Workshop Proceedings

Abstracts, Presentations and Papers

Monday 4th October, 2010

INTERNATIONAL PANORAMA FOR THIN FILM TECHNOLOGY APPLIED TO SRF CAVITIES
(Chairman: Larry Phillips)
Foreword - Why thin films did not yet corner the SRF Market? (Enzo Palmieri - 20')
Speaker: Enzo Palmieri - INFN-LNL   |   Duration: 20 min.

Abstract
Dear Colleagues,
Welcome abord to the thin film enterprise.
Three days of intense work are waiting for us and we expect to come out from the workshop with clear ideas about how to proceed, about new collaborations to start or about selecting the most promising roads.
Thin films for SRF have an explosive potential for Accelerator Technology, but it is clear that they belong to the future, more than to the present. However the if we will not address in the right way the present, the future will be more and more far.
So, with my request to the Workshop Delegates to spend their maximum effort in making this opportunity a fruitful one, I wish you a wonderful time here at Legnaro National Laboratories of the INFN.

Enzo Palmieri

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Thin Films for SRF – What might the future hold? (Charlie Reece - 20')
Speaker: Charlie Reece - Jefferson Lab - Newport News (VA) USA   |   Duration: 20 min.

Abstract
While the great majority of applications which use superconductors for their low surface resistance properties in response to rf fields employ high-purity bulk niobium, there are inherent limitations with this material. One may imagine even a large number of desirable applications if but the high system costs could be brought down by factors of 2 to 10, or the supportable stable fields could be increased by factors of 2 to 10 over the best that niobium can sustain while remaining superconducting. So what do we dare dream of bringing onto the real plane? What might be realizable if we but sort out and control material properties appropriately? I’ll attempt to construct a brief survey taste of what might be possible via SRF thin films, while thoroughly ignoring the technical difficulties that lie in the path ahead.

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Overview of the latest SRF projects at the S-DALINAC (Sven Sievers - 30')
Speaker: Sven Sievers - S-DALINAC, TU Darmstadt   |   Duration: 30 min.

Abstract
S. Sievers, U. Bonnes, J. Conrad, R. Eichhorn, F. Hug, M. Konrad, T. Kürzeder, N. Pietralla, A. Richter

The Superconducting Darmstadt Linear Accelerator S-DALINAC uses 3 GHz niobium cavities for electron acceleration. These cavities suffer from Q-values being three times lower than designed. Part of an explanation for these low Q-values could be surface contaminations, another possibility is the use of magnetostrictive components in the fine tuning system located directly at the cavity. Current research includes the setup of an array for quench detection via 2nd sound to find the contaminated areas, the investigation of the behaviour of piezo actors in superfluid helium to replace the magnetostrictive elements and the operation test of new power couplers.
These experiments are conducted at the vertical bath cryostat that has been commissioned within the last year. The talk will cover the latest results in these projects.

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CAVITY CONSTRUCTION
(Chairman: Larry Phillips)
RRR Niobium Seamless Cavities (Roy Crooks - 20')
Speaker: Roy Crooks - Black Laboratories   |   Duration: 20 min.

Abstract
Roy Crooks, Black Laboratories, L.L.C., Newport News, VA, USA
Waldemar Singer, DESY, Hamburg, Germany

Conventional welding of half-cells deep-drawn from niobium sheet may generate flaws near the cavity equators which limit cavity performance. Cavities have been produced from RRR niobium tube by a combination of spinning and hydroforming, using the facilities at DESY. Seamless cavities were manufactured from tubes produced by two different methods, based on pipe spun from plate and on pipe back-extruded from ingot (at ATI Wah Chang). The microstructures resulting from the two fabrication methods are described, along with SRF properties from recent tests on prototype ILC cavities.

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Discussion ( Conveiner: Larry Phillips - 10')
Speaker: Conveiner: Larry Phillips -   |   Duration: 10 min.

Abstract

RF SC THEORY
(Chairman: Jens Knobloch)
Superconductors and Vortices at Radio Frequency Magnetic Fields (Ernst Helmut Brandt - 50')
Speaker: Ernst Helmut Brandt - Max Planck Institute for Metals Research, D-70506 Stuttgart, Germany   |   Duration: 50 min.

Abstract
After an introduction to superconductivity and Abrikosov vortices, the statics and dynamics of pinned and unpinned vortices in bulk and thin film superconductors is presented. Particular interesting is the case of Niobium, which has a Ginzburg-Landau parameter near 0.71, the boundary between type-I and type-II superconductors. This causes the appearance of a so called type-II/1 state in which the vortex lattice forms round or lamellar domains that are surrounded by ideally superconducting Meissner state. This state has been observed by decoration experiments and by small-angle neutron scattering.
Also considered are the ac losses caused at the surface of clean superconductors, in particular Niobium, in the Meissner state, when no vortices have yet penetrated. The linear ac response is then xpressed by a complex resistivity or complex magnetic penetration depth, or by a surface impedance. At higher amplitudes, several effects can make the response nonlinear and increase the ac losses.
In particular, at sharp edges or scratches of a rough surface the magnetic field is strongly enhanced by demagnetization effects and the induced current may reach its depairing limit, leading to the nucleation of short vortex segments. Strong ac losses appear when such vortex segments oscillate. In high-quality microwave cavities the nucleation of vortices has thus to be avoided. Once nucleated, some vortices may remain in the superconductor even when the applied magnetic field goes through zero. This phenomenon of flux-trapping is caused by weak pinning in the bulk or by surface pinning.

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RF Losses due to incomplete Meissner-Ochsenfeld effect: difference between bulk Nb and Nb/Cu (Enzo Palmieri - 20')
Speaker: Enzo Palmieri - Legnaro National Laboratories of INFN and University of Padua   |   Duration: 20 min.

