Structural and RF properties of niobium films deposited onto annealed niobium resonators

Studies have been performed on the properties of niobium thin films sputtered onto solid niobium TM010 resonators at 1.5 GHz. The purpose of the work is to study the behaviour of the film's RF and str uctural properties as a function of heat treatment temperature in order to determine if and at what treatment temperature the properties of the films merge with those of the bulk. Niobium resonators h ave been heat treated at temperatures up to 1100°C in a vacuum furnace inside a niobium box surrounded by a titanium gettering protection. Subsequently, they have been sputter coated with a niobium fi lm. Following RF measurements of the coated resonators, the cavities have undergone heat treatments as described above at 800°C, 900°C, 1000°C and 1100°C, each time followed by RF measurements. Before heat treatment, the RF response of the film was similar to that of a film coated on a copper substrate. A marked transition towards bulk-like RF behaviour was observed after the 900°C treatment. The c hanges include a sharp variation of the BCS resistance and of the sensitivity to externally applied magnetic field, quantities believed to be closely linked to the amount and nature of defects in the coating

Kohlenstoffbilanzen von renaturierten Hangmooren im Hunsrück, Rheinland-Pfalz

In Moorböden ist ein Großteil des terrestrischen Kohlenstoffs gespeichert. Drainage für land- oder forstwirtschaftliche Nutzung von Mooren führt zu einer Freisetzung von Kohlenstoff aus dem Boden in die Atmosphäre. Renaturierung von genutzten Mooren ist ein weitverbreitetes Steuerungsinstrument um die Treibhausgasemissionen von Moorböden zu reduzieren. Dieses Projekt zielt darauf ab, die Kohlenstoffbilanzen von renaturierten Hangmooren zu quantifizieren. Die sogenannten Hangbrücher im Nationalpark Hunsrück-Hochwald sind charakteristisch für die Region. Sie wurden seit dem 19. Jahrhundert mit Gräben für die forstwirtschaftliche Nutzung (Fichte) entwässert. Seit einigen Jahren werden sie im Rahmen von verschiedenen Projekten durch Grabenverschlüsse wiedervernässt. Außerdem wurde die Entnahme von standortsfremder Bestockung mittels Seilkran durchgeführt. Sechs Moore mit unterschiedlichen Degradierungsstadien wurden ausgewählt und verschiedene bodenchemische, -physikalische und -hydrologische Parameter aufgenommen. Die Ausdehnung der Moorflächen und die Kohlenstoffvorräte (Boden und Holz) werden erfasst und in ein Moorkataster überführt. Geoelektrische Sondierungen und Kartierungen werden zur Charakterisierung des oberflächennahen Untergrundes der Moorwälder und Hangmoore sowie zur Erfassung der hydrologischen Situation der Moorstandorte durchgeführt. Des Weiteren werden luftgestützte Laserscanaufnahmen für hochaufgelöste Geländemodelle und eine Abschätzung des Holzvorrates der Moore verwendet. Das Gesamtziel einer flächendeckenden Kohlenstoffbilanzierung wird erreicht, indem wir auf der Grundlage des Moorkatasters ein Upscaling der Kohlenstoffvorräte bei unterschiedlichen Renaturierungsszenarien durchführen. Das Moorkataster bietet außerdem die Möglichkeit die erhobenen Daten für ein Langzeitmonitoring der Flächen zu nutzen und dient dabei als Grundlage für weitere Untersuchungen

Recent evolution of an ice‐cored moraine at the Gentianes Pass, Valais Alps, Switzerland

International audienceLateral moraines located in permafrost environments often preserve large amounts of both glacier and periglacial ice. To understand how ice‐cored moraines located in high alpine environments evolve in a context of both glacier retreat and permafrost degradation, we performed 11 terrestrial laser‐scanning measurement campaigns between 2007 and 2014 on a highly anthropogenic overprinted moraine prone to instability. Resulting comparison of the subsequent 3D models allowed to qualitatively and quantitatively analyze the morphological evolution of the moraine. The comparisons indicate a very high geomorphic activity of the moraine including large areas affected by downslope movements of blocks and 10 landslides with a volume between 24 ± 1 and 1,138 ± 47 m3. Data also indicated a very strong ice melt with a loss of ice thickness locally reaching 17.7 m at the foot of the moraine. These results, compared with resistivity and thermal measurements of the ground, suggest the combined role of ice loss at the foot of the moraine and the permafrost activity/warming in triggering these processes

Flux pinning characteristics in cylindrical ingot niobium used in superconducting radio frequency cavity fabrication

We present the results of from DC magnetization and penetration depth measurements of cylindrical bulk large-grain (LG) and fine-grain (FG) niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities. The surface treatment consisted of electropolishing and low temperature baking as they are typically applied to SRF cavities. The magnetization data were fitted using a modified critical state model. The critical current density Jc and pinning force Fp are calculated from the magnetization data and their temperature dependence and field dependence are presented. The LG samples have lower critical current density and pinning force density compared to FG samples which implies a lower flux trapping efficiency. This effect may explain the lower values of residual resistance often observed in LG cavities than FG cavities

Low temperature laser scanning microscopy of a superconducting radio-frequency cavity

An apparatus was developed to obtain, for the first time, 2D maps of the surface resistance of the inner surface of an operating superconducting radio-frequency niobium cavity by a low-temperature laser scanning microscopy technique. This allows identifying non-uniformities of the surface resistance with a spatial resolution of about one order of magnitude better than with earlier methods and surface resistance resolution of ~ 1 micro-Ohm at 3.3 GHz. A signal-to-noise ratio of about 10 dB was obtained with 240 mW laser power and 1 Hz modulation frequency. The various components of the apparatus, the experimental procedure and results are discussed in detail in this contribution.Comment: 19 pages, 17 figures, submitted to Rev. Sci. Instru

Successful user operation of a superconducting radio-frequency photoelectron gun with Mg cathodes

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 (LN2) 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