New approaches to anti-multipactor coatings for space applications

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 08 de febrero de 2016El efecto Multipactor es un fenómeno perjudicial en dispositivos de alta potencia de RF en vacío de gran importancia tecnológica, industrial y económica. Es un viejo problema en la industria espacial, en los aceleradores de partículas de gran energía, en dispositivos toroidales de energía termo-nuclear, en generadores de potencia de RF, y muchas tecnologías electrónicas avanzadas. El efecto o descarga Multipactor se genera y alimenta por la Emisión de Electrones Secundarios (SEE) en las superficies del dispositivo. Por eso siempre su solución o mitigación pasa por reducir esta SEE de los materiales usados en las partes críticas del dispositivo. Los laboratorios de los aceleradores SLAC, CERN, KEK,… y ESTEC y VSC de ESA han dedicado un gran esfuerzo a este problema. Nuestro grupo colabora con los grupos en SLAC, CERN y ESA. El objetivo final y global del trabajo de esta tesis doctoral era el desarrollo y aplicación de recubrimientos anti-multipactor para aplicaciones espaciales de alta potencia de RF, para mitigar o suprimir la descarga multipactor con recubrimientos de baja SEE, estables en el aire y con baja resistencia superficial en la banda Ku (alrededor de 12 GHz). La búsqueda de materiales con baja SEE entra en conflicto con otras propiedades requeridas estrictamente para su aplicación en el espacio. Estas son principalmente muy buena conductividad eléctrica y gran estabilidad en el aire. Esta última está claramente en conflicto con la baja SEE y la alta conductividad. Este enfoque basado en las propiedades físico-químicas de las superficies se ha agotado sin encontrar una buena solución al Multipactor. A partir de en algunas observaciones tanto de nuestro laboratorio como de otros y que no habían sido consideradas en toda sus implicaciones, esta tesis doctoral se basa en varias hipótesis de trabajo importantes y novedosas: i) los diferentes requerimientos que no se pueden cumplir con un solo material se pueden satisfacer por capas de diferentes materiales haciendo uso de las diferentes escalas y penetraciones de las diferentes propiedades requeridas (modelo de capas del recubrimiento). ii) la rugosidad superficial de gran relación de aspecto es muy eficiente en reducir fuertemente la SEE aparente o eficaz y esta propiedad es de la forma y no del tamaño. iii) la rugosidad superficial de gran relación de aspecto aumenta la resistencia superficial de RF pero esto es una propiedad de la forma y del tamaño. Reduciendo suficientemente el tamaño puede hacerse compatible (ii) y (iii). Para la obtención de dicho tipo de rugosidad superficial en el material mejor conductor y de referencia en la industria espacial, Ag electrodepositada, se propuso realizar una amplia y concienzuda investigación en técnicas de micro y nano-estructuración de superficies. Otros objetivos eran desarrollar los recubrimientos y sus aplicaciones en dispositivos prácticos en estrecha colaboración con la industria espacial para satisfacer toda una serie de requerimientos industriales y económicos para su aplicación. Desarrollo del trabajo y metodología Este trabajo de investigación se ha desarrollado en dos grandes proyectos. Cada uno formado por un proyecto del Plan Nacional de I+D+i y otro paralelo de ESA. En ellos han colaborado el grupo de la UAM, el grupo de I. Montero en el ICMM del CSIC, la empresa Tesat Spacecom y ESTEC y VSC de ESA. En esta tesis se presenta el trabajo realizado por el doctorando en la UAM. El trabajo en el primer gran proyecto y tema principal de esta Tesis, tuvo varias etapas: Definición de las capas (materiales, tamaños, estructura, ..) del recubrimiento. Desarrollo de técnicas de preparación y caracterización de los procedimientos y los recubrimientos. Se investigaron más de diez técnicas de micro estructuración de superficies. Para la más prometedora se estudiaron nueve variantes, se prepararon unas cien muestras para estudiar más de cinco propiedades, además se prepararon nueve muestras preindustriales. La investigación fue tecnológica: el objetivo era alcanzar los resultados buscados en menos tiempo. El estudio científico de procesos y mecanismos estaba subordinado al método de la bisección o de ensayo y error. Definición de la técnica y procedimiento óptimo: grabado químico poroso y posterior mentalización con Au. Aplicación a cinco dispositivos industriales. Caracterización científica de los resultados. Caracterización tecnológica e industrial de los dispositivos tratados. Discusión y valoración de los resultados y propuestas para nuevas investigaciones. Los excelentes y novedosos resultados de este primer gran proyecto han permitido la realización de los proyectos siguientes. Estas propuestas dieron lugar al segundo gran proyecto también formado por dos proyectos paralelos realizados por los mismos grupos y centros. En este segundo gran proyecto se ha logrado mantener la práctica supresión del Multipactor alcanzada en el primero pero ahora con recubrimientos de conductividad óptima, la máxima posible, la de los recubrimientos de Ag lisos estándar de la industria. Para ello se han obtenido rugosidades superficiales especiales de escala 100 nm. En este proyecto se desarrolló una de las técnicas “descubiertas” en el proyecto anterior pero para la que no hubo suficiente recursos ni tiempo: grabado auto-organizado con haces de iones asistido por deposición de máscara-surfactante por sputtering

