[Show abstract][Hide abstract] ABSTRACT: This tutorial presents in the first part the requirement for the surface
preparation of RF Niobium cavities and its justification in term s of physical
origin of limitation (e.g. cleanliness and field emission, influence of the
surface treatments, morphology and surface damage, etc.). in the second part we
discuss the different models describing the ultimate limits of SRF cavities,
and present one of the possible ways to overcome Niobium monopoly toward higher
[Show abstract][Hide abstract] ABSTRACT: Vortex penetration severely limits high field performances in bulk niobium RF cavities for accelerators. To reach higher fields, Gurevich [Appl. Phys. Lett. 88, 012511 (2006)] proposed to deposit nanometric layers (d < λ) to partially screen of the field sensed by niobium. Model (NbN/MgO)n (n = 0 to 4) samples have been deposited on thick Nb layers. This paper presents the first complete characterization set (HC1 and RF surface resistance) of a new family of composite nanostructured superconducting layers deposited on Nb, which are liable to bring a breakthrough in the technology of superconducting accelerating RF cavities.
[Show abstract][Hide abstract] ABSTRACT: The application of a high electrical field on metallic surfaces leads to the well described phenomena of breakdown. In the classical scenario, explosive electron emission (EEE), breakdown (BD) originates from an emitting site (surface protrusion): the current at the apex vaporizes the emitting tip and the emitting current triggers a plasma in the vapor close to the surface. The plasma in turn melts the emitting site and makes it (hopefully) disappear. The conditioning process consists of “burning” the emitting sites one after another and numerous observations exhibit surfaces covered with molten craters that more or less overlap. In the case of radiofrequency (RF) applied fields, the effects of fatigue are also considered due to the cyclic nature of the applied stress. Nevertheless when dealing with RF cavities for accelerators, where higher fields are now sought, one can legitimately wonder if other physical phenomena should also be taken into account.
In particular, we believe that electromigration, especially at surfaces or grain boundaries cannot be neglected anymore at high field (i.e. 50-100 MV/m). Many publications in the domain of liquid metal emission sources show that very stable and strong emission sources, either ions or electrons, build up on metallic surfaces submitted to electrical fields through a mechanism that is slightly different from the usual localized breakdown evoked in accelerators. This mechanism involves the combination of electromigration and collective motion of surface atoms. In the case of emission source, this effect is sought after and has been extensively studied, whereas in our case it is very detrimental to the possibility of reaching high fields.
The recent results obtained on 30 GHz CLIC (Compact Linear Collider) accelerating structures, altogether with the data exposed hereafter have led us to propose a complementary scenario which could explain early melting of large areas of the surface.
In this paper we will concentrate on the early stages of breakdown, before plasma apparition. We will not discuss the plasma spot formation at the surface as we consider it to be the next step into the formation of the vacuum arc. We have gathered from the literature several examples of the physical phenomena involved on metallic surfaces submitted to very high fields. Definition of well-known concepts and terms used in other research fields will be introduced, like electrosprays, capillary waves…while some others have been left aside; not because they were irrelevant but because they would have requested extensive development which in turn would make this paper heavier. Because these concepts are, in a given community, well known a lot can be found using your favorite search engine and such without having to download the extensive bibliography cited in this paper.
In the introduction (section 2), we describe some of the damage that has been observed in CLIC accelerating structures which led us to suspect that electromigration is involved. We will present an alternative possible scenario for explosive breakdown (BD), which can result into the melting of extended area. In section 3 we will present RF simulations which show that pulse heating cannot be accountable for the observed melting. In section 4 we will describe what electromigration is and how it can lead to the appearance of nanotip and/or surface pre-melting. We will give several example of the occurrence of electromigration in several different experimental situations and we will try to evaluate some figures of merit. In particular we will show that electromigration is liable to occur at room temperature at fields close to 100 MV/m. We will also discuss other surface mechanisms that could also interfere with the breakdown mechanism.
A general discussion will be given in section 5 and the conclusion in section 6.
