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We present single event effects (SEE) evaluation of the Everspin Technologies 1Gb non-volatile ST-DDR4 spin-transfer torque MRAM.
A quantitative model is presented to account for the observed features of the binary pulsar PSR1953+53. A lower giant branch star is suggested to have evolved on a nuclear time scale while orbiting a neutron star, with mass transfer to the neutron star commencing when the giant attained its Roche lobe. A radius-core mass relationship was used to examined the processes that occurred after the hydrogen atmosphere was drained off to the neutron star. Tidal dissipation has circularized the orbit of the companion during mass transfer, while the orbit expanded. Accretion torques caused spin-up of the neutron star, which had a magnetic moment of no more than 3 x 10 to the 27th G/cu cm. The drained star became a degenerate dwarf with very faint luminosity, which accounts for no visual companion having been observed to date.
Transient spin-up and spin-down flows inside of spacecraft fuel tanks need to be analyzed in order to properly design spacecraft control systems. Knowledge of the characteristics of angular momentum transfer to and from the fuel is used to size the de-spin mechanism that places the spacecraft in a controllable in-orbit state. In previous studies, several analytical models of the spin-up process were developed. However, none have accurately predicted all of the flow dynamics. Several studies have also been conducted using Navier-Stokes based methods. These approaches have been much more successful at simulating the dynamic processes in a cylindrical container, but have not addressed the issue of momentum transfer. In the current study, the spin-up and spin-down of a fuel tank filled with gaseous xenon has been investigated using a three-dimensional unsteady Navier-Stokes code. Primary interests have been concentrated on the spin-up/spin-down time constants and the initial torque imparted on the system. Additional focus was given to the relationship between the dominant flow dynamics and the trends in momentum transfer. Through the simulation of both a cylindrical and a spherical tank, it was revealed that the transfer of angular momentum is nonlinear at early times and tends toward a linear pattern at later times. Further investigation suggests that the nonlinear spin up is controlled by the turbulent transport of momentum, while the linear phase is controlled by a Coriolis driven (Ekman) flow along the outer wall. These results indicate that the spinup and spin-down processes occur more quickly in tanks with curved surfaces than those with defined top, bottom, and side walls. The results also provide insights for the design of spacecraft de-spin mechanisms.
Three high-mass X-ray binaries have been discovered recently exhibiting enormous spin-up rates. Conventional accretion theory predicts extremely high-surface dipolar magnetic fields that we believe are unphysical. Instead, we propose quite the opposite scenario; some of these pulsars exhibit weak magnetic fields, so much so that their magnetospheres are crushed by the weight of inflowing matter. The enormous spin-up rate is achieved before inflowing matter reaches the pulsar's surface as the penetrating inner disc transfers its excess angular momentum to the receding magnetosphere, which, in turn, applies a powerful spin-up torque to the pulsar. This mechanism also works in reverse; it spins a pulsar down when the magnetosphere expands beyond corotation and finds itself rotating faster than the accretion disc, which then exerts a powerful retarding torque to the magnetic field and to the pulsar itself. The above scenaria cannot be accommodated within the context of neutron-star accretion processes occurring near spin equilibrium, thus they constitute a step towards a new theory of extreme (far from equilibrium) accretion phenomena.
We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.
Over 5 yr of hard X-ray (20-60 keV) monitoring of the 7.66 s accretion-powered pulsar 4U 1626-67 with the Compton Gamma Ray Observatory/BATSE large-area detectors has revealed that the neutron star is now steadily spinning down, in marked contrast to the steady spin-up and spin-down torques differ by only 15% with the neutron star spin changing on a timescale |v/v| approximately equals 5000 yr in both states. The current spin-down rate is itself decreasing on a timescale |v/v| approximately equals 26 yr. The long-term timing history shows small-amplitude variations on a 4000 day timescale, which are probably due to variations in the mass transfer rate. The pulsed 20-60 keV emission from 4U 1626-67 is well-fitted by a power-law spectrum with photon index gamma = 4.9 and a typical pulsed intensity of 1.5 x 10(exp -10) ergs cm (exp -2)s(exp -1). The low count rates with BATSE prohibited us from constraining the reported 42 minute binary orbit, but we can rule out long-period orbits in the range 2 days < or = P(orb) < or = 900 days. We compare the long-term torque behavior of 4U 1626-67 to other disk-fed accreting pulsars and discuss the implications of our results for the various theories of magnetic accretion torques. The abrupt change in the sign of the torque is difficult to reconcile with the extremely smooth spin-down now observed. The strength of the torque noise in 4U 1626-67, approximately 10(exp -22) Hz(exp 2)s(exp -2) Hz(exp -1), is the smallest ever measured for an accreting X-ray pulsar, and it is comparable to the timing noise seen in young radio pulsars. We close by pointing out that the core temperature and external torque (the two parameters potentially relevant to internal sources of timing noise) of an accreting neutron star are also comparable to those of young radio pulsars.
