Materials Science of the Interstitial Doping Process

Radio-frequency superconducting (SRF) cavities are widely used to accelerate a charged particle beam in particle accelerators. The performance of SRF cavities made of bulk niobium has significantly improved over the last ten years and is approaching the theoretical limit for niobium. Nevertheless, RF tests of niobium cavities are still showing some “anomalous” losses that require a better understanding in order to reliably obtain better performance. These losses are characterized by a marked dependence of the surface resistance on the surface electromagnetic field and can be detected by measuring the quality factor of the resonator as a function of the peak surface field. A low-temperature (100–150°C) “in situ” bake under ultrahigh vacuum has been successfully applied as final preparation of niobium RF cavities by several laboratories over the last few years. The benefits reported consist mainly of an improvement of the cavity quality factor at low field and a recovery from “anomalous” losses (so-called “Q drop”) without field emission at higher field. A series of experiments with a CEBAF single-cell cavity have been carried out at Jefferson Lab to carefully investigate the effect of baking at progressively higher temperatures for a fixed time on all the relevant material parameters. Measurements of the cavity quality factor in the temperature range 1.37–280K and resonant frequency shift between 6–9.3K provide information about the surface resistance, energy gap, penetration depth, and mean free path. The experimental data have been analyzed with the complete BCS theory of superconductivity. The hydrogen content of small niobium samples inserted in the cavity during its surface preparation was analyzed with nuclear reaction analysis. The single-cell cavity has been tested at three different temperatures before and after baking to gain some insight on thermal conductivity and Kapitza resistance and the data are compared with different models. This paper describes the results of these experiments and comments on existing models to explain the effect of baking on the performance of niobium RF cavities.

What are the major goals of the project? Goal 1: Optimize interactively processing of the single-cell cavity by combining plasma treatment and vertical cold bath testing. Continue using single cell cavity as the test bed for new applications. Goal 2: Work on upgrading the plasma etching technology for multi-cell SRF cavities. Goal 3: Develop and adapt discharge configurations to critical cavity regions including the electron beam weld (EBW) and iris areas. Goal 4: Develop tailored methods to prepare surface properties for a specific function prior to any additional modification, such as (but not limited to) surface passivation in preparation of other superconducting materials and structures.

A study is presented on the superconducting properties of niobium used for the fabrication of the SRF cavities after treating by recently discovered nitrogen doping methods. Cylindrical niobium samples have been subjected to the standard surface treatments applied to the cavities (electro-polishing, l 20°C bake) and compared with samples treated by additional nitrogen doping recipes routinely used to reach ultra-high quality factor values (>3· 1010 at 2 K, 16 MV/m). The DC magnetization curves and the complex magnetic AC susceptibility have been measured. Evidence for the lowered field of first flux penetration after nitrogen doping is found suggesting a correlation with the lowered quench fields. Superconducting critical temperatures Tc = 9.25 K are found to be in agreement with previous measurements, and no strong effect on the critical surface field (Bd) from nitrogen doping was found.

Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulæ, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in situ observations, analytical and numerical developments. A particular emphasis is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. It appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. The shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. Recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. A special section is dedicated to new laser plasma experiments probing shock physics.

Nitrogen doping and infusion in SRF cavities: A review

Accelerator Science is both a discipline in its own right within modern physics and provides highly powerful tools for discovery and innovation in many other fields of scientific research. Accelerators do support different disciplines of subatomic physics, material sciences, life sciences and applications in research and industry. Accelerator Science Community does perform R&D to improve operational facilities and prepare technologies for new facilities unveiling new opportunities and unprecedented performance parameters. Having delivered nearly five decades of discovery, TRIUMF has a vibrant reputation globally as Canada’s particle accelerator laboratory and a hub for particle accelerator physics and technology with a wide network of international connections. In collaboration with CLS, the Fedoruk Centre and Canadian Universities, we want to grow the impact of Accelerator Science in Canada on the society and the potential to address key issues of the society of today.

3D carbon coated NiCo2S4 nanowires doped with nitrogen for electrochemical energy storage and conversion

Electropolishing (EP) is a standard process for the treatment of the interior surface of niobium (Nb) superconducting radio-frequency (SRF) cavities used in particle accelerators. We previously conducted a study on vertical electropolishing (VEP), in which the cavity was set vertically during EP of a 1.3 GHz Nb single-cell cavity. In that study, we showed that the major cause of asymmetric removal in VEP is the accumulation of hydrogen gas bubbles, generated during the EP process, on the upper half cell of the cavity. Three types of unique rotating cathodes were tested with the aim to resolve the issues of asymmetric removal and rough surface. Appropriate cathode design and parameters reduced the asymmetric removal in the cell and yielded a smooth surface of the cavity. In this study, we have performed an extensive investigation on VEP of both single- and nine-cell Nb 1.3 GHz cavities with a modified cathode. The VEP parameters with single- and nine-cell coupon cavities were investigated, which facilitate an in situ study of the coupon currents at various positions of the cavities and observation from the viewports located near the iris and equator positions of the cavities. The modified cathode and optimized VEP process for the single-cell cavity yielded uniform removal in the cavity cell and a smooth surface of the interior of the cavity. A novel acid flow method that allows the separation of the acid flow in the cavity and cathode housing was employed in the nine-cell cavity to efficiently remove the gas bubbles from the cavity. The optimized acid flow rates in the cavity and cathode housing, along with other adequate parameters including temperature, cathode rotation speed, and applied voltage, made the surface smooth and significantly reduced the removal nonuniformity in the nine-cell cavity. The single- and nine-cell cavities after an average removal of 46 and $130\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, respectively, were tested in a vertical cryostat. The single- and nine-cell cavities achieved $31\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a ${Q}_{0}$ value of $9\ifmmode\times\else\texttimes\fi{}{10}^{9}$ and $28.3\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a ${Q}_{0}$ value of $6.7\ifmmode\times\else\texttimes\fi{}{10}^{9}$, respectively, in the rf tests at the temperature of 2 K. The achieved SRF performances were as good as the baseline performances achieved after the horizontal electropolishing process of the same cavities.