We hope to provide a new angle of observation that could help the accelerators community to better understand, and possibly overcome, the observed experimental limitation. Although it is very difficult to provide an evaluation of the relative weight of each phenomenon: electromigration, material type, surface state, plasma formation, we strongly think that the electromigration role needs to be explored.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 04/2012; 670:79-94. DOI:10.1016/j.nima.2011.11.032 · 1.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The best rf accelerating cavities are made of bulk niobium. This technology is now close to it full development, with any further improvement limited by the superconducting properties of Nb itself. In the best performing cavities the rf equatorial magnetic field H is close to the Nb thermodynamic critical field (H<sub>C</sub> ≈ 200 mT). In 2006 Gurevich proposed the use of nanoscale layers of superconducting materials having H<sub>C</sub> higher than Nb. This multilayer structure is expected to shield bulk niobium and therefore to increase the cavity breakdown field. In order to explore this pioneering idea, we have studied the penetration of the magnetic field by mean of nanometric sized multi layer structures deposited on flat single crystal substrates. We have deposited high quality "model" samples by dc magnetron reactive sputtering on R-plane cut sapphire substrates. A 250 nm layer of niobium represents 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-ray reflectivity and dc transport measurements, dc magnetization curves have been measured by conventional super conducting quantum interference device (SQUID) to determine the first penetration field B<sub>C1</sub>. For comparison, B<sub>C1</sub> was also mea sured 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.
[Show abstract][Hide abstract] ABSTRACT: The best rf bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H≈200mT close to the thermodynamic critical field Hc. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc>HcNb for magnetic shielding of bulk niobium to increase the breakdown magnetic field of superconducting rf cavities. Depositing good quality layers inside a whole cavity is rather difficult, so as a first step, characterization of single layer coating and multilayers was conducted on high quality sputtered samples by applying the technique used for the preparation of superconducting electronics circuits. The samples were characterized by x-ray reflectivity, dc resistivity (PPMS), and dc magnetization (SQUID) measurements. Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating exhibit different behaviors and clearly show an enhancement of the magnetic penetration field. Because SQUID measurements are influenced by edge and shape effects, we propose to develop a specific local magnetic measurement of HC1 based on ac third harmonic analysis in order to reveal the true screening effect of multilayers.
Review of Modern Physics 12/2010; 13(12). DOI:10.1103/PhysRevSTAB.13.121001 · 42.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electropolishing (EP) in the HF-H2SO4 electrolyte is the most desirable surface treatment for niobium superconducting radio frequency cavities yet demonstrated, in terms of performance and surface finish. However, the efficiency of the electrolyte declines quickly with time (decrease in removal rate, deterioration of the niobium surface, increased sulfur generation). Previous studies at CEA Saclay have highlighted the impact of the water content in EP mixtures rather than the content of dissolved niobium. Knowledge of the electrochemical system was improved thanks to studies using a rotating disk electrode (RDE). Measurements with a RDE give precious information concerning mass transport of the different ionic groups present in the solution. The performed measurements prove that EP is controlled by the diffusion of fluorine ions and the value of the related diffusion coefficient DF- was estimated for different mixtures. Electrochemical impedance spectroscopy (EIS) measurements were also performed with different EP mixtures. Both volt ampere metric and EIS measurements prove the central role of fluorine during EP and show that EP mechanisms evolve with the aging of the bath. Another major problem related to electrolytes is the formation of impurities such as sulfur. We have proved that working at a reduced voltage of 5 V does not alter cavity performance and makes it possible to reduce the undesirable particulate contamination in electrolytes and to increase their lifetime.
Physical Review Special Topics - Accelerators and Beams 08/2010; 13(8). DOI:10.1103/PhysRevSTAB.13.083501 · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The use of large grains or single crystal niobium to improve the Q factor of superconducting rf cavities for particle accelerators, is presently under study. Heat extraction which plays a decisive role in the thermomagnetic stability of these devices depends on the thermal conductivity of niobium K and the thermal boundary (Kapitza) resistance RK at the niobium/superfluid helium interface. Here we present the first measurements of RK performed between 1.5-2.1 K with single crystal (111) niobium, having two different surface morphologies, namely, a surface with a damage layer and a chemically polished surface. The thermal conductivity of the single crystal Nb samples is also simultaneously determined. For monocrystalline niobium we demonstrate that RK is an increasing primary limiting factor with temperature, contrary to the behavior found for polycrystalline cavities. The present investigation reveals for the first time that the presence of impurities (metallic particles and oxygen) within the damage layer leads to a stronger RK, although the effective heat exchange area to the superfluid is increased. We further show the importance of dislocations in the thermal conductivity of monocrystalline niobium.
