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This review paper describes the energy-upgraded Continuous Electron Beam Accelerator Facility (CEBAF) accelerator. This superconducting linac has achieved 12 GeV beam energy by adding 11 new high-performance cryomodules containing 88 superconducting cavities that have operated cw at an average accelerating gradient of 20 MV/m. After reviewing the attributes and performance of the previous 6 GeV CEBAF accelerator, we discuss the upgraded CEBAF accelerator system in detail with particular attention paid to the new beam acceleration systems. In addition to doubling the acceleration in each linac, the upgrade included improving the beam recirculation magnets, adding more helium cooling capacity to allow the newly installed modules to run cold, adding a new experimental hall, and improving numerous other accelerator components. We review several of the techniques deployed to operate and analyze the accelerator performance and document system operating experience and performance. In the final portion of the document, we present much of the current planning regarding projects to improve accelerator performance and enhance operating margins, and our plans for ensuring CEBAF operates reliably into the future. For the benefit of potential users of CEBAF, the performance and quality measures for the beam delivered to each of the experimental halls are summarized in the Appendix.

At the electron linac for beams with high brilliance and low emittance (ELBE) center for high-power radiation sources, the second version of a superconducting radio-frequency (SRF) photoinjector has been put into operation and has been routinely applied for user operation at the ELBE electron accelerator. SRF guns are suitable for generating a continuous wave electron beam with high average currents and high beam brightness. The SRF gun at ELBE has the goal to generate short electron pulses with bunch charges of 200–300 pC at typical repetition rates of 100 kHz for the production of superradiant, coherent terahertz radiation. The SRF gun includes a 3.5-cell, 1.3-GHz niobium cavity and a superconducting solenoid. A support system with liquid nitrogen (LN_{2}) cooling allows the operation of normal-conducting, high quantum efficiency photocathodes. We present the design and performance of the SRF gun as well as beam measurement results of the operation with Mg photocathodes at an acceleration gradient of 8 MV/m (4 MeV kinetic energy). In the last section, we discuss the SRF gun application for production of coherent terahertz radiation at the ELBE facility.

Activities of the past several years in developing the technique of forming seamless (weldless) cavity cells by hydroforming are summarized. An overview of the technique developed at DESY for the fabrication of single cells and multicells of the TESLA cavity shape is given and the major rf results are presented. The forming is performed by expanding a seamless tube with internal water pressure while simultaneously swaging it axially. Prior to the expansion the tube is necked at the iris area and at the ends. Tube radii and axial displacements are computer controlled during the forming process in accordance with results of finite element method simulations for necking and expansion using the experimentally obtained strain-stress relationship of tube material. In cooperation with industry different methods of niobium seamless tube production have been explored. The most appropriate and successful method is a combination of spinning or deep drawing with flow forming. Several single-cell niobium cavities of the 1.3 GHz TESLA shape were produced by hydroforming. They reached accelerating gradients E_{acc} up to 35 MV/m after buffered chemical polishing (BCP) and up to 42 MV/m after electropolishing (EP). More recent work concentrated on fabrication and testing of multicell and nine-cell cavities. Several seamless two- and three-cell units were explored. Accelerating gradients E_{acc} of 30–35 MV/m were measured after BCP and E_{acc} up to 40 MV/m were reached after EP. Nine-cell niobium cavities combining three three-cell units were completed at the company E. Zanon. These cavities reached accelerating gradients of E_{acc}=30–35 MV/m. One cavity is successfully integrated in an XFEL cryomodule and is used in the operation of the FLASH linear accelerator at DESY. Additionally the fabrication of bimetallic single-cell and multicell NbCu cavities by hydroforming was successfully developed. Several NbCu clad single-cell and double-cell cavities of the TESLA shape have been fabricated. The clad seamless tubes were produced using hot bonding or explosive bonding and subsequent flow forming. The thicknesses of Nb and Cu layers in the tube wall are about 1 and 3 mm respectively. The rf performance of the best NbCu clad cavities is similar to that of bulk Nb cavities. The highest accelerating gradient achieved was 40 MV/m. The advantages and disadvantages of hydroformed cavities are discussed in this paper.

Photocathodes are a limiting factor for the next generation of ultrahigh brightness photoinjectors. We studied the behavior of a superconducting Pb cathode in the cryogenic environment of a superconducting rf gun cavity to measure the quantum efficiency, its spatial distribution, and the work function. We will also discuss how the cathode surface contaminants modify the performance of the photocathode as well as the gun cavity and we discuss the possibilities to remove these contaminants.

