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.

title: UPGRADE AND EVALUATION OF THE BUNCH COMPRESSION MONITOR AT THE FREE-ELECTRON LASER IN HAMBURG (FLASH)
abstract: The control and stabilization of RF systems for accelerators are of considerable importance. In case of high-gain free-electron lasers (FEL) with magnetic bunch compressors, the RF phases determine the attainable bunch peak current, which is a relevant parameter for driving the FEL process. In order to measure the bunch peak current in a simple and fast but indirect way, both bunch compressors at FLASH are equipped with compression monitors (BCM) based on pyroelectric detectors and diffraction radiators. They provide substantial information to tune the bunch compression and are used for a beam-based feedback to stabilize the RF phases. This monitor system became more important and more challenging after the installation of a third-harmonic RF system for longitudinal phase space linearization. In this paper, we describe the hardware upgrade of the bunch compression monitor and show the expected performance by simulations of the CDR source and the radiation transport optics. Particle tracking simulations are used for generation of the simulated BCM signal for various compression schemes. Comparison with experimental data will be presented.

title: ELECTRO-OPTIC BUNCH ARRIVAL TIME MEASUREMENT AT FLASH
abstract: Optical synchronization systems, based on modelocked erbium-doped fiber lasers whose pulses are distributed over length stabilized fiber links, are expected to be drift free and to provide femtosecond stability for the next generation of free electron lasers. In this paper we provide a comparison between the stability of two bunch arrival time monitors working on different principles. In the first type the bunch-induced signal in a broad-band beam pick-up invokes an amplitude modulation in a train of short laser pulses, which are detected with a photo diode and a fast ADC. The second type utilizes the electro-optic effect in a 175 μm thick GaP crystal, placed 3 mm away from the electron beam. Both methods show excellent correlation over a change in the bunch compression and acceleration. The achieved arrival time jitter is 80 fs over 20 s, which is near the limit of the present RF stabilization at FLASH.

title: Using Line Profiles to Test the Fraternity of Type Ia Supernovae at High and Low Redshifts
abstract: Using archival data of low-redshift (z 1.7] SNe Ia, which are also subluminous. In addition, we give the first direct evidence in two high-z SN Ia spectra of a double-absorption feature in Ca II λ3945, an event also observed, although infrequently, in low-redshift SN Ia spectra (6 out of 22 SNe Ia in our local sample). Moreover, echoing the recent studies of Dessart & Hillier in the context of Type II supernovae (SNe II), we see similar P Cygni line profiles in our large sample of SN Ia spectra. First, the magnitude of the velocity location at maximum profile absorption may underestimate that at the continuum photosphere, as observed, for example, in the optically thinner line S II λ5640. Second, we report for the first time the unambiguous and systematic intrinsic blueshift of peak emission of optical P Cygni line profiles in SN Ia spectra, by as much as 8000 km s-1. All the high-z SNe Ia analyzed in this paper were discovered and followed up by the ESSENCE collaboration and are now publicly available.

title: An Energetic Afterglow from a Distant Stellar Explosion
abstract: We present the discovery of radio afterglow emission from the high-redshift (z = 6.295) burst GRB 050904. The peak flux density for this burst is similar to typical low-redshift gamma-ray bursts (GRBs). We further show that beyond a redshift of order unity, the flux densities of radio afterglows are largely insensitive to redshift, consistent with predictions. By combining the existing X-ray, near-infrared, and radio measurements, we derive estimates for the kinetic energy and opening angle of the blast wave and for the density of the circumburst medium into which it expands. Both the kinetic and radiated energy indicate that GRB 050904 was an unusually energetic burst (10^52 ergs). More importantly, we are able to make an in situ measurement of the density structure of the circumburst medium. We conclude that GRB 050904 exploded into a constant-density medium with n0 = 680 cm^−3, which is 2 orders of magnitude above the nominal value for low-redshift GRBs. The next generation of centimeter (EVLA) and millimeter radio instruments (ALMA) will be able to routinely detect events such as GRB 050904 and use them to study magnetic fields and the atomic and molecular gas in the high-redshift universe.

title: Cosmological Results from High-z Supernovae* **
abstract: The High-z Supernova Search Team has discovered and observed eight new supernovae in the redshift interval z = 0.3-1.2. These independent observations, analyzed by similar but distinct methods, confirm the results of Riess and Perlmutter and coworkers that supernova luminosity distances imply an accelerating universe. More importantly, they extend the redshift range of consistently observed Type Ia supernovae (SNe Ia) to z ≈ 1, where the signature of cosmological effects has the opposite sign of some plausible systematic effects. Consequently, these measurements not only provide another quantitative confirmation of the importance of dark energy, but also constitute a powerful qualitative test for the cosmological origin of cosmic acceleration. We find a rate for SN Ia of (1.4 ± 0.5) × 10-4 h3 Mpc-3 yr-1 at a mean redshift of 0.5. We present distances and host extinctions for 230 SN Ia. These place the following constraints on cosmological quantities: if the equation of state parameter of the dark energy is w = -1, then H0t0 = 0.96 ± 0.04, and ΩΛ - 1.4ΩM = 0.35 ± 0.14. Including the constraint of a flat universe, we find ΩM = 0.28 ± 0.05, independent of any large-scale structure measurements. Adopting a prior based on the Two Degree Field (2dF) Redshift Survey constraint on ΩM and assuming a flat universe, we find that the equation of state parameter of the dark energy lies in the range -1.48 -1, we obtain w < -0.73 at 95% confidence. These constraints are similar in precision and in value to recent results reported using the WMAP satellite, also in combination with the 2dF Redshift Survey.

title: The Rise time of Nearby Type 1a Supernovae
abstract: The 
work at the University of California, Berkeley, was supported 
by the Miller Institute for Basic Research in Science, by NSF grant AST 94-17213, and by grant GO-7505 from 
the Space Telescope Science Institute, which is operated by 
the Association of Universities for Research in Astronomy, 
Inc., under NASA contract NAS 5-26555.

title: Absolute luminosity measurements with the LHCb detector at the LHC
abstract: Absolute luminosity measurements are of general interest for colliding-beam experiments at storage rings. These measurements are necessary to determine the absolute cross-sections of reaction processes and are valuable to quantify the performance of the accelerator. LHCb has applied two methods to determine the absolute scale of its luminosity measurements for proton-proton collisions at the LHC with a centre-of-mass energy of 7 TeV. In addition to the classic "van der Meer scan" method a novel technique has been developed which makes use of direct imaging of the individual beams using beam-gas and beam-beam interactions. This beam imaging method is made possible by the high resolution of the LHCb vertex detector and the close proximity of the detector to the beams, and allows beam parameters such as positions, angles and widths to be determined. The results of the two methods have comparable precision and are in good agreement. Combining the two methods, an overal precision of 3.5% in the absolute luminosity determination is reached. The techniques used to transport the absolute luminosity calibration to the full data-taking period are presented.