs. For this purpose an optical synchronization system is currently being installed and tested at FLASH. In this paper, we report on the progress of the development of a background-free optical cross-correlation scheme to synchronize two individual mode-locked laser systems of different center wavelengths and repetition rates with an accuracy of better than 10 fs. The scheme is being tested by linking a commercial Ti:sapphire oscillator, used for electro-optical diagnostics at FLASH, to a locally installed erbium-doped fiber laser. Later, this laser will be replaced by an end-point of an actively length-stabilized fiber-link distributing the pulses from the master laser oscillator of the machine to lock the diagnostics laser to the optical synchronization system. THE OPTICAL SYNCHRONIZATION SYSTEM OF FLASH The optical synchronization system for FLASH [1, 2] and under consideration for the European XFEL [3] is based on a ultra-stable, mode-locked master laser (MLO) locked to a low-noise RF oscillator generating sub-ps light pulses at the telecommunication wavelength of 1550 nm. The timing information is contained in the precise repetition frequency of 216.66 MHz of the pulse train and distributed via actively length-stabilized fiber links to the remote locations along the accelerator. At these locations, low-level RF signals can be extracted using photo detectors and bandpass filters or Sagnac loops to pick a harmonic of the MLO’s repetition rate. Another application is the use of the distributed light pulses directly, for example for bunch arrival time monitors [4] or after frequency doubling to seed a Ti:sapphire amplifier. One of the key component of the synchronization system is a balanced cross-correlator, because it provides a mechanism to lock different laser systems, as those used for electron beam generation, diagnostics and pump-probe experiments, to the timing reference with femtosecond accuracy. TWO-COLOR BALANCED OPTICAL CROSS-CORRELATOR The operation principle of such a two-color balanced optical cross-correlator is based on the sum frequency generation (SFG) in a type-I phase-matched nonlinear crystal and the generation of a group delay difference between the two input pulses with the same polarization but different center frequencies using a dispersive material. In our first prototype we use a commercial ultra-short pulse 81.25 MHz Ti:sapphire oscillator with a center wavelength of 800 nm and a home-built erbium-doped fiber laser (EDFL) with a center wavelength of 1550 nm as the timing reference, resulting in a wavelength of 527.7 nm of the corresponding sum frequency. Figure 1: Schematic setup of a balanced optical crosscorrelator and sketched travel of the optical pulses inside the device. See text for details. A more detailed view of the optical setup is given in Fig. 1. After combining both laser beams on a common optical axis the horizontally polarized input pulses enter the cross-correlator through a first dichroic mirror, which is transmissive at 800 nm and 1550 nm, but reflective at the sum frequency of the input pulses. Then the pulses are focussed into the phase-matched sum frequency generating BBO crystal and collimated by a second lens. The resulting sum frequency is transmitted through a second dichroic mirror and passed through an optical bandpass filter onto THPC160 Proceedings of EPAC08, Genoa, Italy 06 Instrumentation, Controls, Feedback & Operational Aspects 3366 T23 Timing and Synchronization the first photo detector. Assuming Gaussian-shaped input pulses with the intensities I1(t) and I2(t), the measured intensity of the SFG component I+(t) is given by the convolution of the input pulses:
55568152
Ultrabroadband terahertz source and beamline based on coherent transition radiation
Coherent transition radiation (CTR) in the THz regime is an important diagnostic tool for analyzing the temporal structure of the ultrashort electron bunches needed in ultraviolet and x-ray free-electron lasers. It is also a powerful source of such radiation, covering an exceptionally broad frequency range from about 200 GHz to 100 THz. At the soft x-ray free-electron laser FLASH we have installed a beam transport channel for transition radiation (TR) with the intention to guide a large fraction of the radiation to a laboratory outside the accelerator tunnel. The radiation is produced on a screen inside the ultrahigh vacuum beam pipe of the linac, coupled out through a diamond window and transported to the laboratory through an evacuated tube equipped with five focusing and four plane mirrors. The design of the beamline has been based on a thorough analysis of the generation of TR on metallic screens of limited size. The optical propagation of the radiation has been computed taking into account the effects of near-field (Fresnel) diffraction. The theoretical description of the TR source is presented in the first part of the paper, while the design principles and the technical layout of the beamline are described in the second part. First experimental results demonstrate that the CTR beamline covers the specified frequency range and preserves the narrow time structure of CTR pulses emitted by short electron bunches.
