25,170 research outputs found

    Twin-spool turbopumps for ''low'' net positive suction pressure operations

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    Modified single-shaft turbopump incorporates inducer and main pump, each separately driven at different speeds through coaxial-shaft arrangement. Inducer operates at low speed for low net positive suction pressure, main pump operates at high speed to generate high pressure. This arrangement requires no external control for the inducer

    Stochastic modelling of intermittent scrape-off layer plasma fluctuations

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    Single-point measurements of fluctuations in the scrape-off layer of magnetized plasmas are generally found to be dominated by large-amplitude bursts which are associated with radial motion of blob-like structures. A stochastic model for these fluctuations is presented, with the plasma density given by a random sequence of bursts with a fixed wave form. Under very general conditions, this model predicts a parabolic relation between the skewness and kurtosis moments of the plasma fluctuations. In the case of exponentially distributed burst amplitudes and waiting times, the probability density function for the fluctuation amplitudes is shown to be a Gamma distribution with the scale parameter given by the average burst amplitude and the shape parameter given by the ratio of the burst duration and waiting times.Comment: 11 pages, 1 figur

    Histidine nutrition and genotype affect cataract development in Atlantic salmon, Salmo salar L.

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    The aim of this study was to investigate effects of dietary levels of histidine (His) and iron (Fe) on cataract development in two strains of Atlantic salmon monitored through parr-smolt transformation. Three experimental diets were fed: (i) a control diet (CD) with 110 mg kg-1 Fe and 11.7 g kg-1 His; (ii) CD supplemented with crystalline His to a level of 18 g kg-1 (HD); and (iii) HD with added iron up to 220 mg kg-1 (HID). A cross-over design, with two feeding periods was used. A 6-week freshwater (FW) period was followed by a 20-week period, of which the first three were in FW and the following 17 weeks in sea water (SW). Fish were sampled for weighing, cataract assessment and tissue analysis at five time points. Cataracts developed in all groups in SW, but scores were lower in those fed high His diets (P < 0.05). This effect was most pronounced when HD or HID was given in SW, but was also observed when these diets were given in FW only. Histidine supplementation had a positive effect on growth performance and feed conversion ratio (P < 0.05), whereas this did not occur when iron was added. Groups fed HD or HID had higher lens levels of His and N-acetyl histidine (NAH), the latter showing a marked increase post-smoltification (P < 0.05). The HD or HID groups also showed higher muscle concentrations of the His dipeptide anserine (P < 0.05). There was a strong genetic influence on cataract development in the CD groups (P < 0.001), not associated with tissue levels of His or NAH. The role of His and His-related compounds in cataractogenesis is discussed in relation to tissue buffering, osmoregulation and antioxidation

    Frustrated Heisenberg antiferromagnet on the honeycomb lattice: Spin gap and low-energy parameters

