18,822 research outputs found
SRF Cavity Fabrication and Materials
The technological and metallurgical requirements of material for highgradient
superconducting cavities are described. High-purity niobium, as the preferred
metal for the fabrication of superconducting accelerating cavities, should meet
exact specifications. The content of interstitial impurities such as oxygen,
nitrogen, and carbon must be below 10{\mu}g/g. The hydrogen content should be
kept below 2{\mu}g/g to prevent degradation of the Q-value under certain
cool-down conditions. The material should be free of flaws (foreign material
inclusions or cracks and laminations) that can initiate a thermal breakdown.
Defects may be detected by quality control methods such as eddy current
scanning and identified by a number of special methods. Conventional and
alternative cavity fabrication methods are reviewed. Conventionally, niobium
cavities are fabricated from sheet niobium by the formation of half-cells by
deep drawing, followed by trim machining and Electron-Beam Welding (EBW). The
welding of half-cells is a delicate procedure, requiring intermediate cleaning
steps and a careful choice of weld parameters to achieve full penetration of
the joints. The equator welds are particularly critical. A challenge for a
welded construction is the tight mechanical and electrical tolerances. These
can be maintained by a combination of mechanical and radio-frequency
measurements on halfcells and by careful tracking of weld shrinkage. The
established procedure is suitable for large series production. The main aspects
of quality assurance management are mentioned. Another cavity fabrication
approach is slicing discs from the ingot and producing cavities by deep drawing
and EBW. Accelerating gradients at the level of 35-45 MV.m-1 can be achieved by
applying Electropolishing (EP) treatment....Comment: 37 pages, contribution to the CAS-CERN Accelerator School:
Superconductivity for Accelerators, Erice, Italy, 24 April - 4 May 2013,
edited by R. Baile
Measurements of Partial Reflections at 3.18 Mhz Using the CW Radar Technique
An equipment for measuring partial reflections using the FM-CW-radar principle at 3.18 MHz, installed at the Ionospheric Observatory Juliusruh of the CISTP (HHI), is described. The linear FM-chirp of 325 kHz bandwidth is Gaussian-weighted in amplitude and gives a height resolution of 1.5 km (chirp length is 0.6 sec). Preliminary results are presented for the first observation period in winter 1982/83
What has NMR taught us about stripes and inhomogeneity?
The purpose of this brief invited paper is to summarize what we have (not)
learned from NMR on stripes and inhomogeneity in La{2-x}Sr{x}CuO{4}. We explain
that the reality is far more complicated than generally accepted.Comment: Accepted for publication in the Proceedings of the LT-23 Conference
(invited
An L^2-Index Theorem for Dirac Operators on S^1 * R^3
An expression is found for the -index of a Dirac operator coupled to a
connection on a vector bundle over . Boundary
conditions for the connection are given which ensure the coupled Dirac operator
is Fredholm. Callias' index theorem is used to calculate the index when the
connection is independent of the coordinate on . An excision theorem due
to Gromov, Lawson, and Anghel reduces the index theorem to this special case.
The index formula can be expressed using the adiabatic limit of the
-invariant of a Dirac operator canonically associated to the boundary. An
application of the theorem is to count the zero modes of the Dirac operator in
the background of a caloron (periodic instanton).Comment: 14 pages, Latex, to appear in the Journal of Functional Analysi
Solubility, stability, and electrochemical studies of sulfur-sulfide solutions in organic solvents
A preliminary study of the sulfur electrode in organic solvents suggests that the system warrants further investigation for use in a low temperature (100 deg to 120 C) Na-S secondary battery. A qualitative screening was undertaken at 120 C to determine the solubilities and stabilities of Na2S and Na2S2 in representatives of many classes of organic solvents. From the screening and quantitative studies, two classes of solvents were selected for work; amides and cyclic polyalcohols. Voltammetric and Na-S cell charge discharge studies of sulfide solutions in organic solvents (e.g., N, N-dimethylformamide) at 120 C suggested that the reversibilities of the reactions on Pt or high density graphite were moderately poor. However, the sulfur electrode was indeed reducible (and oxidizable) through the range of elemental sulfur to Na2S. Reactions and mechanisms are proposed for the oxidation reduction processes occurring at the sulfur electrode
(63)Cu NQR Evidence for Spatial Variation of Hole Concentration in La(2-x)Sr(x)CuO(4)
We report experimental evidence for the spatial variation of hole
concentration x_(hole) in the high Tc superconductor La(2-x)Sr(x)CuO(4) (0.04
<= x <= 0.16) by using (63)Cu NQR for (63)Cu isotope enriched samples. We
demonstrate that the extent of the spatial variation of the local hole
concentration D(x)_(hole) is reflected on (63)1/T1 and deduce the temperature
dependence. D(x)_(hole) increases below 500 - 600K, and reaches values as large
as D(x)_(hole)/x ~ 0.5 below ~ 150K. We estimate the length scale of the
spatial variation in x_(hole) to be R_(hole) >~ 3nm from analysis of the NQR
spectrum.Comment: Accepted for publication in Physical Review Letter
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