T. Noll

Spatio-temporal coherence of free electron laser pulses in the soft x-ray regime

The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two time-delayed partial beams directly on a CCD camera. The partial beams are obtained by wave front beam splitting in an autocorrelator operating at photon energies from h nu = 30 to 200 eV. At zero delay a visibility of (0.63+/- 0.04) is measured. The delay of one partial beam reveals a coherence time of 6 fs at 23.9 nm. The visibility further displays a non-monotonic decay, which can be rationalized by the presence of multiple pulse structure.

New infrared spectroscopic beamline at BESSY II

At BESSY a multipurpose infrared (IR) beamline is being built for biological and materials science investigations. It will provide useful IR intensities over the energy range from about 10 000 down to 50 cm−1 and lower. After commissioning the first beamline section in the spring of this year first quantitative measurements on the performance have been made in the near and mid-infrared wavelength region.

Proposal for a universal test mirror for characterization of slope measuring instruments

The development of third generation light sources like the Advanced Light Source (ALS) or BESSY II brought to a focus the need for high performance synchrotron optics with unprecedented tolerances for slope error and micro roughness. Proposed beam lines at Free Electron Lasers (FEL) require optical elements up to a length of one meter, characterized by a residual slope error in the range of 0.1 μrad (rms), and rms values of 0.1 nm for micro roughness. These optical elements must be inspected by highly accurate measuring instruments, providing a measurement uncertainty lower than the specified accuracy of the surface under test. It is essential that metrology devices in use at synchrotron laboratories be precisely characterized and calibrated to achieve this target. In this paper we discuss a proposal for a Universal Test Mirror (UTM) as a realization of a high performance calibration instrument. The instrument would provide an ideal calibration surface to replicate a redundant surface under test of redundant figure. The application of a sophisticated calibration instrument will allow the elimination of the majority of the systematic error from the error budget of an individual measurement of a particular optical element. We present the limitations of existing methods, initial UTM design considerations, possible calibration algorithms, and an estimation of the expected accuracy.

The XUV split-and-delay unit at beamline BL2 at FLASH

For time-resolved extreme ultraviolet (XUV) pump?XUV probe experiments at the Free electron LASer in Hamburg (FLASH), a split-and-delay unit (SDU) has been built. It is implemented in beamline BL2 which provides a focal spot size of about 20 ?m diameter in the experiment. The beam is divided geometrically into two paths which can be delayed from ?3 to +15?ps with respect to each other. The transmission up to 200?eV photon energy is above 35% in one beam path and 74% in the other. The latter transmits the XUV beam again from 305 to 570?eV (>1% transmission). Thus almost the whole spectral range at FLASH is covered by the SDU with reasonable transmission, including the option to transport high-energy third harmonic radiation in one of the beam paths. Both beam paths are realigned into the original direction of the radiation at the end of the SDU. Thus the utilization of the divided as well as the original beam is enabled by simply moving the optical elements of the SDU into or out of the beam. Using the SDU, the coherence length and the average pulse duration at FLASH was determined to be 0.9?1.8 ?m, depending on the wavelength, and about 30 fs, respectively, for the specific electron bunch parameters.

Advanced metrology: an essential support for the surface finishing of high performance x-ray optics

The performance of x-ray beamlines at 3rd generation synchrotron radiation sources and Free Electron Lasers (FELs) is limited by the quality of the state of the art optical elements. Proposed FEL beamlines require optical components which are of better quality than is available from the optical manufacturing technology of today. As a result of a joint research project (Nanometer Optik Komponenten - NOK) coordinated by BESSY, involving both metrologists and manufacturers it is possible now to manufacture optical components beyond the former limit of 0.1 arcsec rms slope error [1, 2]. To achieve the surface finishing of optical components with a slope error in the range of 0.04 arcsec rms (for flat or spherical surfaces up to 300 mm in length) by polishing and finally by ion beam figuring technology it is essential that the optical surface be mapped and the mapping data used as input for the multiple ion beam figuring stages. Metrology tools of at least five times superior accuracy to that required of the component have been developed in the course of the project. The Nanometer Optical Component measuring Machine (NOM) was developed at BESSY for line and area measurements of the figure of optical components used at grazing incidence in synchrotron radiation beamlines. Surfaces up to 730 cm2 have been measured with the NOM a measuring uncertainty in the range of 0.01 arcsec rms and a correspondingly high reproducibility [3]. Three dimensional measurements were used to correct polishing errors some nanometers high and only millimeters in lateral size by ion beam treatment. The design of the NOM, measurement results and results of NOM supported surface finishing by ion beam figuring will be discussed in detail. The improvement of beamline performance by the use of such high quality optical elements is demonstrated.