Review of Modern Physics 02/2010; 13(2). DOI:10.1103/PhysRevSTAB.13.023201 · 42.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Niobium, with its very high H<sub>C1</sub>, has been used in superconducting radio frequency (SRF) cavities for accelerator systems for 40 years with continual improvement. The quality factor of cavities (Q) is governed by the surface impedance R<sub>BCS</sub>, which depends on the quasiparticle gap, delta, and the superfluid density. Both of these parameters are seriously affected by surface imperfections (metallic phases, dissolved oxygen, magnetic impurities). Loss mechanism and surface treatments of Nb cavities found to improve the Q factor are still unsolved mysteries. We present here an overview of the capabilities of the point contact tunneling spectroscopy and Atomic layer deposition methods and how they can help understanding the High field Q-drop and the mild baking effect. Tunneling spectroscopy was performed on Nb pieces from the same processed material used to fabricate SRF cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap Delta = 1.55 meV, characteristic of clean, bulk Nb, however the tunneling density of states (DOS) was broadened significantly. Nb pieces treated with the same mild baking used to improve the Q-slope in SRF cavities revealed a much sharper DOS. Good fits to the DOS are obtained using Shiba theory suggesting that magnetic scattering of quasiparticles is the origin of the degraded surface superconductivity and the Q-slope problem of Nb SRF cavities.
[Show abstract][Hide abstract] ABSTRACT: The surface morphology and chemical purity of superconducting radio frequency (SRF) niobium cavities are very important for proper accelerator operation. Typically on the order of 120 micrometers of niobium (Nb) is removed from cavities to remove damage done during the forming of Nb sheets and cavities. A study was done to find the effect of cutting or finishing Nb with a band saw, diamond saw, electrical discharge machining (EDM) wire, garnet water jet, sheer, and mill. Surface contamination of the samples was measured before and after buffered chemical polish (BCP) by secondary ion mass spectroscopy (SIMS), energy dispersive spectroscopy (EDS), and by measuring relative resistivity ratios (RRRs). Surface morphology was examined with a digital microscope, a surface profilometer and scanning electron microscope (SEM). It was found that all techniques altered the top 3-5 micrometers of the Nb. It was also found by SIMS that the water jet technique introduced the most hydrogen and oxygen to the Nb in the first 2.5 micrometers of the sample. The EDM wire cutting technique introduced the least amount of hydrogen to the Nb. After 5 micrometers were etched away by BCP on the various samples, no contaminants were found except on the water jet cut samples. Even after 20 micrometers of Nb removal silica could be seen on the surface with EDS. The water jet produced the roughest surface with 50-100 micrometer deep pits made from embedded garnet particles. It was found that the garnet water jet damages the surface to the point where even the typical 120 micrometers of BCP etching may not remove all the defects created.
[Show abstract][Hide abstract] ABSTRACT: Tunneling spectroscopy was performed on Nb pieces prepared by the same processes used to etch and clean superconducting radio frequency (SRF) cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap Delta=1.55 meV, which is characteristic of a clean, bulk Nb. However, the tunneling density of states (DOS) was significantly broadened. The Nb pieces, which were treated with the same mild baking used to improve the Q slope in SRF cavities, reveal a sharper DOS. Good fits to the DOS were obtained by using the Shiba theory, suggesting that magnetic scattering of quasiparticles is the origin of the gapless surface superconductivity and a heretofore unrecognized contributor to the Q-slope problem of Nb SRF cavities.(c) 2008 American Institute of Physics.
