Benvenuti, Cristoforo
Calatroni, Sergio
Campisi, I E
Darriulat, Pierre
Durand, C
Peck, M A
Russo, R
Valente, A M
Magnetic flux trapping in superconducting niobium
In a systematic study of the RF response of superconducting niobium cavities operated in their fundamental TM010 mode at 1.5 GHz, magnetic flux trapping has been used as a tool to diagnose the presenc e of pinning centres. In addition to bulk niobium cavities the study covers copper cavities, the inner walls of which are coated with 1.5 µm thick niobium films grown by magnetron sputtering in a nobl e gas atmosphere. The use of different gases (Xe, Kr, Ar and Ne) or gas mixtures has made it possible to vary the concentration of noble gas atoms in the films. Film contamination is characterised by an electron mean free path l calculated from the results of systematic measurements of the penetration depth at T = 0 K, l0, and from RRR measurements made on samples prepared under similar conditions as the cavity films.
eng
1997
http://preprints.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-97-008;
http://cds.cern.ch/record/352135/files/352135.zip;
http://cds.cern.ch/record/352135/files/est-97-008.pdf;
http://cds.cern.ch/record/352135/files/est-97-008.ps.gz;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Campisi, I E
Darriulat, Pierre
Durand, C
Kneisel, P
Marsh, S
Peck, M A
Russo, R
Sgobba, Stefano
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.
eng
1997
http://preprints.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-97-007;
http://cds.cern.ch/record/352134/files/est-97-007.pdf;
http://cds.cern.ch/record/352134/files/est-97-007.ps.gz;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Campisi, I E
Darriulat, Pierre
Peck, M A
Russo, R
Study of the role of the interface between niobium films and copper RF resonators
Niobium-coated copper resonators are usually produced with an oxide interface between the film and the substrate. This oxide has two sources: the passivation layer inevitably formed on the surface of the cavity after chemical preparation before coating, and the niobium oxide which builds up on the surface of the cathode when it is exposed to air, and is transferred to the cavity surface during coa ting. The oxide layer may influence both the purity and the structural properties of the film, and in turn its RF behaviour. To study its effect, some cavities have been coated with a special two-cath ode sputtering system, allowing for a complete removal of both oxide layers by sputter-etching. For comparison, a few cavities have also been produced with the same coating system without sputter-etch ing, or with a controlled oxidation of the copper surface of the cavity after sputter-etching. Two cavities have also been produced without oxide interface using Kr and Ne as sputter gas instead of Ar .
eng
1997
http://preprints.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-97-005;
http://cds.cern.ch/record/352132/files/est-97-005.pdf;
http://cds.cern.ch/record/352132/files/est-97-005.ps.gz;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Campisi, I E
Darriulat, Pierre
Marino, M
Peck, M A
Russo, R
Valente, A M
Properties of Copper Cavities Coated with Niobium Using Different Discharge Gases
Experimental evidence is presented showing that in conformity with theoretical expectations, discharge gas atoms are trapped in sputtered films whenever a gas of atomic mass smaller than that of the c athode is used. In such a case, discharge gas atoms may be reflected by the cathode as high energy neutrals and get incorporated in the growing film. Niobium films have been produced using Ne, Ar, Kr, Xe and then analysed for rare gas content by thermal extraction. The gas concentrations are found to vary from the several percent range for Ne down to the ppm level for Kr and Xe. The noble gas conce ntration in the film influences the RRR and, in the case of high concentration, also the critical temperature. To study the effect of the implanted noble gas on the superconducting RF parameters, seve ral 1.5 GHz copper cavities have been niobium-coated using the different discharge gases. The noble gases trapped in the film affect the penetration depth, the temperature dependent losses (RBCS), the losses induced by the presence of trapped fluxons, but have no significant influence on the residual resistance.
eng
1997
http://preprints.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-97-004;
http://cds.cern.ch/record/352131/files/est-97-004.pdf;
http://cds.cern.ch/record/352131/files/est-97-004.ps.gz;
Imported from Invenio.