Abstract
Experimentally it is found that for Nb/Cu Sputtered Resonators, conrary to bulk Niobium Cavities the residual resistance due to magnetic flux trapped into the superconductor is independent of the magnetic induction intensity B at the moment of cooling. Because of the large demagnetization factor of the extended surface of a resonator, an incomplete Meissner-Ochsenfeld effect happens, favouring the trapping into the superconductor of any external fied present during the cooling phase as for instance the earth magnetic field or its unscreened fraction.
For Bulk Nb cavities the trapped vortexes dissipate energy since depinned by radiofrequency, while for Nb sputtered cavities the vortexes are frozen on pinning centers and there is no flow resistance.

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Probing the Nonlinear Meissner Effect and the Enhancement of the Field Onset of Penetration of Vortices in Dirty Nb Films Under RF and DC Fields (Alex Gurevich - 30')
Speaker: Alex Gurevich - National High Magnetic Field Laboratory   |   Duration: 30 min.

Abstract
Alex Gurevich
National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32303, USA

The first observation of the nonlinear Meissner effect in dirty Nb films revealed by the measurements of the frequency shift of the Nb thin film 20GHz resonator strip line as a function of the parallel dc magnetic field is reported [1]. The use of films thinner than the London penetration depth has enabled us to circumvent the main obstacle in the observation of the nonlinear Meissner effect due to penetration of vortices. A significant enhancement of the vortex penetration field up to  1T for a 65 nm dirty Nb film is demonstrated. We developed a theory based on the Usadel equations, which describe the experimental data very well. The nonlinear Meissner effect caused by impurities at the surface of Nb cavities can contribute to the medium and high-field Q slope. The thin film strip line resonator method can be very useful for measurements of the surface resistance and the onset of vortex penetration for emerging SRF materials used in thin film coating structures for Nb cavities.

[1]. N. Groll, A. Gurevich, and I. Chiorescu, Phys. Rev. B 81, 020504(R) (2010).

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Laminated Conductor Structure for RF in normal conducting case (Yoshihisa Iwashita - 20')
Speaker: Yoshihisa Iwashita - Kyoto University   |   Duration: 20 min.

Abstract
Laminated conductor structure for RF was proposed by A.M. Clogston in 1951.The motivation was to reduce the skin-effect loss caused by Joule heating.When the currents are well distributed to conductor foils that is thinner than the skin depth, the current density can be reduced and the power dissipation in the conductor can be reduced.This structure, however, has not been practically used, maybe because of some restrictions to apply. When we apply the similar layered structure for superconducting (sc.) surfaces, similar restriction may hit.Some thoughts that may be useful for sc. from the study on normal conducting case will be discussed.

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Discussion (Conveiner: Jens Knobloch - 10')
Speaker: Conveiner: Jens Knobloch -   |   Duration: 10 min.

Abstract

NIOBIUM DEPOSITION: STATUS OF ART
(Chairman: Enzo Palmieri)
A Brief Discussion of Conventional Sputtering and Energetic Condensation for Superconducting cavity applications (Larry Phillips - 25')
Speaker: Larry Phillips - JLab   |   Duration: 25 min.

Abstract
The history of niobium films produced by conventional magnetron sputtering and energetic condensation will be discussed in terms of their impact on SRF cavity performance.
A brief overview of current R&D in the energetic condensation of niobium films for this application will also be discussed.

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The CERN development of Superconducting Sputtered Nb/Cu QWR for the HIE-ISOLDE Project (Giulia Lanza - 20')
Speaker: Giulia Lanza - CERN - Geneva   |   Duration: 20 min.

Abstract
For the foreseen upgrade of the ISOLDE complex a new superconducting LINAC based on sputtered Nb/Cu Quarter Wave Resonators (QWRs) of two different beta families is planned to be installed in the next three to five years. The niobium coating process has been developed at CERN and the newly constructed biased magnetron system has shown that the target performance in terms of RRR and Tc can been achieved on samples. A first prototype resonator, of the higher beta family, has been coated and characterized in RF.
In this talk the latest developments on the sputtering technique and the first cold RF measurements will be reported.

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Reference Material

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Nb sputtered 150 MHz quarter-wave resonators for ANU LINAC Upgrade (Nikolai Lobanov - 20')
Speaker: Nikolai Lobanov - The Australian National University   |   Duration: 20 min.

Abstract

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Literature Review of Nb Electroplating (Dmytro Chirkov - 20')
Speaker: Dmytro Chirkov - Legnaro National Laboratories of INFN   |   Duration: 20 min.

Abstract
The main aim of this work is try to understand if expensive techniques of niobium coating could be replaced by cheaper electroplating method.
Nowadays electrochemical surface treatment is one of the most used approaches in industry.
Aqueous solutions has been the most widely used process mainly because of advantages such us low cost, non-flammability, high solubility of electrolytes, high conductivities resulting in low ohmic losses, high solubility of metal salts and high rates of mass transfer. However, despite these advantages there are several limitations in using aqueous solutions such as limited potential windows, gas evolution processes that can result in hydrogen embrittlement, passivation of substrates, electrodes and deposits, and the necessity for hazardous complex agents such as cyanide, causing environmental contamination. These are the reasons why new non-aqueous solutions to electrodeposit niobium have been researched.
One of the main limitations in using aqueous solutions is their narrow electrochemical window. The major reason for carrying out electrodeposition in non-aqueous electrolytes (such as conventional organic solvents, ionic liquids and molten salts) is water and air stability and the wide electrochemical window of these media.
Alternatively, also high temperature molten salts have been used extensively for niobium electrodeposition. They have wide potential windows, high conductivities and high solubility for metal salts. In fact, they have most of the advantages of aqueous solutions and overcome most of the limitations of aqueous solutions, but their one major limitation is the very high temperature (more than 750 °C).
Therefore, the alternative to high temperature molten salts was an ionic substance that melts at a low temperature. Over the last few decades, room temperature ionic liquids have been widely studied in various scientific fields due to their interesting properties, such as negligible vapour pressures, high chemical and thermal stability, acceptable intrinsic ionic conductivity and wide potential window.
Ionic liquids are molten salts with melting points below 100 °C and they consist entirely of cations and anions. The development of ionic liquids, especially air and water stable types, has attracted extensive attention since they have outstanding physical properties.
The aim of this work will be the analysis of available literature data in order to have precise knowledge about niobium behavior in the different electrolytes and to get new information about possible electrolytes based on ionic liquids.