124862478
BUNCH COMPRESSION MONITOR
The longitudinal charge distribution in the electron bunches has a strong impact on the lasing process in a Free-Electron Laser based on the principle of Self Amplified Spontaneous Emission of radiation. For the ultraviolet and soft X ray FEL FLASH at DESY, structures in the range of ten to hundred micrometers play a crucial role. The investigation of the longitudinal charge distribution in the electron bunches on a bunch-by-bunch basis is an important issue for optimizing the bunch compression and improving the performance of the machine. This paper introduces a new tool for longitudinal diagnostics based on THz spectroscopy of coherent radiation. A novel spectrometer has been designed which permits to analyze the radiation of single electron bunches in a broad spectral range and with high resolution. Preliminary measurements with this spectrometer are presented. A strong variation of the CTR intensity in the 40 μm regime is observed when the off-crest phase in the accelerating cavities preceding the first magnetic bunch compressor is changed in small steps. This variation in the CTR intensity is found to be correlated with a similar variation in the pulse energy of the FEL light. Hence the THz spectrometer appears to be a very sensitive device for controlling the parameters of the bunch compression scheme.
119297111
HATSouth: A Global Network of Fully Automated Identical Wide-Field Telescopes*
HATSouth is the world's first network of automated and homogeneous telescopes that is capable of year-round 24 hr monitoring of positions over an entire hemisphere of the sky. The primary scientific goal of the network is to discover and characterize a large number of transiting extrasolar planets, reaching out to long periods and down to small planetary radii. HATSouth achieves this by monitoring extended areas on the sky, deriving high precision light curves for a large number of stars, searching for the signature of planetary transits, and confirming planetary candidates with larger telescopes. HATSouth employs six telescope units spread over three prime locations with large longitude separation in the southern hemisphere (Las Campanas Observatory, Chile; HESS site, Namibia; Siding Spring Observatory, Australia). Each of the HATSouth units holds four 0.18 m diameter f/2.8 focal ratio telescope tubes on a common mount producing an 8.2° × 8.2° field of view on the sky, imaged using four 4 K × 4 K CCD cameras and Sloan r filters, to give a pixel scale of 3.7'' pixel-1. The HATSouth network is capable of continuously monitoring 128 square arc degrees at celestial positions moderately close to the anti-solar direction. We present the technical details of the network, summarize operations, and present detailed weather statistics for the three sites. Robust operations have meant that on average each of the six HATSouth units has conducted observations on ~500 nights over a 2 years time period, yielding a total of more than 1 million science frames at a 4 minute integration time and observing ~10.65 hr day-1 on average. We describe the scheme of our data transfer and reduction from raw pixel images to trend-filtered light curves and transiting planet candidates. Photometric precision reaches ~6 mmag at 4 minute cadence for the brightest non-saturated stars at r ≈ 10.5. We present detailed transit recovery simulations to determine the expected yield of transiting planets from HATSouth. We highlight the advantages of networked operations, namely, a threefold increase in the expected number of detected planets, as compared to all telescopes operating from the same site.
119881774
Anisotropic low energy electron collision cross sections for methane derived from transport coefficients
Based on a comprehensive set of transport parameters of electrons in methane derived from single electron spectra in the range of field 0.01-15 Td the authors determined a set of elastic and inelastic scattering cross sections up to 3 eV. The necessity to include anisotropic elastic scattering in a Boltzmann analysis of methane is demonstrated. In addition the drift velocity in a mixture of methane with helium was used to enhance the reliability of the derived cross sections. The low energy part of the cross sections for elastic scattering is subject to a MERT analysis and is compared with total cross section data from single scattering experiments.