This paper reports on our efforts to develop a flangeable coaxial coupler for both higher order mode and fundamental coupling for nine-cell ILC-type cavities, which were designed in the early 1990’s for pulsed operation with a duty factor less than 1%. The design of the coupler has been done in such a way that the rf magnetic flux B at the flange connection was minimized and only a field of

The most challenging issue for understanding the performance of superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio >200) niobium is due to a sharp degradation (“Q-drop”) of the cavity quality factor Q_{0}(B_{p}) as the peak surface magnetic field (B_{p}) exceeds about 90 mT, in the absence of field emission. In addition, a low-temperature (100–140°C) in situ baking of the cavity was found to be beneficial in reducing the Q-drop. In this contribution, we present the results from a series of rf tests at 1.7 and 2.0 K on a single-cell cavity made of high-purity large (with area of the order of few cm^{2}) grain niobium which underwent various oxidation processes, after initial buffered chemical polishing, such as anodization, baking in pure oxygen atmosphere, and baking in air up to 180°C, with the objective of clearly identifying the role of oxygen and the oxide layer on the Q-drop. During each rf test a temperature mapping system allows measuring the local temperature rise of the cavity outer surface due to rf losses, which gives information about the losses location, their field dependence, and space distribution. The results confirmed that the depth affected by baking is about 20–30 nm from the surface and showed that the Q-drop did not reappear in a previously baked cavity by further baking at 120°C in pure oxygen atmosphere or in air up to 180°C. These treatments increased the oxide thickness and oxygen concentration, measured on niobium samples which were processed with the cavity and were analyzed with transmission electron microscope and secondary ion mass spectroscopy. Nevertheless, the performance of the cavity after air baking at 180°C degraded significantly and the temperature maps showed high losses, uniformly distributed on the surface, which could be completely recovered only by a postpurification treatment at 1250°C. A statistic of the position of the “hot spots” on the cavity surface showed that grain boundaries are not the preferred location. An interesting correlation was found between the Q-drop onset, the quench field, and the low-field energy gap, which supports the hypothesis of thermomagnetic instability governing the Q-drop and the baking effect.

Three 6-cell 805 MHz superconducting cavity prototypes for acceleration in the velocity range of about 0.4 to 0.53 times the speed of light have been fabricated and tested. The quality factors (Q_{0}) were between 7×10^{9} and 1.4×10^{10} at the design field (accelerating gradient of 8–10 MV/m). The maximum gradients reached were between 11 and 16 MV/m; in each case, the Q_{0} values were ≥3×10^{9} at the maximum gradient. The design, fabrication, surface preparation, and rf testing of the 6-cell cavities are reported in this paper.

Commissioning of two large coherent light facilities (XFELs) at SLAC and DESY should begin in 2008 and 2011, respectively. In this paper we look further into the future, hoping to answer, in a very preliminary way, two questions. First: What will the next generation of XFEL facilities look like? Believing that superconducting technology offers advantages such as high quality beams with highly populated bunches, the possibility of energy recovery and higher overall efficiency than warm technology, we focus this preliminary study on the superconducting option. From this belief the second question arises: What modifications in superconducting technology and in the machine design are needed, as compared to the present DESY XFEL, and what kind of research and development program should be proposed to arrive in the next few years at a technically feasible solution with even higher brilliance and increased overall conversion of ac power to photon beam power? In this paper we will very often refer to and profit from the DESY XFEL design, acknowledging its many technically innovative solutions.

Due to the rise of the Internet, the effects of different science communication formats in which experts appear cannot be neglected in communication research. Through their emotional and more comprehensible communication ‘sciencetubers’—who frequently differ from the stereotypical image of scientists as white, old men—may have a considerable effect on the public’s perceived trustworthiness of scientists as well as their trust in science. Thus, this study aims to extend trust and trustworthiness research to consider the role of emotion in science communication in the context of emerging online video content. Therefore, perceived trustworthiness was examined in an experimental online survey of 155 people aged 18–80. We considered different potential influencing variables for trustworthiness (expertise, integrity, benevolence) and used six different video stimuli about physics featuring scientific experts. The video stimuli varied according to format (TV interviews vs. YouTube videos), gender (male vs. female), and age of the experts depicted (old vs. young). The results suggest that: (1) Scientific experts appearing in TV interviews are perceived as more competent but not higher in integrity or benevolence than sciencetubers—while scientists interviewed on TV are regarded as typical scientists, sciencetubers stand out for their highly professional communication abilities (being entertaining and comprehensible); (2) these emotional assessments of scientists are important predictors of perceived trustworthiness; and (3) significantly mediate the effect of the stimulus (TV interview vs. YouTube video) on all dimensions of perceived trustworthiness of scientific experts.