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Cylindrical Post-Magnetron sputtering for High Rate Niobium deposition (Cristian Pira - 15')
Speaker: Cristian Pira - INFN-LNL   |   Duration: 15 min.

Abstract
The use of Nb/Cu cavity at CERN for the LEP and at the INFN-LNL for Alpi Linac has demonstrated the possibility to use this technology for particles accelerators to substitute the more expensive technology of niobium bulk cavity. The limit of the Nb/Cu cavity is the Q-slope, which decreases the Q factor at high accelerating fields. The accelerators community supposes that it’s possible to eliminate, or to decrease, the problem of Q-slope with high pure films of sputtered niobium. One way to obtain pure films is to decrease the number of impurities enclosed in the growing film.

It’s possible to reduce the number of impurities when the sputtering rate process increases.

We study the possibility to enhance the plasma density in order to increase the sputtering rate and then reduce the impurities in the niobium sputtered film and finally obtain high pure films.

In order to enhance the plasma density we sputter the niobium target with high currents to heat it and get to thermoionic emission. This sputtering method is called high rate sputtering.

First results of Niobium coatings will be presented.

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Round Table (Conviener: Sergio Calatroni - 60')
Speaker: Conviener: Sergio Calatroni -   |   Duration: 60 min.

Abstract
Martinet talk

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FILM CHARACTERIZATION
(Chairman: Claire Antoine )
Nano-mechanical and microstructural characterization of MS-PVD Nb thin films (Marco Sebastiani - 30')
Speaker: Marco Sebastiani - Roma 3 University   |   Duration: 30 min.

Abstract
E. Bemporad1, M. Sebastiani1, F. Carassiti1

1Mechanical and Industrial Engineering Department, University of Rome ‘Roma Tre’, Via della Vasca Navale 79-00146 Rome, Italy


The main objective of the present study was to identify the influence of the applied bias voltage on the microstructural and mechanical properties of magnetron sputtering physical vapour deposition (MS-PVD) niobium thin films for use in superconducting resonant cavities for particle accelerators.
The microstructure and mechanical properties evolution as a function of the applied bias voltage and nature of the substrate (copper or quartz) were investigated by means of micro-hardness and nanoindentation testing, FIB/SEM, AFM and TEM techniques.
The superconducting properties (critical temperature Tc and residual resistivity) were determined by a calibrated four-contact probe and a cryogenic apparatus and then correlated to the mechanical properties.
Significant difference in terms of microstructure, surface roughness and mechanical properties were observed for biased coatings grown on different substrates. The observed differences are likely connected to the low conductivity of quartz that induces a re-sputtering effect and a consequent modification of the superconducting performances.

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Measurements of RF properties of Novel Superconducting Materials (Sami Tantawi - 20')
Speaker: Sami Tantawi - SLAC National Accelerator Laboratory   |   Duration: 20 min.

Abstract
We have developed an X-band SRF testing system using a high-Q copper cavity with an interchangeable flat bottom for the testing of different materials. By measuring the Q of the cavity, the system is capable to characterize the quenching magnetic field of the superconducting samples at different power level and temperature, as well as the surface resistivity. This paper will present the most recent development of the system and testing results.

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Commissioning of the JLab Surface Impedance Characterization (SIC) System (Charles Reece - 20')
Speaker: Charles Reece - Jefferson Lab, Newport News (VA) USA   |   Duration: 20 min.

Abstract
Binping Xiao, Larry Phillips, and Charles Reece

A system for making direct calorimetric measurements of the surface resistance at 7.5 GHz of small samples of variously prepared superconducting surfaces has been commissioned at JLab. The flat, 50 mm diameter sample temperature is regulated independently of the balance of the TE011 sapphire-loaded cavity, enabling Rs and Δλ measurements from 2 K to Tc of the sample. Initial operation, limited by available rf power, has extended to Bpk of 18 mT. The calorimeter resolution is better than 10 nΩ, and the sampled surface area is ~ 0.8 cm2. The SIC has been commissioned with a bulk Nb sample, demonstrating excellent agreement with standard BCS characterizations. Initial application to SRF thin films has begun. We are eager to apply it to non-niobium materials. Preparations for a second generation with extended dynamic range have already begun.

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Texture Analysis of Niobium Thin Films (Josh Spradlin - 20')
Speaker: Josh Spradlin - SRF Institute - jefferson Lab, Newport News (VA) USA   |   Duration: 20 min.

Abstract
High RRR ( > 100) Nb thin films have been frequently fabricated by energetic condensation, via both cathodic arc discharge and ECR Nb plasma method during the Jefferson Lab supported programs. The Nb thin films were deposited on single crystal sapphire (a and c-planes) and MgO on moderate substrate temperate (300C-450C).