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    We use the coupled cluster method implemented to high orders of approximation to investigate the frustrated spin-12\frac{1}{2} J1J_{1}--J2J_{2}--J3J_{3} antiferromagnet on the honeycomb lattice with isotropic Heisenberg interactions of strength J1>0J_{1} > 0 between nearest-neighbor pairs, J2>0J_{2}>0 between next-nearest-neighbor pairs, and J3>0J_{3}>0 between next-next-neareast-neighbor pairs of spins. In particular, we study both the ground-state (GS) and lowest-lying triplet excited-state properties in the case J3=J2≡ÎșJ1J_{3}=J_{2} \equiv \kappa J_{1}, in the window 0≀Îș≀10 \leq \kappa \leq 1 of the frustration parameter, which includes the (tricritical) point of maximum classical frustration at Îșcl=12\kappa_{{\rm cl}} = \frac{1}{2}. We present GS results for the spin stiffness, ρs\rho_{s}, and the zero-field uniform magnetic susceptibility, χ\chi, which complement our earlier results for the GS energy per spin, E/NE/N, and staggered magnetization, MM, to yield a complete set of accurate low-energy parameters for the model. Our results all point towards a phase diagram containing two quasiclassical antiferromagnetic phases, one with N\'eel order for Îș<Îșc1\kappa < \kappa_{c_{1}}, and the other with collinear striped order for Îș>Îșc2\kappa > \kappa_{c_{2}}. The results for both χ\chi and the spin gap Δ\Delta provide compelling evidence for a quantum paramagnetic phase that is gapped over a considerable portion of the intermediate region Îșc1<Îș<Îșc2\kappa_{c_{1}} < \kappa < \kappa_{c_{2}}, especially close to the two quantum critical points at Îșc1\kappa_{c_{1}} and Îșc2\kappa_{c_{2}}. Each of our fully independent sets of results for the low-energy parameters is consistent with the values Îșc1=0.45±0.02\kappa_{c_{1}} = 0.45 \pm 0.02 and Îșc2=0.60±0.02\kappa_{c_{2}} = 0.60 \pm 0.02, and with the transition at Îșc1\kappa_{c_{1}} being of continuous (and probably of the deconfined) type and that at Îșc2\kappa_{c_{2}} being of first-order type

    Tuning Rashba and Dresselhaus spin-orbit couplings: Effects on singlet and triplet condensation with Fermi atoms

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    We investigate the pair condensation of a two-spin-component Fermi gas in the presence of both Rashba and Dresselhaus spin-orbit couplings. We calculate the condensate fraction in the BCS-BEC crossover both in two and in three dimensions by taking into account singlet and triplet pairings. These quantities are studied by varying the spin-orbit interaction from the case with the only Rashba to the equal-Rashba-Dresselhaus one. We find that, by mixing the two couplings, the singlet pairing decreases while the triplet pairing is suppressed in the BCS regime and increased in the BEC regime, both in two and three dimensions. At fixed spin-orbital strength, the greatest total condensate fraction is obtained when only one coupling (only Rashba or only Dresselhaus) is present.Comment: 9 pages, 6 figures, final versio

    Spin-1/2 Heisenberg antiferromagnet on an anisotropic kagome lattice

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    We use the coupled cluster method to study the zero-temperature properties of an extended two-dimensional Heisenberg antiferromagnet formed from spin-1/2 moments on an infinite spatially anisotropic kagome lattice of corner-sharing isosceles triangles, with nearest-neighbor bonds only. The bonds have exchange constants J1>0J_{1}>0 along two of the three lattice directions and J2≡ÎșJ1>0J_{2} \equiv \kappa J_{1} > 0 along the third. In the classical limit the ground-state (GS) phase for Îș<1/2\kappa < 1/2 has collinear ferrimagnetic (N\'{e}elâ€Č') order where the J2J_2-coupled chain spins are ferromagnetically ordered in one direction with the remaining spins aligned in the opposite direction, while for Îș>1/2\kappa > 1/2 there exists an infinite GS family of canted ferrimagnetic spin states, which are energetically degenerate. For the spin-1/2 case we find that quantum analogs of both these classical states continue to exist as stable GS phases in some regions of the anisotropy parameter Îș\kappa, namely for 0<Îș<Îșc10<\kappa<\kappa_{c_1} for the N\'{e}elâ€Č' state and for (at least part of) the region Îș>Îșc2\kappa>\kappa_{c_2} for the canted phase. However, they are now separated by a paramagnetic phase without either sort of magnetic order in the region Îșc1<Îș<Îșc2\kappa_{c_1} < \kappa < \kappa_{c_2}, which includes the isotropic kagome point Îș=1\kappa = 1 where the stable GS phase is now believed to be a topological (Z2\mathbb{Z}_2) spin liquid. Our best numerical estimates are Îșc1=0.515±0.015\kappa_{c_1} = 0.515 \pm 0.015 and Îșc2=1.82±0.03\kappa_{c_2} = 1.82 \pm 0.03
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