123101687
Luminosity determination at HERA-B
Abstract A detailed description of an original method used to measure the luminosity accumulated by the HERA-B experiment for a data sample taken during the 2002–2003 HERA running period is reported. We show that, with this method, a total luminosity measurement can be achieved with a typical precision, including overall systematic uncertainties, at a level of 5% or better. We also report evidence for the detection of δ -rays generated in the target and comment on the possible use of such delta rays to measure luminosity.
15727772
The detailed optical light curve of GRB 030329
We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst (GRB) 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical data set contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. This data set allows us to follow the photometric evolution of the transient with unprecedented detail. We use the data to constrain the light curve of the underlying supernova (SN) 2003dh and show that it evolved faster than and was probably somewhat fainter than the Type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be alpha(1) approximate to 1.1 and alpha(2) approximate to 2. Assuming this single-break power-law model, we constrain the break to lie between similar to3 and similar to8 days after the burst. This simple, singly broken power-law model, derived only from the analysis of our optical observations, may also account for available multiband data, provided that the break happened similar to8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, millimeter, and radio observations of this event. The unique early coverage available for this event allows us to trace the color evolution of the afterglow during the first hours after the burst. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling-break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations (less than or similar to8 days after the burst) are asymmetric, with a steep rise followed by a relatively slower ( by a factor of about 2) decline. The variations maintain a similar timescale during the first 4 days and then get significantly longer. The structure of these variations is similar to those previously detected in the afterglows of several GRBs.
40178169
A Comprehensive Study of GRB 070125, A Most Energetic Gamma-Ray Burst
We present a comprehensive multiwavelength analysis of the bright, long-duration gamma-ray burst GRB 070125, comprised of observations in gamma-ray, X-ray, optical, millimeter, and centimeter wave bands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation and used to derive an upper limit of 2.4 × 10^(17) cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self-absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a windlike profile (R^(−2)). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that GRB 070125 is a burst with high radiative efficiency (>60%).
119231582
Measurement of the D ± production asymmetry in 7 TeV pp collisions
Heavy quark production in 7 TeV centre-of-mass energy pp collisions at the LHC is not necessarily flavour symmetric. The production asymmetry, A(P), between D-s(+) and D-s(-) mesons is studied using the phi pi(+/-) decay mode in a data sample of 1.0 fb(-1) collected with the LHCb detector. The difference between pi(+) and pi(-) detection efficiencies is determined using the ratios of fully reconstructed to partially reconstructed D*(+/-) decays. The overall production asymmetry in the D-s(+/-) rapidity region 2.0 to 4.5 with transverse momentum larger than 2 GeV is measured to be A(P) = (-0.33 +/- 0.22 +/- 0.10)%. This result can constrain models of heavy flavour production. (C) 2012 CERN. Published by Elsevier By. All rights reserved.
59127326
Measurement of the charge asymmetry in $B^{\pm}\rightarrow \phi K^{\pm}$ and search for $B^{\pm}\rightarrow \phi \pi^{\pm}$ decays
The \CP-violating charge asymmetry in $B^{\pm}\rightarrow \phi K^{\pm}$ decays is measured in a sample of $pp$ collisions at 7 TeV centre-of-mass energy, corresponding to an integrated luminosity of 1.0 fb$^{-1}$ collected by the LHCb experiment. The result is $\mathcal{A}_{CP}(B^{\pm}\rightarrow \phi K^{\pm}) = \rm 0.022\pm 0.021 \pm 0.009$, where the first uncertainty is statistical and the second systematic. In addition, a search for the $B^{\pm}\rightarrow \phi \pi^{\pm}$ decay mode is performed, using the $B^{\pm}\rightarrow \phi K^{\pm}$ decay rate for normalization. An upper limit on the branching fraction $\mathcal{B}(B^{\pm}\rightarrow \phi \pi^{\pm})< 1.5\times 10^{-7}$ is set at 90% confidence level.
209834344
Measurement of the CKM angle $\gamma$ using $B^\pm \to D K^\pm$ with $D \to K^0_{\rm S} \pi^+\pi^-, K^0_{\rm S} K^+ K^-$ decays