Advanced X-ray Diffraction and Electron Back-scattering Diffraction (EBSD) techniques were applied to reveal crystal structures of these Nb thin films. This study particularly used Pole Figures and EBSD to visualize the Reciprocal Lattice Space of the Nb thin films. These representations yielded a new understanding of the Nb thin films, such as the materials crystal texture in two probing depth: 50nm (in the range of SRF London penetration depth) by EBSD, and 2 micron in depth via XRD (covering the Nb/sapphire interface and entire thin films).

Variants of crystal structural symmetries were observed in the pole figures. We assigned them to 3 (or 6) folder Rotation Symmetry or Twinning Symmetry. To confirm the Twinning symmetry, we conduct a computational fitting of the empirical PF plot. For further discussion, twelve Nb B.C.C. Twinning systems are deduced here after a crystallographic study.

By complying with the well-known rule of "Three Dimensional Registry" of Nb/sapphire epitaxy, we could rationalize the observed texture (twinning symmetry, or rotation symmetry) by referring to the Island-Growth model and substrate initiatives. Nevertheless, we witnessed a violation of the law by coating the Nb thin films on c-plane sapphire.

Phenomenological relevance of RRR and texture are presented as is. The high RRR thin films unanimously have near single-crystal-structure (no texture, only monolithic Nb (110) orientation). This provoked us to speculate that the low RRR of Nb thin films might be caused by the high-defect-density zones among the grain boundaries, which in-turn are determined by the island growth model.

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Surface topography investigation for thin film SRF (Genfa Wu - 20')
Speaker: Genfa Wu - Fermilab   |   Duration: 20 min.

Abstract
The general surface topography will be discussed for various niobium cavities. The field enhancement factor for both niobium or thin film cavities will have different effects. Their implications for thin film based cavities will require investment in extensive surface preparations for cavity substrate cavities. The surface preparation effort at Fermilab will be discussed for future new material effort.

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Discussion (Conveiner: Claire Antoine - 20')
Speaker: Conveiner: Claire Antoine -   |   Duration: 20 min.

Abstract

Registration     (8:40 - 9:20)
Coffee Break     (11:00 - 11:20)
Lunch     (13:20 - 14:20)
Coffee Break     (16:00 - 16:20)
Bus Transfer     (19:30 - 19:40)

Tuesday 5th October, 2010

SUPERCONDUCTING RF LOSSES
(Chairman: Alex Gurevich)
Localized magnetism on the surface of Niobium: experiments and theory (Thomas Proslier - 40')
Speaker: Thomas Proslier - Argonne National Laboratory   |   Duration: 40 min.

Abstract
The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios (rods, foils, wires, powders) it was found that the impurity contribution is correlated with surface area. Tunneling measurements which utilize the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field > 4T, consistent with the Appelbaum-Anderson model for spin flip tunneling. Viewed together, the experiments strongly suggest that the native oxides of Nb are intrinsically defective, and consistently exhibit localized paramagnetic moments, likely caused by oxygen vacancies in Nb2O5. The computation of the surface impedance (RS) in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of Rs, suggesting that magnetic impurities are responsible for the so-called residual resistance.

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Intrinsic limitations of Niobium under RF (Tobias Junginger - 30')
Speaker: Tobias Junginger - CERN   |   Duration: 30 min.

Abstract
Calorimetric measurements on bulk niobium samples aim to reveal its intrinsic limitations for RF accelerator application. We tested the surface resistance vs. the RF magnetic field at 400 and 1200 MHz for the same sample and cooldown cycle. The ambient DC magnetic field was less than 0.1 ?T. The surface resistance shows an increase of 50 % when the field level is raised from 0 to 35 mT, independent of frequency and temperature. This is consistent with measurements on film cavities at 1500 MHz. We show that the residual surface resistance depends on frequency. The maximum field under RF exposure is consistent with the superheating field and does not depend on frequency. Our device has been commissioned using bulk niobium samples and studies of thin film samples are already being prepared.

Elastic recoil detection and positron annihilation studies of the mild baking effect (Alexander Romanenko - 30')
Speaker: Alexander Romanenko - Fermilab   |   Duration: 30 min.

Abstract
In this contribution we will present the results of mild baking experiments performed at Fermilab and through FNAL-University collaboration, which include investigations of vacancies and hydrogen in niobium coupons and cavity cutouts utilizing positron annihilation spectroscopy and elastic recoil detection analysis. The implications of the findings to the high field Q-slope problem mitigation will be discussed.

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Application of Muon Spin Rotation to studies of cavity performance limitations (Anna Grassellino - 20')
Speaker: Anna Grassellino - TRIUMF - Vancouver, Canada   |   Duration: 20 min.

Abstract
In this contribution a new experiment to investigate magnetic flux entry in Nb coupons and HFQS limited cutout samples will be presented. The experimental technique, called muSR (muon spin rotation), utilizes a probe magnetic moment to reveal local magnetic fields in the sample under study. Through the use of low energy spin polarized muons, the experiment can probe near surface local magnetic fields with extreme sensitivity. Being a ‘local’ rather than external and global technique, it offers a different and precise way to measure the field of first penetration in type-II superconductors. The experiment will study the nature of the transition from superconducting to mixed state in the marginal type II superconductor Nb, for samples with different treatment and grain size, and for RF characterized (via thermometry) HFQS limited cutout samples. Studying the latest will provide an opportunity to look for correlation of the onset of HFQS with the appearance of flux entry into the sample, detectable via the extremely sensitive muSR probe. Models for HFQS and MFQS which muSR can help probing will be discussed.

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Discussion (Conveiner: Alex Gurevich - 10')
Speaker: Conveiner: Alex Gurevich -   |   Duration: 10 min.