Schucan, G M
Benvenuti, Cristoforo
Calatroni, Sergio
Niobium films produced by magnetron sputtering using an Ar-He mixture as discharge gas
Superconducting RF accelerating cavities have been produced at CERN by sputter-coating, with a thin niobium layer, cavities made of copper. In the present work, the discharge behaviour and niobium film properties have been investigated when part of the argon sputtering gas is replaced with helium. Helium is chosen because of its low mass, which reduces the energy lost by the niobium atoms colliding with the sputter gas atoms. The higher niobium atom energy should lead to higher adatom mobility on the substrate and, hence, to a larger grain size, a feature which is highly desirable to reduce the cavity surface resistance. It has been found that helium addition effectively helps to maintain the discharge at considerably lower argon pressures, via metastable-neutral ionisation and high secondary electron yield. However, a large amount of helium is trapped in the film, amount which is proportional to the helium partial pressure during the discharge, resulting in a reduction of both Residual Resistivity Ratio and grain size.
eng
1995
http://preprints.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=MT-95-013;
http://cds.cern.ch/record/295022/files/mt-95-013.pdf;
http://cds.cern.ch/record/295022/files/mt-95-013.ps.gz;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Chiaveri, Enrico
Orlandi, G
Weingarten, Wolfgang
Superconducting cavities for particle accelerators: achievements and problems
Discharge gas trapping in thin films produced by sputtering is known to be due to high energy neutrals bouncing back from the cathode. Qualitatively, the phenomenon is enhanced by raising the discharge voltage and is strongly dependent on the atomic masses of the discharge gas and of the cathode material. In addition to these known effects it is shown that, for a given gas, the trapped amount decreases with increasing the melting temperature of the deposited material. The results obtained both by sample melting and laser ablation are presented and discussed.
eng
2001
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-1999-005;
http://cds.cern.ch/record/437646/files/est-1999-005.pdf;
http://cds.cern.ch/record/437646/files/est-1999-005.ps.gz;
Imported from Invenio.
Valente, A M
Benvenuti, Cristoforo
Calatroni, Sergio
Darriulat, Pierre
Peck, M A
Van't Hof, C A
Study of the Residual Resistance of Superconducting Niobium Films at 1.5 GHz
A study of the main potential contributions to the residual surface resistance of niobium-coated cavities is reviewed. They are the formation of hydride precipitates, the contamination by discharge gas atoms and the presence of macroscopic defects in the film, induced by defects in the substrate. It will be shown that residual resistances as low as those obtained for bulk niobium can be achieved, together with a very small dependence on the amplitude of the RF field.
eng
1.5;
1999
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-1999-002;
http://cds.cern.ch/record/433395/files/est-1999-002.pdf;
http://cds.cern.ch/record/433395/files/est-1999-002.ps.gz;
Imported from Invenio.
Calatroni, Sergio
Benvenuti, Cristoforo
Darriulat, Pierre
Peck, M A
Valente, A M
Van't Hof, C A
High-Q, high gradient niobium-coated cavities at CERN
Superconducting cavities made by sputter-deposition of a thin niobium film onto copper have proven over the years to be a viable alternative to bulk niobium, the best example being the very successful operation of LEP at 200 GeV. It will be shown that this technology, investigated at 1.5 GHz by a dedicated R&D effort at CERN, can be developed to unprecedented performance, proving that no fundamental limitation prevents high quality factors to be maintained over a broad range of accelerating field.
eng
1999
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-1999-001;
http://cds.cern.ch/record/432866/files/est-1999-001.pdf;
http://cds.cern.ch/record/432866/files/est-1999-001.ps.gz;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Campisi, I E
Darriulat, Pierre
Peck, M A
Russo, R
Valente, A M
Study of the surface resistance of niobium sputter-coated copper cavities
IEEE Trans. Appl. Supercond.
