Johannes Bahrdt

Circularly polarized synchrotron radiation from the crossed undulator at BESSY

The first experimental results from a double undulator with crossed magnetic fields for producing circularly polarized synchrotron radiation in the vacuum ultraviolet‐soft x‐ray range are presented. The observed variation of the extent of circularly polarized radiation with photon energy is discussed. A strong dependence of the state and degree of polarization on the exact details of the tuning of the two undulators and the monochromator is observed. This probably accounts for the measured degree of polarization being smaller than theoretically expected.

The exactly focusing spherical grating monochromator for undulator radiation at BESSY

The design goals and the performance data for the exactly focusing spherical grating monochromator at BESSY which is installed on a crossed field undulator for circularly polarized undulator radiation is described. By employing a variable deviation angle at the grating it is possible to exactly focus the entrance slit on the exit slit at all wavelengths. In addition, the optical aberrations are very small, leading to a high resolution over the energy range of the monochromator. The performance has been verified by analysis of the He (1s−1u2009np) and the Kr (3d−15/2,3/2u2009np) resonance series. A resolution of E/ΔE≊17u2009500 at 65 eV in second order has been achieved. Since commissioning, this monochromator has been used for a variety of experiments requiring both plane‐ and circularly polarized undulator radiation.

Monochromators for the undulator U49 at the BESSY II storage ring

Two types of monochromators will be built for use on the planned soft x‐ray undulator U49 of the BESSY II storage ring: A plane grating monochromator using the plane grating focusing condition and a spherical grating monochromator using the spherical grating focusing condition. The optical layout of the instruments and calculated performance data, i.e., spectral resolution, grating efficiencies, and available photon flux are presented for the photon energy range available from the U49, 137–1516 eV in the first, third, and fifth harmonic.

The Energy Materials in-Situ Laboratory Berlin (EMIL) at BESSY II

The Helmholtz Center Berlin (HZB) and the Max-Planck Society (MPG) strengthen their research in renewable energies with the implementation of the joint Energy Material in-Situ Lab Berlin (EMIL) at the third generation light source BESSY II. The new facility is dedicated to the in-situ and in-system x-ray analysis of materials and devices for photovoltaic applications, (photo-) catalytic processes, energie conversion and storage. To obtain a comprehensive understanding of the involved materials, spectroscopic methods with x-rays from the soft- up to the hard x-ray regime reveal an almost complete picture of their chemical and electronic properties. The contribution presents the layout of the x-ray beamlines and their performance in terms of photon flux, energy resolution and spot sizes.

Circular polarization measurements at the BESSY crossed field undulator

The crossed field undulator independently proposed by Moiseev et al. and K. J. Kim generates short wavelength (photon energy above 35 eV) circularly polarized radiation. It is based on the coherent superposition of two mutually orthogonally polarized wavetrains generated in two `ordinary undulators arranged one after the other along with electron beam axis. The superposition is achieved in a monochromator of sufficient spectral resolution. The relative phase is determined by the so called modulator located in between the two undulators. It is a three pole magnetic structure returning the electrons to the axis with a time delay with respect to the straight path of the order of an optical period.

In-Vacuum APPLE II Undulator

APPLE II undulators are widely used in many synchrotron radiation facilities for the generation of arbitrarily polarized light, because they provide the highest magnet fields among all planar variably polarizing permanent magnet undulators (PMUs). So far, in-vacuum permanent magnet undulators (IVUs) have a fixed polarization, either planar or elliptical / helical. A variably polarizing in-vacuum undulator was never built due to the engineering challenges. We present the design of a new invacuum APPLE II, which will extend the photon energy range to tender X-rays in the 1.7 GeV storage ring BESSY II.