Abstract

MULTILAYER FILMS
(Chairman: Tsuyoshi Tajima)
Enhancement of First Penetration Field in Superconducting Multi-layers Samples (Claire Antoine - 30')
Speaker: Claire Antoine - CEA   |   Duration: 30 min.

Abstract
In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > Hc (Nb) for magnetic shielding of bulk niobium to increase the breakdown field of Nb RF cavities.

We have deposited high quality “model” samples by DC magnetron reactive sputtering on R-plane cut sapphire substrates. A 250 nm layer of niobium figures the bulk material as in rf cavities. Such Nb layers were coated with a single or multiple stacks of NbN layers (25 nm or 12 nm) separated by 15 nm MgO barriers, and characterized by X-rays reflectivity and DC transport measurements.

The first magnetic penetration field HC1 has been measured with dc magnetization curves in a SQUID system and with a local probe method based on 3rd harmonic analysis. The Nb samples coated with NbN multi-layers clearly exhibit a higher first penetration field, and the screening effect of the NbN layer was evidenced.

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Surface and Thin Film Characterization of Superconducting Multilayer films for application in RF (Roland Schulze - 30')
Speaker: Roland Schulze - Los Alamos National Laboratory   |   Duration: 30 min.

Abstract
The use of multilayer ultra-thin films on the interior surfaces of Nb superconducting RF cavities shows great promise in substantially improving the performance characteristics of superconducting RF cavities into the 100 MV/m range by increasing the RF critical magnetic field, HRF, through careful choice of new materials and thin film structures. However, there are substantial materials science challenges associated with producing such complex film structures, particularly for conformal application of uniform thin films on the interior surfaces of RF cavities. Here we present surface and thin film analysis of ultra-thin films of two candidate materials, MgB2 and NbN superconductors, deposited through several different methods, along with multilayers produced with alternating superconductor and dielectric films. We report on the analysis methods and techniques, using primarily x-ray photoelectron spectroscopy and Auger spectroscopy with ion sputter depth profiling, and describe results from variety of thin film samples. The materials stability, microstructure, chemistry, and thin film morphology are highly dependent on methods and parameters used in the thin film deposition. From our analysis, important factors for producing quality superconducting and dielectric films include chemical stoichiometry, impurity content, deposition temperature, substrate choice and conditioning, choice of dielectric material, and the nature of the thin film interfaces. These factors will be discussed in the context of the production methods used for these ultra-thin superconducting films.

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Atomic Layer Deposition of NbN thin films for SRF applications (Larry Phillips - 15')
Speaker: Larry Phillips - Jefferson Lab - Newport News - Virginia   |   Duration: 15 min.

Abstract
Niobium Nitride is a 17K superconductor investigated since early eighthies for Superconducting Radiofrequency applications.
Atomic Layer deposition is instead a technique that only recently starts to be considered for industrial applications.

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Discussion (Conveiner: Tsuyoshi Tajima - 40')
Speaker: Conveiner: Tsuyoshi Tajima -   |   Duration: 40 min.

Abstract

A15 AND MgB2
(Chairman: AnneMarie Valente)
Magnesium Diboride Thin Films for superconducting RF cavities (Xiaoxing Xi - 40')
Speaker: Xiaoxing Xi - Temple University   |   Duration: 40 min.

Abstract
MgB2 has a Tc of 40 K, a low residual resistivity, and a high Hc . RF cavities coated with MgB2 films have the potential for a higher Q and gradient than Nb cavities with an operation temperature of 4.2 K or higher. At Temple University, we have started a project to study issues related to the application of MgB2 to RF cavities, and to coat single-cell RF cavities with MgB2 film for characterization by the collaborators in accelerator-compatible environment. The key technical thrust of this project is the deposition of high quality clean MgB2 films and coatings using a hybrid physical-chemical vapor deposition technique. I will review the progress to date in this project.

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DC and RF Measurements of MgB2 thin films (Tsuyoshi Tajima - 30')
Speaker: Tsuyoshi Tajima - Los Alamos National Laboratory   |   Duration: 30 min.

Abstract
In order to overcome the fundamental limit of an accelerating gradient of ~50 MV/m for Nb SRF cavities, thin film coating of MgB2 has been studied. Results of DC measurements using Magnetic Property Measurement System (MPMS) SQUID at LANL and of RF measurements using 11.4 GHz high-power pulsed Klystron with a TE013-mode copper cavity at SLAC will be presented. While DC measurements show very promising results, i.e., Bc1>200 mT at 4.5 K, two RF measurements have shown a quench field of ~25 mT at 3 K.

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Status of KEK studies on MgB2 (Shinji Mitsunobu - 20')
Speaker: Shinji Mitsunobu - KEK   |   Duration: 20 min.

Abstract
We have been engaged in fabrication of high-Tc or MgB thin films on
metallic substrates. At the international workshop on thin films in
Padova, we showed our basic idea to make an accelerating-mode cavity.
In the last of this paper, we report a subsequent development, mainly a
partial success in fabricating superconducting film on a quadrant
cavity of an accelerator structure. In the beginning, we describe some
results concerning fabrication of films on Niobium and Titanium
surfaces.
Studies of Nb and Nb3Sn thin films by PLD will be shown.


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Nb3Sn Work at Cornell (Sam Posen - 30')
Speaker: Sam Posen - Cornell Unviersity   |   Duration: 30 min.

Abstract
This paper will describe the active research program at Cornell University devoted to superconducting Nb3Sn coatings. This includes a new furnace system for coating 5-inch diameter niobium samples using the vapor diffusion technique, designed after the systems built at Wuppertal University. The first coated samples have undergone surface studies as well as RF tests conducted with a newly constructed TE cavity. The results will be presented. Once the coating procedure is optimized, Nb3Sn-coated cavities will be fabricated at Cornell, and both CW and pulsed measurements will be done. Recently, a cavity coated at Wuppertal more than 20 years ago was tested using these techniques. The results will be shown here.