900-903
9
A systematic study of the superconducting properties of niobium films deposited on the inner wall of copper radiofrequency cavities is presented. Films are grown by sputtering with different discharge gases (Xe, Kr, Ar and Ar/Ne mixtures) on substrates prepared under different conditions. The measured quantities include the surface resistance at 1.5 GHz, the critical temperature and the penetration depth. The surface resistance is analyzed in terms of its dependence on temperature, RF field and the density of trapped fluxons. Once allowance for electron scattering is made by means of a single mean free path parameter, good agreement with BCS theory is observed. The residual resistance is observed to be essentially noncorrelated with the superconducting properties, although influenced by specific coating conditions. On occasions, very low residual resistances, in the nano-ohm range, have been maintained over a broad range of RF field, indicating the absence of fundamental limitations specific to the film technology in practical applications. (7 refs).
eng
1999
http://documents.cern.ch/cgi-bin/setlink?base=generic&categ=public&id=cer-000336900;
http://cds.cern.ch/record/409760/files/cer-000336900.pdf;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Campisi, I E
Darriulat, Pierre
Peck, M A
Russo, R
Valente, A M
Study of the surface resistance of superconducting niobium films at 1.5 GHz
Physica C
153-88
316
A systematic study of superconducting properties of niobium films sputtered on the inner wall of radiofrequency cavities is presented. The measured quantities include in particular the response to 1.5 GHz microwaves, the critical temperature, the penetration depth and the magnetic penetration field. In addition to films grown in different gas discharges (Xe, Kr, Ar and Ar/Ne mixtures) and to films grown on substrates prepared under different conditions, the study also includes bulk niobium cavities. The surface resistance is analysed in terms of its dependence on temperature, on RF field and, when relevant, on the density of trapped fluxons. A simple parameterisation is found to give a good fit to the data. Once allowance for the presence of impurities and defects is made by means of a single parameter, the electron mean free path, good agreement with BCS theory is observed. The fluxon-induced losses are studied in detail and their dependence on RF field, on temperature and on the density of trapped fluxons is analysed. The residual resistance is observed to be essentially uncorrelated with the other variables, suggesting that it is dominantly extragranular. In occasions very low residual resistances, in the n Omega range, have been maintained over a broad range of RF field, indicating the absence of significant fundamental limitations specific to the film technology in practical applications such as the production of accelerating cavities for particle accelerators. (130 refs).
eng
1999
http://preprints.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-98-002;
http://cds.cern.ch/record/409287/files/est-98-002.pdf;
Imported from Invenio.
Aberle, O
Boussard, Daniel
Calatroni, Sergio
Chiaveri, Enrico
Häbel, E
Hanni, R
Losito, R
Marque, S
Tückmantel, Joachim
Technical Developments on Reduced $\beta$ Superconducting Cavities at CERN
Several authors proposed the construction of superconducting proton linacs using the LEP2 cavities once LEP will be decommissioned. However only a fraction (about half) of these cavities can be used as they are for the high-energy part (b~1) of such a linac, the low energy part requiring the development of accelerating structures optimized for lower values of the particle velocity. At CERN an R&D programme on reduced-b single-cell cavities started in 1996 in order to study and explore the limits of the technology successfully used for the production of LEP2 cavities (copper cavities niobium-plated using the magnetron sputtering technique). Four different geometries were extensively investigated, each representing part of a multicell structure optimized for particles having b=0.48, b=0.625, b=0.66 and b=0.8 respectively. The results were encouraging for the last two types and therefore a new phase of R&D aimed at the production of multicell cavities for b=0.66 and b=0.8 was started. The goal is to demonstrate simultaneously the feasibility of such cavities and the possibility of producing them by low-cost modification of LEP cavities.
eng
1999
http://documents.cern.ch/archive/electronic/other/uploader/PAC99/mop121.pdf;
http://cds.cern.ch/record/387156/files/mop121.pdf;
http://cds.cern.ch/record/387156/files/sl-99-024.ps.gz;
http://cds.cern.ch/record/387156/files/sl-99-024.pdf;
http://cds.cern.ch/record/387156/files/CERN-SL-99-024-CT.pdf;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Campisi, I E
Darriulat, Pierre
Durand, C
Peck, M A
Russo, R
Studies of RF-superconductivity properties of niobium film-coated cavities at CERN
Adv. Cryog. Eng., A
77-85
43
Niobium sputter-coated copper resonators are successfully employed in operating particle accelerators, the most outstanding example being LEP2 at CERN. In this review we present recent progress in the understanding of the basic principles governing their behaviour, based on an extensive R&D programme carried out at CERN on 1.5 GHz resonators operated in the TM010 mode. At the present stage of the s tudy, no fundamental limitation has been found which would prevent the use of this technology for future high-field, high-Q accelerating cavities.
eng
1998
http://preprints.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-97-002;
http://cds.cern.ch/record/353335/files/est-97-002.pdf;
http://cds.cern.ch/record/353335/files/est-97-002.ps.gz;
Imported from Invenio.