[Show abstract][Hide abstract] ABSTRACT: Niobium is the metal of choice for superconducting radio-frequency cavities for the future International Linear Collider. We present the results of atomic-scale characterization of the oxidation of niobium utilizing local-electrode atom-probe tomography employing picosecond laser pulsing. Laser pulsing is utilized to prevent a tip from fracturing as a buried niobium oxide/niobium interface is dissected on an atom-by-atom basis. The thickness of niobium oxide is about 15 nm, the root-mean-square chemical roughness is 0.4 nm, and the composition is close to Nb2O5, which is an insulator, with an interstitial oxygen concentration profile in Nb extending to a depth of 12 nm.
[Show abstract][Hide abstract] ABSTRACT: A method to treat the surface of Nb is described which potentially can improve the performance of superconducting RF cavities. We present tunneling and x-ray photoemission spectroscopy (XPS) measurements at the surface of cavity-grade niobium samples coated with a 3 nm alumina overlayer deposited by Atomic Layer Deposition (ALD). The coated samples baked in ultra high vacuum (UHV) at low temperature reveal at first degraded superconducting surface. However, at temperatures above 450C, the tunneling conductance curves show significant improvements of the superconducting density of states (DOS) compared with untreated surfaces. Comment: 3 pages, 2 figures
[Show abstract][Hide abstract] ABSTRACT: We have investigated the dissolution of a natural oxide layer on a Nb(110) surface upon heating, combining x-ray reflectivity, grazing incidence diffuse scattering, and core-level spectroscopy. The natural oxide reduces after heating to 145 °C partially from Nb2O5 to NbO2, and an enrichment in subsurface interstitial oxygen by ∼ 70% in a depth of 100 Å is observed. After heating to 300 °C, the oxide reduces to NbO and the surplus subsurface oxygen gets dissolved into the bulk. Our approach can be applied for further investigation of the effect of subsurface interstitial oxygen on the performance of niobium rf cavities.
[Show abstract][Hide abstract] ABSTRACT: A condition for Electron Cyclotron Resonance (ECR) can be established inside a fully assembled RF cavity without the need for removing high-power couplers. As such, plasma generated by this process can be used as a final cleaning step, or as an alternative cleaning step in place of other techniques. Tests showed filtered dry air plasma can successfully remove sulfur particles on niobium surface while the surface oxygen content remains intact.
[Show abstract][Hide abstract] ABSTRACT: The key technology for the linear collider is the high gradient superconducting radio-frequency (SRF) cavity, approximately 20,000 of which will make up the accelerator. The preferred technology is to make the cavities from high-purity niobium-sheet. From the RF superconductivity point-of-view, the interface between the native niobium oxide on the surface of the cavity and near sub-surface region is the most important one. Superconducting properties of cavities depend on the chemistry and microstructure of the surface oxide and the concentration and location of impurity elements. Little is known, however, about this information and the effect of low-temperature baking on the surface region. Atom-probe tomography (APT) provides chemical information of the analysed materials on an atomic scale utilizing time-of-flight (TOF) mass spectrometry, with the field evaporation of materials permitting atom-by-atom dissection. We employ a 3-D local-electrode atom-probe (LEAP) tomography to analyse the chemistry of niobium tips, from the surface niobium oxide to underlying bulk niobium.
[Show abstract][Hide abstract] ABSTRACT: In the framework of SRF cavity development, Fermilab is creating the infrastructure needed for the characterization of the material used in the cavity fabrication. An important step in the characterization of "as received" niobium sheets is eddy current scanning. Eddy current scanning is a non-destructive technique first adopted and further developed by DESY with the purpose of checking the cavity material for sub-surface defects and inclusions. Fermilab has received and further upgraded a commercial eddy current scanner previously used for the SNS project. This scanner is now used daily to scan the niobium sheets for the Fermilab third harmonic, the ILC, and the Proton Driver cavities. After optical inspection, more than 400 squares and disks have been scanned and when necessary checked at the optical and electron microscopes, anodized, or measured with profilometers looking for surface imperfections that might limit the performance of the cavities. This paper gives a status report on the scanning results obtained so far, including a discussion of the classification of signals being detected.