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Discussion (AnneMarie Valente - 60')
Speaker: AnneMarie Valente -   |   Duration: 60 min.

Abstract
List of Contributions:
K. Atroschenko: "Presence of residual Tin drops in Thermally diffused Nb3Sn" (10 min)

Presence of Residual Tin drops on Thermally Diffused Nb3Sn (Atroschenko Konstantin - 10')
Speaker: Atroschenko Konstantin - INFN-LNL   |   Duration: 10 min.

Abstract

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Session Opening     (9:10)
Coffee Break     (11:20 - 11:40)
Lunch     (13:20 - 14:20)
Bus Transfer     (17:20)
Gala Dinner     (20:20 - 23:00)

Wednesday 6th October, 2010

HIGH PEAK POWER MAGNETRON SPUTTERING
(Chairman: Larry Phillips)
ENERGETIC DEPOSITION OF NIOBIUM FILMS FOR SUPERCONDUCTING RF CAVITIES USING HIGH POWER IMPULSE MAGNETRON SPUTTERING (Andre Anders - 60')
Speaker: Andre Anders - Lawrence Berkeley National Laboratory   |   Duration: 60 min.

Abstract
André Anders
Lawrence Berkeley National Laboratory, Berkeley, California

Niobium coatings on copper cavities have widely been seen as a cost-efficient replacement of bulk niobium cavities, however, niobium coatings made by sputtering and other techniques have not lived up to expectations. Energetic condensation from the plasma phase has been tried using filtered cathodic arc, which is difficult given macroparticle filter and cavity geometries. High power impulse magnetron sputtering is a promising alternative technology. In this contribution, a cavity coatings concept with HIPIMS is explained, and preliminary data on niobium HIPIMS and resulting coatings are presented. One of the finding was the superior adhesion of niobium films on aluminum (with its native oxide), which suggest that cavities should be made from aluminum.
This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

First HIPIMS tests for superconducting Niobium films (Anna Gustafsson - 20')
Speaker: Anna Gustafsson - CERN - Geneva   |   Duration: 20 min.

Abstract
Sergio Calatroni, Anna Gustafsson*, Wil Vollenberg
CERN-TE-VSC-SCC; CH-1211 Geneva 23, Switzerland.

Niobium films are widely used at CERN for the coating of superconducting cavities. The performances of these cavities are highly influenced by the thin film properties. High Power Impulse Magnetron Sputtering (HIPIMS) gives new opportunities for conventional magnetron sputtering equipment. It can generate dense plasma with high target material ion content. The ion flux to the work piece can be used as a parameter for tuning the film properties. Reported peak power densities range from 1 – 3 kW/cm2, with discharge voltages between 500 – 2000 V. Pulses up to 200 µs with a repetition frequency up to 500 Hz are used.
The aim of this study is to compare the superconducting properties of Niobium films made by conventional DC magnetron sputtering, with Niobium films made by HIPIMS.
A commercial available Power Supply is used with a 150 mm planar magnetron source. The films are characterized by measuring TC, Residual Resistivity Ratio and morphology, either on quartz or copper substrates. X-ray diffraction is used to study the texture of the coatings. Working gas content of the coated layers will be measured by laser ablation.
First results of Niobium coatings will be presented.

(Conveiner: Larry Phillips - 20')
Speaker: Conveiner: Larry Phillips -   |   Duration: 20 min.

Abstract

VACUUM ARC DEPOSITION
(Chairman: Xiaoxing Xi )
Energetic Condensation Growth of Nb films for SRF accelerators (Mahadevan Krishnan - 30')
Speaker: Mahadevan Krishnan - Alameda Applied Sciences Corporation   |   Duration: 30 min.

Abstract
AASC, Jefferson Lab and NSU conduct research into new SRF thin-film coatings by first characterizing the materials properties such as morphology, grain size, crystalline structure, defects, and impurities, then measuring properties such as Tc and RRR and following this with ‘in-cavity’ RF measurements of the Surface Impedance of the films at cryogenic temperatures. These progressive steps are essential to the eventual design of SRF accelerator structures and to measure Q-slope and other performance parameters at high fields.
This paper describes recent results from pure Nb thin-films grown on a-plane and c-plane sapphire, MgO as well as on amorphous substrates. Substrate preparation is shown to be critical to good electrical properties of the film. The sapphire and MgO substrates were heated up to 700 deg C and subsequently coated at 300, 500 and 700 deg C. Film thickness was varied from ~0.25µm up to >3µm. RRR and Tc were measured. The XRD data yielded pole figures, intensity vs. 2-θ and intensity vs. φ plots. These data were complemented by EBSD and SEM images. RRR values ranging from ~10 up to ~333 have been measured and correlated with the XRD data. Good crystallinity is associated with high RRR. Single crystalline (110) epitaxial layers of Nb films are grown well on a-plane sapphire substrates at different temperatures. Nb films have also been grown on Cu substrates, as well as on MgO and borosilicate substrates. The significance of crystalline structure observed on amorphous substrates is discussed in light of its implications for future, lower-cost SRF cavities.

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RF and structural characterization of SRF thin films produced by energetic condensation (AnneMarie Valente - 30')
Speaker: AnneMarie Valente - Jefferson Lab - Newport News (VA) USA   |   Duration: 30 min.

