Production And Studies Of Photocathodes For High Intensity Electron Beams

For short, high-intensity electron bunches, alkali-tellurides have proved to be a reliable photo-cathode material. Measurements of lifetimes in an RF gun of the CLIC Test Facility II at field strengths greater than 100 MV/m are presented. Before and after using them in this gun, the spectral response of the Cs-Te and Rb-Te cathodes were determined with the help of an optical parametric oscillator. The behaviour of both materials can be described by Spicer's 3-step model. Whereas during the use the threshold for photo-emission in Cs-Te was shifted to higher photon energies, that of Rb-Te did not change. Our latest investigations on the stoichiometric ratio of the components are shown. The preparation of the photo-cathodes was monitored with 320 nm wavelength light, with the aim of improving the measurement sensitivity. The latest results on the protection of Cs-Te cathode surfaces with CsBr against pollution are summarized. New investigations on high mean current production are presented.Comment: Submission to LINAC2000 conference, Paper number MOB08, 3 pages, 6 figure

Photo-cathodes for the CERN CLIC Test Facility

Since 1993 the CLIC Test Facility (CTF) has used laser-illuminated Tellurium Alkali photo-cathodes as intense electron sources (up to 50 nC in 10 ps), for the Drive Beam of a two-beam accelerator. These cathodes have been produced and tested in our photo­emission laboratory and transported under vacuum to the CTF. They are placed in a 3 GHz RF gun with a 100 MV/m electric field. This RF gun produces a train of 48 pulses, each of 13.4 nC charge and 10 ps length. The CTF Probe Beam has used air­transportable cesium iodide + germanium photo­cathodes in another RF gun, which produces a single pulse of the same duration but with only 1 nC charge. The optical damage threshold in the laser is the main limitation of energy available on the photo­cathode. From an operational point of view, the photo­cathode lifetime is defined to be the time during which the cathode is able to produce the nominal charge with the nominal laser energy. After having recalled the main characteristics of the photo-cathodes tested, this note describes in more detail the performa nce obtained in operation. The possibility of photo-cathode production at the RF gun in a simplified evaporation chamber will also be discussed

The photo-injector option for CLIC: past experiments and future developments

The Compact Linear Collider (CLIC) drive beam requires a long bunch train (92 us) consisting of 42880 bunches with a bunch charge of 17.5 nC in a bunch length of less than 20 ps. This train might be produced by an RF-photo-injector equipped with a Cs-Te cathode. After a short review of experience with such cathodes in the present CLIC Test Facility (CTF2), mainly focused on the production of short trains of high-charge bunches, we will present the scheme foreseen for CLIC and CTF3. The laser will be a diode-pumped MOPA (Master Oscillator Power Amplifier), operating at a repetition rate of 469 MHz for CLIC and 1.5 GHz for CTF3. The specific requirements of an RF-gun for this high-current operation are discussed. New experimental results on the photocathode lifetime at high average current are summarized

CLIC: a Two-Beam Multi-TeV e±e\pm Linear Collider

The CLIC study of a high-energy (0.5 - 5 TeV), high-luminosity (1034 - 1035 cm-2 sec-1) e+e- linear collider is presented. Beam acceleration using high frequency (30 GHz) normal-conducting structures operating at high accelerating fields (150 MV/m) significantly reduces the length and, in consequence, the cost of the linac. Using parameters derived from general scaling laws for linear colliders, the beam stability is shown to be similar to lower frequency designs in spite of the strong wake-field dependency on frequency. A new cost-effective and efficient drive beam generation scheme for RF power production by the so-called "Two-Beam Acceleration" method is described. It uses a thermionic gun and a fully-loaded normal-conducting linac operating at low frequency (937 MHz) to generate and accelerate the drive beam bunches, and RF multiplication by funnelling in compressor rings to produce the desired bunch structure. Recent 30 GHz hardware developments and CLIC Test Facility (CTF) results are described

Use of cultivation-dependent and -independent techniques to assess contamination of central venous catheters: a pilot study

<p>Abstract</p> <p>Background</p> <p>Catheters are the most common cause of nosocomial infections and are associated with increased risk of mortality, length of hospital stay and cost. Prevention of infections and fast and correct diagnosis is highly important.</p> <p>Methods</p> <p>In this study traditional semiquantitative culture-dependent methods for diagnosis of bacteria involved in central venous catheter-related infections as described by Maki were compared with the following culture-independent molecular biological methods: Clone libraries, denaturant gradient gel electrophoresis, phylogeny and fluorescence in situ hybridization.</p> <p>Results</p> <p>In accordance with previous studies, the cultivation of central venous catheters from 18 patients revealed that <it>S. epidermidis </it>and other coagulase-negative staphylococci were most abundant and that a few other microorganisms such as <it>P. aeruginosa </it>and <it>K. pneumoniae </it>occasionally were found on the catheters. The molecular analysis using clone libraries and sequencing, denaturant gradient gel electrophoresis and sequencing provided several important results. The species found by cultivation were confirmed by molecular methods. However, many other bacteria belonging to the phyla <it>Proteobacteria, Firmicutes, Actinobacteria </it>and <it>Bacteroidetes </it>were also found, stressing that only a minor portion of the species present were found by cultivation. Some of these bacteria are known to be pathogens, some have not before been described in relation to human health, and some were not closely related to known pathogens and may represent new pathogenic species. Furthermore, there was a clear difference between the bacterial species found in biofilm on the external (exluminal) and internal (luminal) side of the central venous catheter, which can not be detected by Maki's method. Polymicrobial biofilms were observed on most of the catheters and were much more common than the cultivation-dependent methods indicated.</p> <p>Conclusion</p> <p>The results show that diagnosis based on molecular methods improves the detection of microorganisms involved in central catheter-related infections. The importance of these microorganisms needs to be investigated further, also in relation to contamination risk from improper catheter handling, as only in vivo contaminants are of interest. This information can be used for development of fast and more reliable diagnostic tools, which can be used in combination with traditional methods.</p

A nomenclature for restriction enzymes, DNA methyltransferases, homing endonucleases and their genes

A nomenclature is described for restriction endonucleases, DNA methyltransferases, homing endonucleases and related genes and gene products. It provides explicit categories for the many different Type II enzymes now identified and provides a system for naming the putative genes found by sequence analysis of microbial genomes