Search results for "Helium"
showing 10 items of 1689 documents
Neutronic and photonic analysis of the single box water-cooled lithium lead blanket for a DEMO reactor
1998
Abstract The water-cooled Pb–17Li demonstration plant (DEMO) breeding blanket line was selected in 1995 as one of the two EU lines to be further developed in the next decade. In this paper the results of a neutronic and photonic analysis of the `single box' concept is presented. A full three-dimensional model, including the whole assembly and many of the DEMO reactor components, has been developed, together with a three-dimensional neutron source. A tritium breeding ratio (TBR) value of 1.16, with no ports and a Li6 enrichment of 90%, has been obtained and a further analysis has been performed to determine Li6 enrichment that would still ensure tritium breeding self-sufficiency. Selected po…
Hydrogen and helium films as model systems of wetting
1997
Optical experiments on the wetting properties of liquid 4He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non-wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films are droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Ou…
Theory of hydrogen and helium impurities in metals
1984
A powerful computational scheme is presented for calculating the static properties of light interstitials in metallic hosts. The method entails (i) the construction of the potential-energy field using the quasiatom concept, (ii) the wave-mechanical solution of the impurity distribution ("zero-point motion"), (iii) calculation of the forces exerted on the adjacent host atoms and their displacements, and (iv) iteration to self-consistency. We investigate self-trapping phenomena in bcc and fcc metals in detail, and calculate both the ground and low-lying excited states. Implications of the wave-mechanical or band picture to diffusion mechanisms and inelastic scattering experiments are discusse…
Production of zero energy radioactive beams through extraction across superfluid helium surface
2003
A radioactive Ra-223 source was immersed in superfluid helium at 1.2-1.7 K. Electric fields transported recoiled Rn-219 ions in the form of snowballs to the surface and further extracted them across the surface. The ions were focussed onto an aluminium foil and alpha particle spectra were taken with a surface barrier spectrometer. This enabled us to determine the efficiency for each process unambiguously. The pulsed second sound wave proved effective in enhancing the extraction of positive ions from the surface. Thus we offer a novel method for study of impurities in superfluid helium and propose this method for production of zero energy nuclear beams for use at radioactive ion beam facilit…
Hydrogen- and helium-implanted silicon: Low-temperature positron-lifetime studies
1991
High-purity single-crystal samples of float-zoned Si have been implanted with 6.95-MeV protons and with 25-MeV {sup 3}He{sup 2} ions at 15 K, and the positron-lifetime technique has been used to identify the defects created in the samples, and to study the effects of H and He on the annealing of point defects in Si. The results have been compared with those of proton-irradiated Si. A 100--300-K annealing stage was clearly observed in hydrogen (H{sup +}) -implanted Si, and this stage was almost identical to that in the {ital p}-irradiated Si. The final annealing state of the H{sup +}-implanted Si started at about 400 K, and it is connected to annealing out of negatively charged divacancy-oxy…
Extraction of radioactive positive ions across the surface of superfluid helium: A new method to produce cold radioactive nuclear beams
2003
Alpha-decay recoils 219Rn were stopped in superfluid helium and positive ions were extracted by electric field into the vapour phase. This first quantitative observation of extraction was successfully conducted using highly sensitive radioactivity detection. The efficiency for extraction across the liquid surface was 23 ± 4% at 1.60 K, the release time was 90 ± 10 ms at 1.50 K and the barrier for positive ions through a free superfluid-helium surface was 19.4 ± 4.5 K. The pulsed second sound proved to be effective in enhancing the extraction.
Near-Infrared Spectra of Water Confined in Silica Hydrogels in the Temperature Interval 365−5 K
2002
We have used a sol−gel technique to obtain optically transparent hydrogels in which water is trapped within a tridimensional disordered silica matrix. A suitable aging of these hydrogels enables to have transparent noncracking samples down to cryogenic temperatures. We report the optical absorption spectra, in the near-infrared region, of water trapped in our silica hydrogels, measured in the temperature range 365−5 K, and we compare them with the same spectra of liquid water, measured in the temperature range 365−263 K. The data show that it is possible to have noncrystallizing water even at 5 K: indeed, the overtone bands at ∼1.41 μm and at ∼1.155 μmtypical of “weakly bonded” water molec…
Measurement of the velocity and attenuation of fourth sound in helium II
1979
The velocity and the attenuation of fourth sound have been measured in liquid helium at temperatures between 0.8 K and the λ point, along the vapor pressure curve. The measurements were made using the resonance technique and the helium was contained between small pores in packed powder specimens. From the velocity, it could be determined that the sound propagates under “adiabatic” conditions. According to theory, the attenuation of fourth sound consists of two contributions: surface losses due to heat exchange with the resonator body and volume losses due to dissipative processes associated with the viscosity coefficients η and ζ3. The results of our attenuation measurements are in agreemen…
Spherical fused silica cells filled with pure helium for nuclear magnetic resonance-magnetometry.
2016
High magnetic fields (> 1 T) are measured by NMR magnetometers with un-rivaled precision if the precessing spin sample provides long coherence times. The longest coherence times are found in diluted ${}^{3}$He samples, which can be hyperpolarized for sufficient signal strength. In order to have minimal influence on the homogeneity and value of the measured magnetic field the optimal container for the ${}^{3}$He should be a perfect sphere. A fused silica sphere with an inner diameter of 8 mm and an outer diameter of 12 mm was made from two hemispheres by diffusion bonding leaving only a small hole for cleaning and evacuation. This hole was closed in vacuum by a CO${}_{2}$ laser and the inner…
Modeling of ITER TF cooling system through 2D thermal analyses and enthalpy balance
2017
Abstract The winding pack of the ITER Toroidal Field (TF) coils is composed of 134 turns of Nb3Sn Cable in Conduit Conductor (CICCs) wound in 7 double pancakes and cooled by supercritical helium (He) at cryogenic temperature. The cooling of the Stainless Steel (SS) case supporting the winding pack is guaranteed by He circulation in 74 parallel channels. A 2D approach to compute the temperature distribution in the ITER TF winding pack is here proposed. The TF is divided in 32 poloidal segments, for each segment the corresponding 2D model is built and a thermal analysis is performed applying the corresponding nuclear heating computed with MCNP code considering the latest design updates, such …