Abstract
In the past years, energetic vacuum deposition methods have been developed in different laboratories to further improve thin film technology for superconducting cavities. Energetic condensation produces high density plasma with singly or quasi-singly charged ions and allows growth modes that favour a better structure and performance in thin films. In the framework of a collaboration with surrounding universities, JLab is pursuing energetic condensation deposition particularly via Electron Cyclotron Resonance (ECR). As part of this study, the influence on the material and RF properties of Nb thin film of the deposition parameters like energy, coating temperature, interface with the substrate is investigated. The film surface and structure analyses are conducted with various techniques like X-ray diffraction, Transmission Electron Microscopy, Auger Electron Spectroscopy and RHEED. The microwave properties of the films are characterized on 50 mm disk samples with a 7.5 GHz surface impedance characterization system. This paper presents results on RF and cryogenic measurements in correlation with surface and material characterization for Nb films produced on insulating and metallic substrates. Emerging opportunities for developing multilayer SRF films with a new deposition system will also be highlighted.

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The film Pb photocathodes prepared with the cathodic arc (Robert Nietubyc - 20')
Speaker: Robert Nietubyc - The Andrzej Soltan Institute for Nuclear Studies   |   Duration: 20 min.

Abstract
Superconducting thin film lead photocathodes became recently a prospective approach in constructing an electron source dedicated for long pulse and high repetition rate operating accelerators.
We report our recent works and achievements in construction and tests of such a source. In the branch of lead deposition, the efforts have been put into increase the transmission of the cathodic arc system which provides a satisfactorily macro-droplet removal from the plasma transit channel. A number of trial processes have been performed where polycrystalline and single-crystalline niobium plugs were coated. Next the surface characterization followed by cleaning procedure and quantum efficiency measurements using UV wavelength range of initiating laser were applied for deposited layers. Those experiments revealed an optimal procedure for an arc operation as well as for a laser cleaning of the Pb surface. In such conditions the value of QE equal to 0.0033 was found.
Following the preparation phase, a 100 nm lead layer was deposited onto the back wall of 1.6-cell Tesla-like accelerating structure of electron injector. Cold rf tests were performed for the cavities with and without lead. In both cases the resonant quality higher than 1010 was observed for an accelerating gradient of 46 MVm-1

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Discussion (Xiaoxing Xi - 20')
Speaker: Xiaoxing Xi -   |   Duration: 20 min.

Abstract

NEW IDEAS FOR ADVANCED CAVITY CLEANING
(Chairman: Enzo Palmieri)
Automatic Electropolishing (Vanessa Rampazzo - 15')
Speaker: Vanessa Rampazzo - Legnaro National Laboratories of INFN   |   Duration: 15 min.

Abstract
Electropolishing is an electrochemical process adopted in order to reduce the internal surface roughness. The process involves a special equilibrium between the reagents and the products, that creates a differential electrical resistance through the micrometric peaks and valleys of surface. This regime is detected collecting the current in function of voltage. Observing dinamically the changes of the characteristic curve during the process it’s possible to ensure the best electropolishing setting.

The Dynamic Control of the best electropolishing point int I-V Plane is done by an automatic program, that collects continuoulsy the voltage and current and calculates in real time the best setting. Roughness smoothing is accompanied by the removal of anomalies on the surface, as pitting and stratches. The automatic program is written in Labview language and works on a industrial computer, a PLC.

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Status of KEKB Electropolishing system (Shinji Mitsunobu - 15')
Speaker: Shinji Mitsunobu - KEK Tsukuba-Shi Ibaraki-Ken Japan   |   Duration: 15 min.

Abstract
Long time KEK superconducting cavities were treated by electro polish at Nomura plating Co.. For developing more reliable EP for ILC SC cavity,KEK decided to construct in-house new EP system. One L-band system and one universal system for 500 MHz and L-band system are gathered in one place.
L-band system have been used routinely ILC cavities EP successfully.
Anther new EP system for 500 MHz KEKB SC cavity have been constructed. KEKB test cavity was successfully processed.

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Squeezing the mystery out of electropolishing niobium (Charlie Reece - 30')
Speaker: Charlie Reece - Jefferson Lab - Newport News (VA) USA   |   Duration: 30 min.

Abstract
Squeezing the mystery out of electropolishing niobium - Charlie Reece

While application of electropolishing to bulk niobium has been a key element in the recent progress in pushing routinely attained performance nearer to Hc1, actual understanding of the process, which one might anticipate being useful for its control and optimization, has lagged behind. In the last couple of years dedicated analyses have made significant progress in describing and quantifying the electrochemical processes which are happening when one “electropolishes” a niobium surface. This talk will attempt to distill and interpret recent developments in this area. Confident control of surface topography is key to attaining niobium’s best, and it may also be critical in circumstances where niobium serves the role of substrate for thin film coatings aiming at even higher performance.

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An Approach to Chemical Free Surface Processing for High Gradient SRF Cavities (Frederick Mako - 30')
Speaker: Frederick Mako - FM Technologies   |   Duration: 30 min.

Abstract
Frederick Mako1, Ph.D., Bing Xiao1, Ph.D. and Larry Phillips2, Ph.D.

