In niobium superconducting radio frequency (SRF) cavities for particle acceleration, a decrease of the quality factor at lower fields-a so-called low field Q slope or LFQS-has been a long-standing ...unexplained effect. By extending the high Q measurement techniques to ultralow fields, we discover two previously unknown features of the effect: (i) saturation at rf fields lower than E_{acc}∼0.1 MV/m; (ii) strong degradation enhancement by growing thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential nonaccelerator low-field applications of SRF cavities.
The article deals with a linear-quadratic optimal control problem governed by a parabolic equation with time-fractional derivative taken in Caputo definition. We impose pointwise constraints on state ...and control functions. The derivative in time of state function is taken in the Caputo derivative form of fractional order α ϵ (0,1). The mesh approximation is based on the using of finite-difference implicit scheme for the state equation. The Lagrange function's saddle point satisfies the saddle point problem. Uzawa-type iterative method for numerical solution is used. The results of numerical tests are presented.
We have studied the seasonal dynamics of the structural and functional characteristics of heterotrophic bacterioplankton in the mouth area of the small Il’d River (inflowing into the Rybinsk ...Reservoir) and sections of bordering water bodies—the rivers themselves and reservoirs during the growing seasons of 2007 and 2009–2011. The abundance, biomass, and production of planktonic bacteria varied greatly, reaching maximum values in the mouth area of the inflow. Statistical processing of the results confirmed the dependence of the number of bacteria on organic matter in spring and early summer and on water temperature in summer. In terms of the abundance and production of bacterioplankton, the mouth area of the inflow is characterized by the manifestation of an edge effect, most pronounced in its frontal zone.
The spatial and seasonal distribution of heterotrophic bacterioplankton has been studied in Lake Pleshcheyevo. Over the past 30 years, the abundance of microorganisms has increased threefold. The ...maximum values have been recorded at littoral stations. The main part of bacterial production is also produced there. Trophic levels have increased, but the ecosystem of the lake withstands the increased load.
The 7.8 eV nuclear isomer transition in 229thorium has been suggested as a clock transition in a new type of optical frequency standard. Here we discuss the construction of a 'solid-state nuclear ...clock' from thorium nuclei implanted into single crystals transparent in the vacuum ultraviolet range. We investigate crystal-induced line shifts and broadening effects for the specific system of calcium fluoride. At liquid nitrogen temperatures, the clock performance will be limited by decoherence due to magnetic coupling of the thorium nuclei to neighboring nuclear moments, ruling out the commonly used Rabi or Ramsey interrogation schemes. We propose clock stabilization based on a fluorescence spectroscopy method and present optimized operation parameters. Taking advantage of the large number of quantum oscillators under continuous interrogation, a fractional instability level of 10−19 might be reached within the solid-state approach.
Nb3Sn is a promising next-generation material for superconducting radiofrequency cavities, with significant potential for both large scale and compact accelerator applications. However, so far, Nb3Sn ...cavities have been limited to continuous wave accelerating fields <18 MV m−1. In this paper, new results are presented with significantly higher fields, as high as 24 MV m−1 in single cell cavities. Results are also presented from the first ever Nb3Sn-coated 1.3 GHz 9-cell cavity, a full-scale demonstration on the cavity type used in production for the European XFEL and LCLS-II. Results are presented together with heat dissipation curves to emphasize the potential for industrial accelerator applications using cryocooler-based cooling systems. The cavities studied have an atypical shiny visual appearance, and microscopy studies of witness samples reveal significantly reduced surface roughness and smaller film thickness compared to typical Nb3Sn films for superconducting cavities. Possible mechanisms for increased maximum field are discussed as well as implications for physics of RF superconductivity in the low coherence length regime. Outlook for continued development is presented.
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