Mounting observational evidence and theoretical results suggest that most pulsing X-ray sources, including those with massive companions, are disk fed when significant mass transfer is taking place. Regardless of whether or not substantial braking has occurred before the onset of disk accretion, once it begins the resulting strong braking torque on fast rotators is sufficient to spin-down a neutron star from an initial period approx. 1 sec to a period approx. greater than 100 sec in a time short compared to the main-sequence lifetime of a massive companion and even, in many cases, in a time short compared to the star's lifetime as a bright X-ray source. It is argued that sources with longer periods have larger magnetic moments, or lower mean luminosities, or both, whereas sources with shorter periods have smaller magnetic moments, or higher mean luminosities, or both.
Techniques and conceptual devices for carrying out detumbling operations are examined, and progress in the development of these concepts is discussed. Devices which reduce tumble to simple spin through active linear motion of a small mass are described, together with a Module for Automatic Dock and Detumble (MADD) that could perform an orbital transfer from the shuttle in order to track and dock at a preselected point on the distressed craft. Once docked, MADD could apply torques by firing thrustors to detumble the passive vehicle. Optimum combinations of mass-motion and external devices for various situation should be developed. The need for completely formulating the automatic control logic of MADD is also emphasized.
An analysis of the accretion process in an X-ray pulsar, whereby angular momentum is transferred to the star and its rotation period is changed, is presented, and an expression for the fractional rate of change of the pulse period in terms of X-ray luminosity and other star parameters is derived. It is shown that observed characteristic spin-up time scales for seven X-ray pulsars strongly support the view that in every source (1) the pulse period reflects the rotation period of a compact object, (2) the accretion is mediated by a disk surrounding the compact object and rotating in the same sense, and (3) the compact object is a neutron star rather than a white dwarf.
Some of the challenges associated with developing electric aircraft propulsion systems include developing powertrain components that are both efficient and light-weight. In particular, electric motors must simultaneously achieve high efficiency by minimizing electrical and mechanical losses while also achieving high specific power by increasing the torque and/or speed. Normally increasing torque or speed will increase electrical and mechanical losses. The High Efficiency Megawatt Machine (HEMM) minimizes electrical losses by incorporating a superconductor to enable increased current on the rotor. And the rotor spins in a vacuum to minimize thermal and mechanical losses. Some organizations have been developing superconducting rotors for similar reasons using either cryogenic fluid transfer systems, fully immersed cryogenic cooling, and in a few cases utilized built-in cryogenic cooling on the rotor using a Brayton or Stirling system but the implementation was too large or inefficient for effective motor integration. Instead, a new approach for cryogenically cooling the superconducting rotor coil with an embedded rotating cryocooler is presented that fits completely within the rotating shaft.
Some of the challenges associated with developing electric aircraft propulsion systems include developing powertrain components that are both efficient and light-weight. In particular, electric motors must simultaneously achieve high efficiency by minimizing electrical and mechanical losses while also achieving high specific power by increasing the torque and/or speed. Normally increasing torque or speed will increase electrical and mechanical losses. The High Efficiency Megawatt Machine (HEMM) minimizes electrical losses by incorporating a superconductor to enable increased current on the rotor. And the rotor spins in a vacuum to minimize thermal and mechanical losses. Some organizations have been developing superconducting rotors for similar reasons using either cryogenic fluid transfer systems, fully immersed cryogenic cooling, and in a few cases utilized built-in cryogenic cooling on the rotor using a Brayton or Stirling system but the implementation was too large or inefficient for effective motor integration. Instead, a new approach for cryogenically cooling the superconducting rotor coil with an embedded rotating cryocooler is presented that fits completely within the rotating shaft.
