Photon beams of 99 eV energy carrying orbital angular momentum (OAM) have been observed in the 2nd harmonic off-axis radiation of a helical undulator at the 3rd generation synchrotron radiation light source BESSY II. For detection, the OAM carrying photon beam was superimposed with a reference beam without OAM. The interference pattern, a spiral intensity distribution, was recorded in a plane perpendicular to the propagation direction. The orientation of the observed spiral structure is related to the helicity of the undulator radiation. Excellent agreement between measurements and simulations has been found.

Extreme-ultraviolet to x-ray free-electron lasers (FELs) in operation for scientific applications are up to now single-user facilities. While most FELs generate around 100 photon pulses per second, FLASH at DESY can deliver almost two orders of magnitude more pulses in this time span due to its superconducting accelerator technology. This makes the facility a prime candidate to realize the next step in FELs-dividing the electron pulse trains into several FEL lines and delivering photon pulses to several users at the same time. Hence, FLASH has been extended with a second undulator line and self-amplified spontaneous emission (SASE) is demonstrated in both FELs simultaneously. FLASH can now deliver MHz pulse trains to two user experiments in parallel with individually selected photon beam characteristics. First results of the capabilities of this extension are shown with emphasis on independent variation of wavelength, repetition rate, and photon pulse length.

We describe a simple, reproducible, and generally applicable method to assess the performance of log amplifiers by using a fluorescent sample that provides multiple peaks of different intensities. The channel differences between multiple peaks are used to evaluate the logarithmic behavior of the fluorescence signal amplifier on the flow cytorneter. A calibration curve can be created to correct the channel numbers for deviations from true logarithmic behavior and then convert data into relative linear intensities. By using these linear fluorescent intensities, we compared the capacity of different antisera against HIV-1 (human immunodeficiency virus type 1) peptides to inhibit the binding of HIV-1 to CEM, a CD4-positive T-cell line. A wide range of applications for this calibration procedure can be envisioned and the method is valuable for monitoring instrument performance over time.Key terms: Flow cytometry, log amplifiers, log to linear conversion Flow cytometry has been used for over a decade to analyze immunofluorescently stained cells (9). Most newer flow cytometers are equipped with log amplifiers, and this type of signal processing has widely replaced linear fluorescence amplification because of a number of advantages (10). Thus, a three-or four-decade logarithmic scale in 256 channels covers a 1,000-or 10,000-fold range in fluorescent signal (3). Linear amplifiers offer only an effective range starting from the channel above the discriminator level (usually set between channels 6 and 25 depending on the flow cytometer) to channel 256 at any given gain setting. Furthermore, the shape of a distribution with a particular coefficient of variation is the same over the whole signal range, which allows easier comparison of frequency distribution shapes in different samples (5). Perhaps most important, they allow a more relevant presentation of data than a linear scale, because biologic parameters often appear as a lognormal distribution (11). Unfortunately, logarithmic display of data does not directly show the true intensity relationship between samples. However, if the amplifier were really logarithmic and the numbers of channels per decade were known, the true intensity relationship between samples could be easily calculated, as described below in the mathematical model. Unfortunately, log amplifiers are, in fact, not perfectly logarithmic, and the number of channels per decade varies over the range of the amplifier, making it risky to infer true intensity relationships between samples. Despite attempts to introduce calibration methods to compare linear and log amplification (71, the presentation of results by using arbitrary fluorescence units, e.g., mean channel or peak channel, is still a very common practice (1,6).In order to simplify conversion of logarithmic intensity of fluorescence data, we used a simple mathematical model to describe the relationship between log channel and relative intensity. To test the accuracy of this relationship in a practical setting, we evaluated the calibration of our flow c...

The Metrology Light Source is a recently constructed 630 MeV electron storage ring, operating as a synchrotron radiation source for the THz to extreme UV spectral range. It is the first storage ring optimized for generating intense, broadband, coherent THz radiation, based on a bunch shortening mode. Stable (''steady state'') or bursting THz radiation up to an average power of about 60 mW can be obtained. The applied machine operation mode is achieved by manipulating the momentum compaction factor by a novel tuning scheme. The underlying low-scheme is of general interest for operating a storage ring in a short bunch mode and is the main subject of this paper.

A new ultrahigh-resolution photoemission electron microscope called PEEM3 is being developed at the Advanced Light Source (ALS). An electron mirror combined with a sophisticated magnetic beam separator is used to provide simultaneous correction of spherical and chromatic aberrations. Installed on an elliptically polarized undulator beamline, PEEM3 will be operated with very high spatial resolution and high flux to study the composition, structure, electric and magnetic properties of complex materials.