X-ray Photoemission Spectroscopy Studies of Cesium Antimonide Photocathodes for Photoinjector Applications

AbstractWithin the CLIC (Compact Linear Collider) project, feasibility studies of a photoinjector option for the drive beam as an alternative to its baseline design using a thermionic electron gun (Geschonke et al. [1]) are on-going. This R&D program covers both the laser and the photocathode side. Cesium antimonide cathodes were produced at CERN by co-deposition onto copper substrates and characterized by photoemission and by XPS (X-ray Photoemission Spectroscopy) analysis. A systematic study on newly produced and used photocathodes was conducted in order to correlate the surface composition to the photoemissive properties

First accelerator test of vacuum components with laser-engineered surfaces for electron-cloud mitigation

Electron cloud mitigation is an essential requirement for high-intensity proton circular accelerators. Among other solutions, laser engineered surface structures (LESS) present the advantages of having potentially a very low secondary electron yield (SEY) and allowing simple scalability for mass production. Two copper liners with LESS have been manufactured and successfully tested by monitoring the electron cloud current in a dipole magnet in the SPS accelerator at CERN during the 2016 run. In this paper we report on these results as well as the detailed experiments carried out on samples—such as the SEY and topography studies—which led to an optimized treatment in view of the SPS test and future possible use in the HL-LHC

Surface Characterization at CERN of Photocathodes for Photoinjector Applications

R&D; on photocathodes takes place at CERN within the CLIC (Compact Linear Collider) project. Photocathodes are produced as thin films on Oxygen Free copper substrate using a co-deposition technique, and characterized in a dedicated laboratory with a DC photo-electron gun. A new UHV carrier vessel compatible with CERN’s XPS (X-ray Photoelectron Spectroscopy) analysis equipment has been commissioned and is used to transport photocathodes from the production laboratory to perform a systematic study of different compounds used as photoemissive materials. In this paper photocathodes used in a RF photoinjector will be characterized and the correlation of their surface properties with their performance will be investigated

Adsorption of lead(II) ions from aqueous solutions onto Cr-pillared clays.

International audienceThe present paper presents the synthesis and characterization of porous nanomaterials, the Cr(III) being the metal oxide used as pillar intercalated between the layers of montmorillonite. The raw material used to obtain the pillared clays was a Romanian natural calcium bentonite, which was provided by S.C.Bentonita S.A. The main parameters were varied (metal ion for ion exchange process, aging temperature and duration for pillaring agent preparation, meta/clay ratio) in the aim of obtaining nanomaterials with high adsorption capacity of lead ions from waters. The modified clays were characterized by nitrogen adsorption technique, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The determination of Pb(II) ions concentration was realized by atomic absorption spectrophotometry.The obtained results showed that the basal distance and specific surface area varied with the parameters of materials preparation. The nanomaterial with the best textural, structural and morphological properties was chosen in the aim of its using in Pb(II) adsorption from aqueous solution.The Langmuir and Freundlich models were used to fit the experimental data and these showed good correlations

Study of the Performance of Cs2Te Cathodes in the PHIN RF Photoinjector using Long Pulse Trains