This paper looks at the early history of accelerator development with a particular focus on the important discoveries that opened the door for the application of permanent-magnet materials this area of science. Researchers began to permanent-magnet materials in particle accelerators soon after the invention of the alternating gradient principle, that showed magnetic fields could be used to control the transverse envelope of charged-particle beams. Since that time, permanent-magnet materials have found wide application in the modern charged particle accelerator. A brief history of permanent-magnet use in accelerator physics and technology is outlined, some of the general design considerations are presented, and several material of concern for particle accelerator are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A contact system for silicon solar cells is described in which silver is coated with a layer of dopant and alloyed with silicon, thereby simultaneously doping the silicon substrate and forming a low-resistance ohmic contact to it. The concept has been demonstrated using evaporated silver with commercially available phosphorus and boron liquid dopants. Silicon surface topography, along with I-V and SIMS analyses all indicate that the Ag-Si eutectic temperature (835/spl deg/C) must be exceeded and the dopant coating must be present for the contact to be self-doping. The concept has also been implemented in the form of screen-printable silver pastes with phosphorus. A fritless version of the paste exhibited only 3 m/spl Omega/-cm/sup 2/ contact resistance directly to 7 /spl Omega/-cm n-type dendritic web silicon. For contacting lightly-doped n/sup +/ layers (<100 /spl Omega///spl square/) through SiN/sub x/, a fritted version is used. The resultant contact metal is highly conductive (3 /spl mu//spl Omega/-cm) and solderable. No carrier lifetime degradation associated with the alloying process has been observed in dendritic web silicon solar cells.

The physics of particle acceleration in turbulent plasmas is a topic of broad interest, which is making rapid progress thanks to dedicated, large-scale numerical experiments. The first part of this paper presents an effective theory of stochastic Fermi acceleration, which subsumes all forms of nonresonant acceleration in ideal electric fields and is applicable in generic settings. It combines an exact equationconnecting the energization rate to the statistics of the velocity field with a statistical model of particle transport through the structures (i.e., the regions of strong velocity gradients). In a second part, this formalism is applied to magnetohydrodynamic turbulence to obtain a comprehensive assessment of the scale-by-scale contributions to the advection and diffusion coefficients. Acceleration peaks on scales where particles can be trapped inside structures for an eddy turnaround time, or in intense structures associated with sharp bends of the magnetic field lines in large-amplitude turbulence (as reported earlier). These spatially inhomogeneous, rapid acceleration regimes pave the way for a rich phenomenology. We discuss the scalings obtained, their interpretation, and we show that the findings compare satisfactorily with existing numerical results.

Si/SiGe/SiGe:C/SiGe/Si heterostructures are investigated by Raman spectroscopy, electroreflectance method, and secondary-neutral mass spectrometry. It is shown that doping of a SiGe layer lying between undoped SiGe layers with C (1.5%) leads to almost complete stress relaxation in the doped layer. It is found that high-temperature photon annealing is responsible for a partial stress relaxation in the lower SiGe buffer layer. However, such annealing increases the Si content in this layer. Low-temperature treatment in the radio-frequency (RF) hydrogen plasma leads to considerable stress relaxation in the lower buffer layer without varying its composition. The results obtained from the electroreflectance and secondary-neutral-mall spectra correlate with the Raman spectroscopy data.

In proton exchange membrane fuel cells, the utilization of supporting materials for Pt-based catalysts is an effective approach to improve the oxygen reduction reaction (ORR) performance. In this study, a doped porous carbon (DPC) as a support was prepared with polyaniline as the main carbon source, iron nitrate nonahydrate as a metal doping source, and dicyandiamide as a nitrogen doping source in the presence of 20 nm silica bead as a template. The carbon support showed fairly high specific surface area (~ 740 m2 g−1), well-defined pore structure, and a proper nitrogen doping (~ 4.9 at.%). The Pt cathode catalyst deposited on the doped porous carbon using an NaBH4 reduction method (Pt/DPC) exhibited significantly improved ORR performance in the half and single cells, compared with the conventional carbon-supported Pt catalyst. The superior ORR stability of Pt/DPC may be as a result of the particular interaction between Pt catalyst and DPC support as a doped carbon.

Design of half-reentrant SRF cavities

Densely pillared holey-graphene block with high-level nitrogen doping enabling ultra-high volumetric capacity for lithium ion storage