Observation of wave-packet propagation in the ion cyclotron range of frequencies in a tokamak plasma

Experimental observation of wave-packet propagation in the ion cyclotron range of frequencies in a tokamak plasma is reported. Studies were carried out in the Caltech Research Tokamak [Phys. Fluids 23, 614 (1980)] in a pure hydrogen plasma and in a regime where fast-wave damping was sufficiently small to permit multiple toroidal transits of the wave packet. Waves were launched by exciting a small loop antenna with a short burst of radio-frequency current and were detected with shielded magnetic probes. Probe scans revealed a large increase in wave-packet amplitude at smaller minor radii, and the packet velocity was found to be independent of radial position. Measurement of the packet transit time yielded direct information about the wave group velocity. Packet velocity was investigated as a function of the fundamental excitation frequency, plasma density, and toroidal magnetic field. Results are compared with the predictions of a cold plasma model that includes a vacuum layer at the edge

The in-vacuo torque performance of dry-lubricated ball bearings at cryogenic temperatures

The performance of dry-lubricated, angular contact ball bearings in vacuum at a temperature of 20 degrees K has been investigated, and is compared with the in-vacuo performance at room temperatures. Bearings were lubricated using dry-lubricant techniques which have been previously established for space applications involving operations at or near room temperature. Comparative tests were undertaken using three lubricants: molybdenum disulphide, lead, and PTFE. Results obtained using the three lubricants are presented

Photometric Selection of QSO Candidates From GALEX Sources

We present a catalog of 36,120 QSO candidates from the Galaxy Evolution Explorer (GALEX) Release Two (GR2) UV catalog and the USNO-A2.0 optical catalog. The selection criteria are established using known quasars from the Sloan Digital Sky Survey (SDSS). The SDSS sample is then used to assign individual probabilities to our GALEX-USNO candidates. The mean probability is ~50%, and would rise to ~65% if better morphological information than that from USNO were available to eliminate galaxies. The sample is ~40% complete for i<=19.1. Candidates are cross-identified in 2MASS, FIRST, SDSS, and XMM-Newton Slewing Survey (XMMSL1), whenever such counterparts exist. The present catalog covers the 8000 square degrees of GR2 lying above 25 degrees Galactic latitude, but can be extended to all 24,000 square degress that satisfy this criterion as new GALEX data become available.Comment: AASTeX v5.2, 31 pages, 9 figures. Accepted for publication in ApJ. Extended tables available in the online edition of the journa

Annual league tables of mortality in neonatal intensive care units: longitudinal study. International Neonatal Network and the Scottish Neonatal Consultants and Nurses Collaborative Study Group.[see comment]

OBJECTIVE: To assess whether crude league tables of mortality and league tables of risk adjusted mortality accurately reflect the performance of hospitals. DESIGN: Longitudinal study of mortality occurring in hospital. SETTING: 9 neonatal intensive care units in the United Kingdom. SUBJECTS: 2671 very low birth weight or preterm infants admitted to neonatal intensive care units between 1988 and 1994. MAIN OUTCOME MEASURES: Crude hospital mortality and hospital mortality adjusted using the clinical risk index for babies (CRIB) score. RESULTS: Hospitals had wide and overlapping confidence intervals when ranked by mortality in annual league tables; this made it impossible to discriminate between hospitals reliably. In most years there was no significant difference between hospitals, only random variation. The apparent performance of individual hospitals fluctuated substantially from year to year. CONCLUSIONS: Annual league tables are not reliable indicators of performance or best practice; they do not reflect consistent differences between hospitals. Any action prompted by the annual league tables would have been equally likely to have been beneficial, detrimental, or irrelevant. Mortality should be compared between groups of hospitals using specific criteria-such as differences in the volume of patients, staffing policy, training of staff, or aspects of clinical practice-after adjusting for risk. This will produce more reliable estimates with narrower confidence intervals, and more reliable and rapid conclusions

Interaction of a Modulated Electron Beam with a Plasma

The results of a theoretical and experimental investigation of the high-frequency interaction of an electron beam with a plasma are reported. An electron beam, modulated at a microwave frequency, passes through a uniform region of a mercury arc discharge after which it is demodulated. Exponentially growing wave amplification along the electron beam was experimentally observed for the first time at a microwave frequency equal to the plasma frequency. Approximate theories of the effects of 1) plasma-electron collision frequencies, 2) plasma-electron thermal velocities and 3) finite beam diameter, are given. In a second experiment the interaction between a modulated electron beam and a slow electrostatic wave on a plasma column has been studied. A strong interaction occurs when the velocity of the electron beam is approximately equal to the velocity of the wave and the interaction is essentially the same as that which occurs in traveling-wave amplifiers, except that here the plasma colum replaces the usual helical slow-wave circuit. The theory predicting rates of growth is presented and compared with the experimental results

Large magnetoresistance effect due to spin-injection into a non-magnetic semiconductor

A novel magnetoresistance effect, due to the injection of a spin-polarized electron current from a dilute magnetic into a non-magnetic semiconductor, is presented. The effect results from the suppression of a spin channel in the non-magnetic semiconductor and can theoretically yield a positive magnetoresistance of 100%, when the spin flip length in the non-magnetic semiconductor is sufficiently large. Experimentally, our devices exhibit up to 25% magnetoresistance.Comment: 3 figures, submitted for publicatio

Galactic X-ray Sources

Bremsstrahlung and synchrotron hypotheses considered as possible mechanisms for galactic X-ray productio

Kinematic response of the outer stellar disk to a central bar

We study, using direct orbit integrations, the kinematic response of the outer stellar disk to the presence of a central bar, as in the Milky-Way. We find that the bar's outer Lindblad resonance (OLR) causes significant perturbations of the velocity moments. With increasing velocity dispersion, the radius of these perturbations is shifted outwards, beyond the nominal position of the OLR, but also the disk becomes less responsive. If we follow Dehnen (2000) in assuming that the OLR occurs just inside the Solar circle and that the Sun lags the bar major axis by ~20 degrees, we find (1) no significant radial motion of the local standard of rest (LSR), (2) a vertex deviation of \~10 degrees and (3) a lower ratio sigma_2/sigma_1 of the principal components of the velocity- dispersion tensor than for an unperturbed disk. All of these are actually consistent with the observations of the Solar-neighbourhood kinematics. Thus it seems that at least the lowest-order deviations of the local-disk kinematics from simple expectations based on axisymmetric equilibrium can be attributed entirely to the influence of the Galactic bar.Comment: 10 pages, 8 figures, accepted for publication in A&