The drive beam of CLIC requires unusually high peak and average currents which is challenging for the electron source. As an alternative to the thermionic electron gun foreseen in the baseline design, a photoinjector option is under study at CERN using the PHIN photoinjector, which was designed for a bunch charge of 2.3 nC and 1200 ns train length. During operation with nominal train length in 2014, a large pressure increase in the vacuum system, attributed to a heating of the Faraday cup, caused a degradation of the photocathode. To overcome this problem a vacuum window has been installed to separate the Faraday cup from the rest of the vacuum system. In addition the train length has been further increased to 1600 ns to advance the beam parameters towards CLIC requirements. In this paper recent improved photocathode lifetime measurements carried out under these new conditions will be presented and compared with earlier measurements. Furthermore, the utilized Cs2Te cathode has been analyzed with X-ray Photoelectron Spectroscopy (XPS) before and after its usage in PHIN to get a better understanding of photocathode surface deterioration effects, which will also be discussed

ACTIVATED ADSORPTION ON CLAY OF MICROPOLLUTANTS FROM PAPER PRINTING INDUSTRY

The paper presents a preliminary study of chemisorption onto anionic and cationic clays, in order to reduce the content of pollutants from a paper printing effluent, collected after technological step named: printing of paper fabric manufacturing. The procedure of filtration fallowed by adsorption process is an effective, fast and low cost technique for treatment of black effluent resulting from paper printing industry. The key parameters tested to achieve a high efficiency for the movement of micropollutants from printing fluid were substrate dose and contact time. The highest treatment performance was obtained for cationic substrate at pH = 6.80, in contact and agitated magnetically for 30 respectively 90 minutes at room temperature

Multipactor suppression by micro-structured gold/silver coatings for space applications

The secondary electron emission (SEE) from materials used in high power RF devices in space is the main trigger and sustaining mechanism of the resonant avalanche electron discharge known as the multipactor effect. It limits the attainable power of those devices. During recent decades, some scientific research has been focused on material properties for obtaining anti-multipactor coatings of low secondary emission yield (SEY). The European Space Agency (ESA) is leading a technological research on a new approach based on surface roughness that might perform as a kind of blackbody or Faraday cage effect. A multilayer coating structure was adopted for fulfilling the stringent requirements of the space. The surface of a standard silver plating was modified by a two-step treatment. First, a wet chemically etching process created a roughness of high aspect ratio, in the scale of microns. Secondly, the surface was coated with a protective 2 μm overlayer of gold, using magnetron sputtering. This anti-multipactor coating has been tested on several types of Ku-band WR75 waveguide transformers and band-pass filters, with excellent results. The multipactor effect was suppressed for two waveguides, even when applying the maximum available power levels. As for the other final four, the increase of multipactor power level was in the range of 4-6 dB. These results were obtained after more than one year of air exposure. In spite of the strong roughness, the insertion losses were diminished by 25% with respect to the values attained in the tests of the standard anti-multipactor coating, Alodine.This work was supported by the European Space Agency (ESA)/European Space Research and Technology Centre (ESTEC), under the research program “Porous Inert Metal Coatings for Controlling Secondary Electron Emission”, ESA 17025/03/NL/EC (CCN-02) and by the Ministry of Economy and Competitiveness of Spain, the National Plan of R&D&I, coordinated Projects no. AYA2012-39832-C02-01 and -02

Factorial Design to Stimulate Biomass Development with Chemically Modified Starch

The present study is focused on mathematical modeling by testing the benefits of modified potato starch in the biomass production of microorganisms, such as the fungus type. Microorganisms need a carbon source for the biomass development. In different industries, microorganisms, such as the Penicillium type, are used for the extraction of different important compounds utilized in biotechnologies. The aim of this study is to establish some important parameters in order to stimulate the biomass production in the presence of chemically modified starch. The carbon sources used in this research are glucose, native potato starch, and chemically modified potato starch. The chemical modification of potato starch was realized with green chemical compounds in order to not influence biomass development. The chemical characterization of starch and modified starch was important in order to confirm the chemical modification of starch. The response function in mathematical modeling is the amount of biomass developed when there are varied parameters. The varied parameters for the factorial design are as follows: time of biomass development, mass report of glucose:starch (G:S), and mass report of glucose:modified starch (G:MS). The results obtained for the optimal values are as follows: 6 days for the biomass development, 1:1.35 for the mass report of G:S, and 1:1.27 for the report of G:MS