Benvenuti, Cristoforo
Calatroni, Sergio
Darriulat, Pierre
Peck, M A
Valente, A M
Van't Hof, C A
Fluxon induced surface resistance and field emission in niobium films at 1.5 GHz
Physica C
31-9
361
The surface resistance of superconducting niobium films induced by the presence of trapped magnetic flux, presumably in the form of a pinned fluxon lattice, is shown to be modified by the presence of a field emitting impurity or defect. The modification takes the form of an additional surface resistance proportional to the density of the fluxon lattice and increasing linearly with the amplitude of the microwave above a threshold significantly lower than the field emission threshold. Such an effect, a precursor of electron emission, is observed for the first time in a study using radiofrequency cavities operating at their fundamental 1.5 GHz frequency. The measured properties of the additional surface resistance severely constrain possible explanations of the observed effect. (23 refs).
eng
2001
Imported from Invenio.
Calatroni, Sergio
Benvenuti, Cristoforo
Neupert, H
Prada, M
Valente, A M
A short summary of the R&D activity carried out at CERN in the past five years and ideas for further R&D based on the present state of the art
We study the flavour changing neutral Higgs boson decays that can be induced from genuine supersymmetric particles at the one-loop level and within the context of the Minimal Supersymmetric Standard Model. We consider all the possible flavour changing decay channels of the three neutral Higgs bosons into second and third generation quarks, and focus on the Supersymmetric-QCD corrections from squark-gluino loops which are expected to provide the dominant contributions. We assume here the more general hypothesis for flavour mixing, where there is misalignment between the quark and squark sectors, leading to a flavour non-diagonal squark mass matrix. The form factors involved, and the corresponding Higgs partial decay widths and branching ratios, are computed both analytically and numerically, and their behaviour with the parameters of the Minimal Supersymmetric Standard Model and with the squark mass mixing are analyzed in full detail. The large rates found, are explained in terms of the non-decoupling behaviour of these squark-gluino loop corrections in the scenario with very large supersymmetric mass parameters. Our results show that if these decays are seen in future colliders they could provide clear indirect signals of supersymmetry.
eng
2003
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=hep-ph&id=0210335;
http://cds.cern.ch/record/588428/files/0210335.pdf;
http://cds.cern.ch/record/588428/files/0210335.ps.gz;
Imported from Invenio.
Arbet-Engels, V P
Benvenuti, Cristoforo
Calatroni, Sergio
Darriulat, Pierre
Peck, M A
Valente, A M
Van't Hof, C A
Superconducting niobium cavities, a case for the film technology
Nucl. Instrum. Methods Phys. Res., A
1-8
463
Evidence is presented for niobium film cavities performing as well as niobium bulk cavities, at variance with a widespread belief that their much smaller grain size should be a fundamental limitation preventing high quality factors to be maintained over a wide range of accelerating fields. By comparing the relative merits of the bulk and film technologies, a strong case is presented in favour of the latter. (19 refs).
eng
2001
http://documents.cern.ch/cgi-bin/setlink?base=generic&categ=public&id=cer-002263851;
http://cds.cern.ch/record/508322/files/cer-002263851.pdf;
Imported from Invenio.
Calatroni, Sergio
Benvenuti, Cristoforo
Darriulat, Pierre
Peck, M A
Valente, A M
Influence of the nature of the substrate on the growth of superconducting niobium films
The superconducting properties of niobium films sputtered onto the inner walls of radiofrequency cavities, including their surface resistance to 1.5 GHz microwaves, have been studied as a function of the nature of the substrate. Films grown on oxide-free copper or niobium behave differently from films grown on other substrates. The results are analysed in terms of the film texture and internal stresses.
eng
2000
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-2000-005;
http://cds.cern.ch/record/477073/files/est-2000-005.pdf;
http://cds.cern.ch/record/477073/files/est-2000-005.ps.gz;
Imported from Invenio.