Chemical treatment such as buffered chemical polishing (BCP) or electro polishing (EP) followed by high pressure rinsing (HPR) of niobium (Nb) superconducting RF (SRF) cavities is expensive and complex multistep process. Furthermore, the cavity RF surfaces after the treatment still have numerous bubbles and pits that result from welding. These quench-producing weld defects together with the particulate contamination, result in significant scatter of the multi-cell Nb SRF cavities performance characteristics. This scatter is the major problem in the current manufacturing of the Nb SRF cavities. FM Technologies proposes a new approach to chemical-free processing for multi-cell Nb SRF cavities using an internal electron beam (IEB). Specifically, FMT proposes to develop a new electron gun system that will perform electron beam melting over the entire interior surface of Nb SRF cavities to produce a smooth surface, free from voids, bubbles, and other imperfections. This may allow manufacturing of the Nb SRF cavities with a reduction in the above chemical treatment procedures and increase the cavities high gradient performance. FM Technologies will design, build, test the new IEB system and process samples/cavities and Thomas Jefferson Laboratory will measure RF performance of processed samples/cavities. Preliminary electron beam melting results will be presented. 1FM Technologies, Inc., Chantilly, VA, USA, 2Thomas Jefferson Laboratory, Newport News, VA, USA

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Electropolishing of 6GHz cavities by ionic liquids (Vanessa Rampazzo - 20')
Speaker: Vanessa Rampazzo - Legnaro National Laboratories of INFN   |   Duration: 20 min.

Abstract
The electropolishing of niobium using RTIL without fluorine shows good surface improvements, and the original recipe based on Urea and Choline Chloride is under study for application on 6 GHz niobium cavities. The goal is to obtain a uniform electrical field on the internal surface, despite of the big differencies in distances from the cathode inserted into the cavity. Morevoer, the electrical power injected into the cavity degrades the ionic liquid, if this isn't efficiently cooled down. All of this pratical problems are partially solved adding various special reagents in IL, that raise the uniformity in electrical field and decrease the working current of electropolishing. Another goal is the use of a continuos flux of liquid, that flows through the cavity.

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Plasma Etching of Niobium surfaces: Studies on samples and Single-Cell Cavities (AnneMarie Valente - 30')
Speaker: AnneMarie Valente - Jefferson Lab - Newport News (VA) USA   |   Duration: 30 min.

Abstract
Plasma based surface modification provides an excellent opportunity to eliminate impurities and defects in the penetration depth region of Nb SRF cavity surfaces. It also allows a better control of the final SRF surface as final surface modifications like oxidation or nitridation can be done in the same process cycle.

In the framework of a collaboration between ODU and Jefferson Lab, we are pursuing the use of environmentally friendly dry etching of SRF cavity in an Ar/Cl2 discharge. The experimental conditions in the microwave glow discharge system with a barrel-type reactor have been optimized. The viability of plasma etching as an alternative surface preparation method for bulk Nb surfaces has been demonstrated on flat samples by achieving etching rates comparable to wet processes, such as BCP or EP.

The optimized experimental conditions are now being applied to the preparation of single cell cavities. The geometry of SRF cavities made of bulk Nb defines the use of asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of a driven and grounded electrode creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the plasma sheath attached to the cavity surface. A specially designed single cell cavity with sample holders is used to study these asymmetric discharges. The sample holder ports can be used for both diagnostics and sample etching purposes. The approach is to combine radially and spectrally resolved profiles of optical intensity of the discharge with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions and sequences.

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Atmospheric Microwave plasma torches for advanced cleaning (Giovanni Terenziani - 30')
Speaker: Giovanni Terenziani - Legnaro National Laboratories - INFN   |   Duration: 30 min.

Abstract
During the last years of research in the plasma field, more and more importance has been focused on Atmospheric Plasma sources. One of the biggest and most important possible application is to use them like a ultra-cleaning device. In fact experiments on Uranium or Thorium contaminated samples have shown that an Atmospheric Plasma surface treatments can decrease significantly the amount of contaminant species without adding any unwanted contamination. These experiments have been taken during the creation of an ultra-cleaning protocol for the C.U.O.R.E. experiment.
This kind of results can also open great possibilities in the field of superconductivity physics. In fact devices like Atmospheric Plasma torches could be used in the future to clean the inside surface of resonant cavities used in the particle accelerators.
During my work at Legnaro National Laboratories in the National Institute of Nuclear Physics I’ve developed an Atmospheric Plasma Semi-Metallic Torch. The principal aim of this device was to lower the content of radioactive contaminant in some Copper samples. The experimental results were encouraging and the study in this field seems to be very fruitful.
One of the possible improvement that we took into account was the possibility of increase the ionized species population by adding to the carrier gas different miscellaneous of various gases or liquid. In particular it’s been noticed, from the literature, the strong effect produced by the O+ and O++ particles. Because of this, we thought that adding water vapor in the gas carrier flux can provide good improvement in the O+ and O++ population. So we reach what we called “The Burning Water Configuration”. The main idea was to introduce a water vapor flux, produced by a vapor generator, directly into the plasma in order to “burn” the biggest quantity of the water possible. Problems with pressure and temperature of the vapor was solved. In fact the condensation of the water on the quartz tube walls, even in the plasma region, could decrease the quantity of “burned” vapor. Increasing then the working temperature and decreasing the flux we reached a minimum in the condensation regime and the plasma with vapor has been obtained.
A new development of this plasma source, for treating cavities, can be the creation of a bended plasma. This can be obtained by using a particular angular tube placed by the end of the plasma torch. This, with the help of a turbulent flux regime, can guide the plasma through the bended tube without hitting walls, avoiding in this case the melting of the tubes. With the possibility to bend the plasma flame we can be able to reach the most difficult part of the inner cavities surface. This should ensure more flexibility of operation to the whole system referring also to any other application field.

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Workshop Conclusion (Enzo Palmieri - 10')
Speaker: Enzo Palmieri -   |   Duration: 10 min.

Abstract

A nice warm cup of coffee ( - 20')
Speaker: -   |   Duration: 20 min.

Abstract
No abstract is required in order to get Coffee

Session Opening     (9:20)
Coffee Break     (11:00 - 11:30)
Lunch     (13:10 - 14:10)
Bus Transfer     (17:30 - 17:50)
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