Metrology/viewing system for next generation fusion reactors

Next generation fusion reactors require accurate measuring systems to verify sub-millimeter alignment of plasma-facing components in the reactor vessel. A metrology system capable of achieving such accuracy must be compatible with the vessel environment of high gamma radiation, high vacuum, elevated temperature, and magnetic field. This environment requires that the system must be remotely deployed. A coherent, frequency modulated laser radar system is being integrated with a remotely operated deployment system to meet these requirements. The metrology/viewing system consists of a compact laser transceiver optics module which is linked through fiber optics to the laser source and imaging units that are located outside of the harsh environment. The deployment mechanism is a telescopic-mast positioning system. This paper identifies the requirements for the International Thermonuclear Experimental Reactor metrology and viewing system, and describes a remotely operated precision ranging and surface mapping system

Frustration-Induced Two Dimensional Quantum Disordered Phase in Piperazinium Hexachlorodicuprate

Piperazinium Hexachlorodicuprate (PHCC) is shown to be a frustrated quasi-two-dimensional quantum Heisenberg antiferromagnet with a gapped spectrum. Zero-field inelastic neutron scattering and susceptibility and specific heat measurements as a function of applied magnetic field are presented. At T = 1.5 K, the magnetic excitation spectrum is dominated by a single propagating mode with a gap, Delta = 1 meV, and bandwidth of approximately 1.8 meV in the (h0l) plane. The mode has no dispersion along the b* direction indicating that neighboring a-c planes of the triclinic structure are magnetically decoupled. The heat capacity shows a reduction of the gap as a function of applied magnetic field in agreement with a singlet-triplet excitation spectrum. A field-induced ordered phase is observed in heat capacity and magnetic susceptibility measurements for magnetic fields greater than H_c1 approximately equal to 7.5 Tesla. Analysis of the neutron scattering data reveals the important exchange interactions and indicates that some of these are highly frustrated.Comment: 13 pages with 14 figures, 7 pages of text, 6 pages of figures. Submitted to Phys. Rev. B 4/7/2001. email comments to [email protected] or [email protected]

Unconventional ferromagnetic and spin-glass states of the reentrant spin glass Fe0.7Al0.3

Spin excitations of single crystal Fe0.7Al0.3 were investigated over a wide range in energy and reciprocal space with inelastic neutron scattering. In the ferromagnetic phase, propagating spin wave modes become paramagnon-like diffusive modes beyond a critical wave vector q0, indicating substantial disorder in the long-range ordered state. In the spin glass phase, spin dynamics is strongly q-dependent, suggesting remnant short-range spin correlations. Quantitative model for S(energy,q) in the ``ferromagnetic'' phase is determined.Comment: 4 pages, 5 figure

Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties

We revisited the problem of the stability of the superconducting state in RbxWO3 and identified the main causes of the contradictory data previously published. We have shown that the ordering of the Rb vacancies in the nonstoichiometric compounds have a major detrimental effect on the superconducting temperature Tc.The order-disorder transition is first order only near x = 0.25, where it cannot be quenched effectively and Tc is reduced below 1K. We found that the high Tc's which were sometimes deduced from resistivity measurements, and attributed to compounds with .25 < x < .30, are to be ascribed to interfacial superconductivity which generates spectacular non-linear effects. We also clarified the effect of acid etching and set more precisely the low-rubidium-content boundary of the hexagonal phase.This work makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we approach this boundary (x = 0.20), if no ordering would take place - as its is approximately the case in CsxWO3. This behaviour is reminiscent of the tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism is responsible for this large increase of Tc despite the considerable associated reduction of the electron density of state ? By reviewing the other available data on these bronzes we conclude that the theoretical models which are able to answer this question are probably those where the instability of the lattice plays a major role and, particularly, the model which call upon local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review

Less than 50% sublattice polarization in an insulating S=3/2 kagome' antiferromagnet at low T

We have found weak long range antiferromagnetic order in the quasi-two-dimensional insulating oxide $KCr_3(OD)_6(SO_4)_2$ which contains Cr$^{3+}$ S=3/2 ions on a kagom\'{e} lattice. In a sample with $\approx$ 76% occupancy of the chromium sites the ordered moment is 1.1(3)$\mu_B$ per chromium ion which is only one third of the N\'{e}el value $g\mu_BS=3\mu_B$. The magnetic unit cell equals the chemical unit cell, a situation which is favored by inter-plane interactions. Gapless quantum spin-fluctuations ($\Delta/k_B >$T_N$= 1.6K are the dominant contribution to the spin correlation function,$S(Q,\omega)$ in the ordered phase.Comment: 18 pages, RevTex/Latex, with 6 figure

Spin Pseudo Gap in La2-xSrxCuO4 Studied by Neutron Scattering

Spin excitations of La2-xSrxCuO have been studied using inelastic neutron scattering techniques in the energy range of 2 meV =< w =< 12 meV and the temperature range of 8 K =< T =< 150 K. We observed a signature of a spin pseudo gap in the excitation spectrum above Tc for the slightly overdoped sample with x = 0.18. On heating, the spin pseudo gap gradually collapses between T = 80 K and 150 K. For the x = 0.15 and 0.20, although the visibility of gap-like structure at T ~ Tc is lower compared to the x = 0.18 sample, the broad bump of kai"(w) appears at w ~ 5 meV,close to the spin-gap energy at base temperature, suggests the existence of the spin pseudo gap in the normal state.Comment: revtex, 7 pages, 8 eps figures, PRB (2003) in pres

Frustrated 3-Dimensional Quantum Spin Liquid in CuHpCl

Inelastic neutron scattering measurements are reported for the quantum antiferromagnetic material Cu_2(C_5H_12N_2)_2Cl_4 (CuHpCl). The magnetic excitation spectrum forms a band extending from 0.9 meV to 1.4 meV. The spectrum contains two modes that disperse throughout the a-c plane of the monoclinic unit cell with less dispersion along the unique b-axis. Simple arguments based on the measured dispersion relations and the crystal structure show that a spin ladder model is inappropriate for describing CuHpCl. Instead, it is proposed that hydrogen bond mediated exchange interactions between the bi-nuclear molecular units yield a three-dimensional interacting spin system with a recurrent triangular motif similar to the Shastry-Sutherland Model (SSM). Model independent analysis based on the first moment sum rule shows that at least four distinct spin pairs are strongly correlated and that two of these, including the dimer bond of the corresponding SSM, are magnetically frustrated. These results show that CuHpCl should be classified as a frustration induced three dimensional quantum spin liquid.Comment: 13 pages, 17 figures (Color) ReSubmitted to Phys. Rev. B 9/21/2001 resubmission has new content email comments to [email protected] or [email protected]

Machine learning outperforms clinical experts in classification of hip fractures

Hip fractures are a major cause of morbidity and mortality in the elderly, and incur high health and social care costs. Given projected population ageing, the number of incident hip fractures is predicted to increase globally. As fracture classification strongly determines the chosen surgical treatment, differences in fracture classification influence patient outcomes and treatment costs. We aimed to create a machine learning method for identifying and classifying hip fractures, and to compare its performance to experienced human observers. We used 3659 hip radiographs, classified by at least two expert clinicians. The machine learning method was able to classify hip fractures with 19% greater accuracy than humans, achieving overall accuracy of 92%