Calatroni, Sergio
Benvenuti, Cristoforo
Darriulat, Pierre
Peck, M A
Valente, A M
Van't Hof, C A
Fluxon induced resistance and field emission
The surface resistance of superconducting niobium films induced by the presence of trapped magnetic flux, presumably in the form of a pinned fluxon lattice, is shown to be modified by the presence of a field emitting impurity or defect. The modification takes the form of an additional surface resistance proportional to the density of the fluxon lattice and increasing linearly with the amplitude of the microwave above a threshold significantly lower than the field emission threshold. Such an effect, precursor of electron emission, is observed here for the first time in a study using radiofrequency cavities operated at their fundamental 1.5 GHz frequency. The measured properties of the additional surface resistance severely constrain possible explanations of the observed effect.
eng
2000
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-2000-004;
http://cds.cern.ch/record/477072/files/est-2000-004.pdf;
http://cds.cern.ch/record/477072/files/est-2000-004.ps.gz;
Imported from Invenio.
Calatroni, Sergio
Benvenuti, Cristoforo
Darriulat, Pierre
Peck, M A
Valente, A M
Fluxon Pinning in Niobium Films
Physica C
429-37
351
Resistive losses induced by the presence of trapped magnetic flux in niobium superconducting films have been studied using 1.5 GHz microwaves. They are measured to span a very broad spectrum depending on the film-substrate interface and on the gas used in the sputtering discharge. An interpretation in terms of pinning by noble gas clusters is considered.
eng
2001
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-2000-003;
http://cds.cern.ch/record/477071/files/est-2000-003.pdf;
http://cds.cern.ch/record/477071/files/est-2000-003.ps.gz;
Imported from Invenio.
Calatroni, Sergio
Benvenuti, Cristoforo
Darriulat, Pierre
Peck, M A
Valente, A M
Van't Hof, C A
Study of the residual surface resistance of niobium films at 1.5 GHz
Physica C
421-8
351
Potential contributions to the residual surface resistance of niobium films exposed to 1.5 GHz microwaves are reviewed and studied. These include the oxidation of the film surface, the formation of hydride precipitates, the contamination by noble gas atoms and the presence of macroscopic film defects such as those resulting from the roughness of the substrate. Particular attention is given to the dependence of the residual resistance on the amplitude of the microwave. Results similar to those obtained for bulk niobium provide strong evidence against the conjecture that the small size of the film grains should be a fundamental limitation to the production of films having a low residual resistance.
eng
2001
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-2000-002;
http://cds.cern.ch/record/477070/files/est-2000-002.pdf;
http://cds.cern.ch/record/477070/files/est-2000-002.ps.gz;
Imported from Invenio.
Calatroni, Sergio
Arbet-Engels, V P
Benvenuti, Cristoforo
Darriulat, Pierre
Peck, M A
Valente, A M
Van't Hof, C A
Superconducting niobium cavities, a case for the film technology
Evidence is presented for niobium film cavities performing as well as niobium bulk cavities, at variance with a widespread belief that their much smaller grain size should be a fundamental limitation preventing high quality factors to be maintained over a wide range of accelerating fields. By comparing the relative merits of the bulk and film technologies, a strong case is presented in favour of the latter.
eng
2000
http://documents.cern.ch/cgi-bin/setlink?base=preprint&categ=cern&id=est-2000-001;
http://cds.cern.ch/record/477069/files/est-2000-001.pdf;
http://cds.cern.ch/record/477069/files/est-2000-001.ps.gz;
Imported from Invenio.
Calatroni, Sergio
Bacher, J P
Benvenuti, Cristoforo
Cosso, R
Dalin, J M
Guérin, J
Lacarrère, D
Lasserre, A
Orlandi, G
Radicioni, E
Scalambrin, F
Taufer, M
Influence of copper substrate treatments on properties of niobium coatings
http://cds.cern.ch/record/256047/files/CERN-MT-93-11-SM.pdf;
Imported from Invenio.