Ballistic and Diffuse Electron Transport in Nanocontacts of Magnetics

The transition from the ballistic electron transport to the diffuse one is experimentally observed in the study of the magnetic phase transition in Ni nanocontacts with different sizes. It is shown that the voltage $U_C$ needed for Joule heating of the near-contact region to the critical temperature does not depend on the contact size only in the diffuse mode. For the ballistic contact it increases with decrease in the nanocontact size. The reduction of the transport electron mean free path due to heating of NCs may result in change of the electron transport mode from ballistic to diffusive one.Comment: 7 pages, 2 figures accepted for the publication in JETPL (http://www.jetpletters.ac.ru). Will be published on 25 april 201

General method for prediction of thermal conductivity for well-characterized hydrocarbon mixtures and fuels up to extreme conditions using entropy scaling

A general and efficient technique is developed to predict the thermal conductivity of well-characterized hydrocarbon mixtures, rocket propellant (RP) fuels, and jet fuels up to high temperatures and high pressures (HTHP). The technique is based upon entropy scaling using the group contribution method coupled with the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state. The mixture number averaged molecular weight and hydrogen to carbon ratio are used to define a single pseudo-component to represent the compounds in a well-characterized hydrocarbon mixture or fuel. With these two input parameters, thermal conductivity predictions are less accurate when the mixture contains significant amounts of iso-alkanes, but the predictions improve when a single thermal conductivity data point at a reference condition is used to fit one model parameter. For eleven binary mixtures and three ternary mixtures at conditions from 288 to 360 K and up to 4,500 bar, thermal conductivities are predicted with mean absolute percent deviations (MAPDs) of 16.0 and 3.0% using the two-parameter and three-parameter models, respectively. Thermal conductivities are predicted for three RP fuels and three jet fuels at conditions from 293 to 598 K and up to 700 bar with MAPDs of 14.3 and 2.0% using the two-parameter and three-parameter models, respectively

Effect of Composition, Temperature, and Pressure on the Viscosities and Densities of Three Diesel Fuels

In this work, a Rolling-Ball Viscometer/Densimeter is used to measure high-pressure, hightemperature (HPHT) density and viscosity data from 298.2 to 532.6 K and pressures up to 300.0 MPa for three different diesel fuels. The densities and viscosities have combined expanded uncertainties of 0.6% and 2.5%, respectively, with a coverage factor, k = 2. Two of the diesels, Highly Paraffinic (HPF) and Highly Aromatic (HAR), contain a larger paraffinic and aromatic content relative to the others, and are standard engine test fuels. The third is a Ultra-Low Sulfur Diesel (ULSD) that resembles an unfinished commercial diesel. Detailed compositional information is also reported for each diesel that provides a basis for interpreting the impact of composition on density and viscosity at high pressures. Both density and viscosity data are correlated to Tait-type equations with uncertainties of 0.6% and 4.0%, respectively. The Tait equations provide a facile means to compare observed differences in the density-pressure and viscosity-pressure profiles of the three different diesels. Density data are modeled with the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state (EoS) with pure component parameters calculated representing diesel as a single, pseudo-component only requiring average molecular weight (Mave) and hydrogen to carbon ratio (RH/C) as inputs. Viscosity data are modeled reasonably well using entropy scaling coupled with the PC-SAFT EoS and information on the diesel Mave and RH/C. The HPHT viscosity data are also modeled reasonably well with Free Volume Theory (FVT) with model parameters correlated to Mave and RH/C

The Next Linear Collider machine protection system

The Next Linear Collider (NLC) electron and positron beams are capable of damaging the linac accelerating structure and beamline vacuum chambers during an individual aberrant accelerator pulse. Machine protection system (MPS) considerations, outlined in this paper, have an impact on the engineering and design of most machine components downstream of the damping ring injector complex. The MPS consists of two functional levels. The first is a system that provides a benign, single bunch, low intensity, high emittance beam that will be used for commissioning and at any time that the integrity or the settings of the downstream component are in doubt. This level also provides for the smooth transition back and forth between high power operation and the benign diagnostic pilot bunch operation. The pilot bunch parameters in the main linac are estimated on the basis of the expected stress in the accelerator structure copper. Beam tests have been done at the SLAC linac to examine the behaviour of the copper at the damage stress threshold. Typical pilot beam parameters (compared with nominal) are: 10 times reduced intensity, 10 times increased horizontal emittance and 1000 times increased vertical emittance, resulting in a reduction in charge density of 105. The second level is the primary protection against a single aberrant pulse. It’s goal is to reduce the possibility that a substantial transverse field changes the trajectory of the high power beam from one pulse to the next. All devices that could produce such a field are 1) monitored by a fast response network and 2) have deliberately slowed response times. A ‘maximum allowable interpulse difference ’ is evaluated for each such device as well as the beam trajectory monitors in each interpulse period.

Radiation Protection Studies for LCLS Tune Up Dump

The Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center is a pioneer fourth generation hard x-ray free electron laser that shall start to deliver laser pulses in 2009. Among other components of LCLS that present radiation protection concerns, the tune up dump (tdund) is of special interest because it also constitutes an issue for machine protection, as it is placed close to radiation sensitive components, like electronic devices and permanent magnets in the undulators. This paper first introduces the stopper of tdund looking at the heat load, and then it describes the shielding around the dump necessary to maintain the prompt and residual dose within design values. Next, preliminary comparisons of the magnetization loss in a dedicated on-site magnet irradiation experiment with FLUKA simulations serve to characterize the magnetic response to radiation of magnets like those of LCLS. The previous knowledge, together with the limit for the allowed demagnetization, are used to estimate the lifetime of the undulator. Further simulations provide guidelines on which lifetime can be expected for an electronic device placed at a given distance of tdund

Direct Mass Measurements in the Light Neutron-Rich Region Using a Combined Energy and Time-of-Flight Technique

Historically, masses have been determined one at a time via decay or reaction Q-value measurements. In general only after several such individual measurements could one establish a trend in the binding energies and thus develop an improved understanding of the nuclear mass surface. With the advancement of online mass spectrometers, it became possible to measure long isotopic sequences. Although only alkali elements have been extensively measured to date, this technique has proved successful in revealing sudden changes in nuclear structure and in pointing out systematic deficiencies of various mass models. In the experiment described herein, we have worked to extend mass measurement capabilities one step further with the development of a more general approach in which a whole series of nuclei can be measured simultaneously, independent of their N or Z. Below we demonstrate that direct mass measurements can be performed for fast recoiling nuclei using a combined energy and time-of-flight (i.e., M proportional to ET/sup 2/) technique