Status of the Cryogenic Undulator CPMU-17 for EMIL at BESSY II / HZB

The CPMU-17 is the hard X-ray radiation source of a canted double undulator system for the Energy Materials In-situ Laboratory EMIL at BESSY II [1]. Various ambitious concepts are realized in this undulator such as Dy-hardened PrFeB-magnets, direct liquid Nitrogen cooling, dual loop feedback gap drive based on an optical micrometer and a low permeability stainless steel In-Vacuum(IV)-girder without keepers. The magnets are sorted according to Helmholtz coil and stretched wire data. Reproducibility and accuracy measurements of two IV-measurement tools needed for the CPMU-17 are presented: an IV-Hall probe bench and an IV-Moving Wire. THE UNDULATOR CPMU-17 The basic parameter set of the cryogenically cooled permanent magnet undulator is documented in [2]. Field Tuning Strategy The field quality of the magnetic structure will be tuned by means of several procedures: i) each girder base plate carries four comb shaped gauges for a precise positioning of the magnets and poles. The two combs on each side must be positioned longitudinally to an accuracy of 10μm. This is achieved via CMM measurements of the mounted combs and the insertion of specifically machined keys, which compensate for geometric fabrication tolerances; ii) all magnets are measured in an automated Helmholtz coil; iii) additionally, the side facing the electron beam is moved along a fixed, 0.5 m long wire, for the characterization of the inhomogeneities; iv) these data are used in a simulated annealing code, which optimizes for minimum trajectory errors and low phase error. The sorting is done for a pure permanent magnet structure without poles, hence, minor deviations from the sorting results are expected. However, the starting point for shimming is improved as compared to an unsorted structure; v) the trajectory straightness is shimmed with a pole height adjustment via a replacement of the pole clamps; vi) residual field integrals are shimmed with in-vacuum magic fingers (Fig. 1), which are similar to the magic fingers usually used for all BESSY II-undulators [3]; vii) gap dependent dipole errors are compensated with air coils at the flexible taper section. Endpole Compensation Triggered by a modification of magnet and pole clamps, the endpole termination was re-designed. The four tuning parameters are: the vertical sizes of two end magnets and the vertical positions of these magnets. With an appropriate choice, a field integral variation over the gap for one endpole can be as low as 0.035 Tmm, which is well within the tuning range of the air dipole coils at either end, which provide field integrals of ±0.3 Tmm in both planes at 3 A (Fig. 2 and Fig. 3). Figure 1: Magic fingers at the four magnet girder ends. Each slot is filled with a transversally quadratic permanent magnet of a specific thickness. Figure 2: Endpole configuration of the CPMU-17. Figure 3: 1 vertical field integrals versus gap of one end section (symmetric structure). Green: nominal vertical position of the last magnet. Blue, red: last magnet is moved by 0.1 and 0.2mm towards the gap. Optical Windows for Gap Measurement The magnetic gap is measured with a Keyence optical micrometer. A light band with a height of 40 mm is generated in the transmitter, passed through the 1 window TUPAB026 Proceedings of IPAC2017, Copenhagen, Denmark ISBN 978-3-95450-182-3 1372 Co py rig ht

The EMIL project at BESSY II: Beamline design and performance

The Energy Materials In-Situ Laboratory Berlin (EMIL) at BESSY-II is currently under construction. Two canted undulators for soft- and hard X-rays will be installed into the BESSY II storage ring in one straight section, complex beamlines with more than twenty optical elements will be set up and a new laboratory building attached to BESSY II will host three endstations and a large UHV-transfer system connecting various HV- and UHV-deposition systems. The undulators, UE48 and U17, provide a broad energy spectrum of 80 - 10000 eV, of which the harder radiation (>700 eV) is provided by a cryogenic in-vacuum device. Three monochromators (two plane grating monochromators (PGM) and one LN2-cooled double crystal monochromator (DCM)) disperse the radiation into separate pathways of 65u2005m length, while downstream of the monochromators split-mirror chambers distribute the photon beam to one (or simultaneously to two) of five upcoming endstations. Three of these endstations are designed for the full energy range with...

Measurements of the Lattice Modifications for the Cryogenic Undulator CPMU17

A 2 mrad-canted double undulator system is in preparation as the wide energy range light source for the Energy Material in-situ Laboratory EMIL at the HZB storage ring BESSY II. The cryogenic undulator CPMU17 is the hard X-ray device of the double undulator system. The soft X-ray undulator UE-48 is of the APPLE II type. It was installed and commissioned a few months ago, whereas the CPMU-17 is under fabrication. The CPMU-17 will employ a minimum magnetic gap of 5.5mm. Including a CuNi-foil for RF-shielding and geometric tolerances the free aperture is planned to be 5.0 mm. The BESSY II lattice has been modified locally in order to cope with the small gap device. The adapted betatron functions with a shifted vertical beam waist were measured and fitted with LOCO. The new optics agrees with the predicted performance. The free aperture at the installation place of the CPMU-17 was measured with four vertical scrapers. It is compatible with the projected minimum undulator gap. Finally, the measured injection efficiency with the new EMIL optics switched on is compatible with top-up operation (injection efficiency ≥ 90 %).

First Results from Two Novel In-vacuum Magnetic Field Measurement Devices as Built at HZB

The characterization of cryogenic in vacuum permanent magnet undulators with periods less than 20 mm and correspondingly narrow gaps requires new in-vacuum measurement systems. The positioning accuracy of the HZB in-vacuum Hallprobe bench has substantially been improved (a few μm) with appropriate feedback systems. A new in-vacuum cable tray has been developed. Another system for field integral measurements, an in-vacuum moving wire, is under commissioning. Both devices are presented. IN-VACUUM HALLPROBE BENCH Positioning Accuracy The general layout of the bench has been presented a few years ago [1]. The accuracy of the device is based on five piezo actuators and five optical measurement devices for feedback: three laser interferometer channels and two 2D-position sensitive detectors. The positioning accuracy of the in-vacuum Hallprobe bench has been successfully tested over a length of 0.5 m with the laser interferometer feedback switched on [2]. The range was limited because the cable tray was not operational, yet. Meanwhile extensive tests of the new in-vacuum cable tray have been performed. The tests will be described in the next section.