The internal structures of a moraine complex mostly provide information about the manner in which they develop and thus they can transmit details about several processes long after they have taken place. While the occurrence of glacier–permafrost interactions during the formation of large thrust moraine complexes at polar and subpolar glaciers as well as at marginal positions of former ice sheets has been well understood, their role in the formation of moraines on comparatively small alpine glaciers is still very poorly investigated. Therefore, the question arises as to whether evidence of former glacier–permafrost interactions can still be found in glacier forefields of small alpine glaciers and to what extent these differ from the processes in finer materials at larger polar or subpolar glaciers. To investigate this, electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) surveys were carried out in the area of a presumed alpine thrust moraine complex in order to investigate internal moraine structures. The ERT data confirmed the presence of a massive ice core within the central and proximal parts of the moraine complex. Using GPR, linear internal structures were detected, which were interpreted as internal shear planes due to their extent and orientation. These shear planes lead to the assumption that the moraine complex is of glaciotectonic origin. Based on the detected internal structures and the high electrical resistivity values, it must also be assumed that the massive ice core is of sedimentary or polygenetic origin. The combined approach of the two methods enabled the authors of this study to detect different internal structures and to deduce a conceptual model of the thrust moraine formation.

In the last few years superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio>200) niobium achieved accelerating gradients close to the theoretical limits. An obstacle towards achieving reproducibly higher fields is represented by “anomalous” losses causing a sharp degradation of the cavity quality factor when the peak surface magnetic field (B_{p}) is above about 90 mT, in the absence of field emission. This effect, called “Q drop” has been measured in many laboratories with single- and multicell cavities mainly in the gigahertz range. In addition, a low-temperature (100–140 °C) “in situ” baking of the cavity was found to be beneficial in reducing the Q drop. In order to gain some understanding of the nature of these losses, a single-cell cavity has been tested in the TM_{010} and TE_{011} modes at 2 K. The feature of the TE_{011} mode is to have zero electric field on the cavity surface, so that electric field effects can be excluded as a source for the Q drop. This article will present some of the experimental results for different cavity treatments and will compare them with existing models.

Drainage and rewetting of peatlands changes their carbon (C) dynamics. We measured C stocks in the soil and biomass of drained and rewetted peatland forests in the region of the National Park Hunsrück-Hochwald, Germany. A detailed soil map was produced, showing the important soil properties for soil C stocks. Based on the horizon-substrate combination as well as LiDAR data, the spatial distribution of soil and forest C stocks was analysed. Our results show that our peatland sites have shallow soils with a heterogeneous spatial distribution. Mean (± SE) soil C stocks in the monitoring sites are 121(± 8) t ha-1 whereas the forest stores on average 71(± 0.2) t ha-1 of C in the biomass. Tree removal in the drained peatlands reduced the total C stocks by 4,430 t. The quality and longevity of the wood products that arise from tree removal will determine whether or not the harvested portion of the C captured by the trees is sequestered over long timescales. Additionally, the success of rewetting activities and the potential soil carbon sequestration or (non-) soil carbon loss will determine whether these ecosystems are carbon sinks or sources. An extrapolation of our data resulted in estimated C stocks of 171,530 t and 544,282 t for peatland soils and for the spruce forest, respectively, for the whole area of the National Park (about 10,000 ha). [ABSTRACT FROM AUTHOR]

The coordinated use of ecological data is critical to the proper management of invasive species in the coastal wetlands of the Laurentian Great Lakes. Researchers and government programs have been increasingly calling for the use of data in management activities to increase the likelihood of success and add transparency in decision making. Web-enabled databases have the potential to provide managers working in Great Lakes coastal wetlands with relevant data to support management decisions. To assess the potential value of these databases to managers in Laurentian Great Lakes states, we surveyed wetland managers to determine their current data usage as well as their future data interests and catalogued the online databases currently available. Surveys were disseminated via email to managers in 56 different organizations overseeing invasive species management efforts in Great Lakes coastal wetlands; 46 responses were included in this analysis. Of the survey respondents, all reported using raw biotic data for decision making, (i.e. presence of target species) but many indicated that they would prefer to incorporate a greater variety of data, as well as more complex information. Our survey found that managers used web-enabled databases, but most databases that we catalogued only provided presence data for wetland biota. We concluded that databases can provide the types of data sought by invasive species managers but have unmet potential to be integrated into responsive management processes. [ABSTRACT FROM AUTHOR]