Elliptically Bent X-Ray Mirrors with Active Temperature Stabilization

We present details of design of elliptically bent Kirkpatrick-Baez mirrors developed and successfully used at the Advanced Light Source for submicron focusing. A distinctive feature of the mirror design is an active temperature stabilization based on a Peltier element attached directly to the mirror body. The design and materials have been carefully optimized to provide high heat conductance between the mirror body and substrate. We describe the experimental procedures used when assembling and precisely shaping the mirrors, with special attention paid to laboratory testing of the mirror-temperature stabilization. For this purpose, the temperature dependence of the surface slope profile of a specially fabricated test mirror placed inside a temperature-controlled container was measured. We demonstrate that with active mirror-temperature stabilization, a change of the surrounding temperature by more than 3K does not noticeably affect the mirror figure. Without temperature stabilization, the surface slope changes by approximately 1.5 ?mu rad rms (primarily defocus) under the same conditions.

Adaption of the BESSY I-3m normal incidence monochromators to the BESSY II source

Abstract The optical and mechanical lay-out of pre- and post-monochromator optics as well as calculated performance data of the 3m-normal incidence monochromators transferred from BESSY I to BESSY II will be presented. For the design, special emphasis was laid on high resolving power and small spot size at the experiment.

Three axis rotational flexure joints of high axial stiffness

The mechanisms for mirror positioning in synchrotron radiation beamlines at BESSY have been shown to have unique stability, precision, and reproducibility. For these mechanisms play-free flexure joints are used exclusively. The elastic material for these joints needs to have a low stiffness against bending and torsion and a high stiffness in the axial direction. It should allow a large number of moving cycles and must not fracture spontaneously. Fiber materials and especially stainless steel rope segments of small length accomplish these requirements. The paper describes a patented (Patent DE10042801.0) arrangement and is intended to provide useful information as an aid in applying this design to other mechanisms.

An x-ray autocorrelator and delay line for the VUV-FEL at TTF/DESY

In order to do jitter-free X-ray pump and probe experiments at the VUV-FEL at DESY / Hamburg (TTF2) as well as to characterize the temporal structure of its high power pulses an X-ray autocorrelator has been designed and is being engineered for photon energies up to 200 eV. The optomechanical design is based on geometrical beam splitting of the incomming FEL beam by a sharp mirror edge. Due to the limited reflection and the strong absorption of soft X-ray radiation an all-reflective geometry with grazing incidence angles at the mirrors has been chosen. The actual design represents a compromise between size and total delay range, on the one hand, and efficiency on the other hand. Thus the optomechanical device allows to handle high power X-ray pulses with high efficiency (50 %). The total delay is about 25 ps with a femtosecond resolution. A further advantage of the special autocorrelator design is the lack of any angle deviation of the outgoing beam direction. Thus the autocorrelator can be integrated permanently into one of the FEL beamlines and measurements can be done with or without the beam splitter by slighly moving the whole chamber without breaking the vacuum. First experiments are planned in 2006 utilizing two-photon photoemission from noble gases in order to measure the temporal width of the FEL pulses at 40 eV.

Six-strut arrangements for cartesian movements of mirrors

Abstract At BESSY a new six-strut arrangement for general small travel mirror adjustment mechanisms has been developed. This patented (Patent DE 10042802.5) arrangement allows very simple movements in all six linear and rotational degrees of freedom. The movements of the mirror are simply determined by moving either one drive, or up to three drives by the same amount. The first mirror adjustment systems of this design is successfully in operation since the start of BESSY II. Their performance and reliability is very satisfactory. This contribution will present the concepts.