The pulse-timing projects supported by NASA Grant NAG8-695 were motivated in large part by our long-standing interest in the physics of rotating neutron stars and particularly the variations in rotation due to internal and external fluctuating torques. For accretion-powered pulsars, observed as compact galactic X-ray sources, our work has been motivated by questions regarding the physics of matter accretion, mass transfer, and mass loss in these X-ray binary systems. For rotation-powered pulsars using X-ray, optical, and radio observations, we have examined the internal structure of neutron stars and mechanisms for secular spin-down. These issues are still central to the continuing process of understanding the complex behavior of these fascinating systems. The work supported by this grant is based mainly on Ginga observations of three pulsating X-ray sources. We observed two of these sources (Her X-1 and SMC X-1) within the framework of a NASA-ISAS program for U.S.-Japan collaborations to obtain and analyze Ginga observations. In addition, we joined with members of the Ginga team to apply our pulse-timing methods to PSR 0540-69 data obtained as a collateral benefit of the regular monitoring by Ginga of SN 1987A. In addition to these Ginga observations, there exist other relevant data on all three of these sources. The direction of our investigations has been affected by the necessity of including these supporting data in our analyses, and setting up the necessary collaborations has sometimes entailed extra work that was not anticipated in our proposals. Indeed, we have often taken the initiative in establishing these joint projects.
Magnetic activity cycles of a convective star in a close binary system may lead to orbit period changes through tidal spin-orbit coupling. An increase in the mean magnetic field throughout the convection zone provides an additional pressure support and increases the star's moment of inertia. In order for the system to conserve angular momentum instantaneously, the convective star must reduce its rotation rate. When this happens, the system is no longer synchronous; in particular, the equilibrium tide now suffers a phase lag. A tidal torque must act, transferring angular momentum from the star's rotation to the orbit, lengthening the period. When the field decays, these events reverse and the period shortens. Observed period changes in RS CVn-like systems are of the order delta p/p = 10 to the -6th, implying variations in the mean field strength at the base of the convection zone of approximately 1000 G over time scales of 10-30 yr. It is predicted that the strength of magnetic activity indicators will track orbital period changes in a given system: the period will be largest during the active portion of the magnetic cycle and smallest during the quiescent part.
Recent advances in the theory of accreting magnetic white dwarf stars are described and several of the unresolved issues concerning them are discussed. Recently reported Einstein satellite observations are examined and the possibility is considered that the 'soft X-ray puzzle' in the AM Her stars may have disappeared. Based on these observations, the evidence is considered that the region emitting the polarized optical light is separate from the X-ray emission region. Among the theoretical advances described are extensive new calculations of the X-ray emission from accreting magnetic white dwarfs, including soft and hard X-ray pulse profiles and time-dependent behavior; improvements in the treatment of cyclotron emission and the transfer of radiation in magnetoactive plasmas, with application to the polarized optical light from the AM Her stars; new ideas about the stream-magnetosphere interaction region that produces the forest of strong emission lines in the AM Her stars; explanation of the synchronization of the white dwarf in the AM Her stars in terms of an MHD torque; and determination of the magnetic fields in the AM Her and DQ Her stars through analysis of their spin-up behavior.
This paper describes an approach based on using both bias momentum and multiple control moment gyros for controlling the attitude of statically unstable thrust-levitated vehicles in hover or slow translation. The stabilization approach described in this paper uses these internal angular momentum transfer devices for stability, augmented by thrust vectoring for trim and other outer loop control functions, including CMG stabilization/ desaturation under persistent external disturbances. Simulation results show the feasibility of (1) improved vehicle performance beyond bias momentum assisted vector thrusting control, and (2) using control moment gyros to significantly reduce the external torque required from the vector thrusting machinery.
The Galileo scan platform is controlled in two degrees of freedom. A clock (Spin Bearing) actuator controls the relative position between the rotor and stator, and a cone actuator controls the position between the stator and the platform. Instruments on the platform are required to point to 140 micro-rad accuracy and 50 micro-rad per second stability. The system identification objectives were to identify dominant structural resonance frequencies, mode shapes, and damping ratio which exist in the transfer function between the clock actuator and the gyro sensor; and position the notch filter to limit undesirable actuator torque output to ensure stability and performance.
An oven-controlled crystal oscillator at 16.368 MHz provides clock signals to all GP-B components and synchronizes the data collection, transmission and processing. The sampled science data signals are stamped with the vehicle time, a counter of the 10Hz data strobe divided down from the clock. The GPS receiver supplies an external reference for time transfer between the vehicle time and coordinated universal time. Ground and space flight tests show the time transfer error is within 1 microsecond. The time latency between the effective sample time of science signals and the stamped vehicle time is verified to 1 ms in the ground tests. The GP-B satellite is controlled to roll with a period of 77.5 sec about an axis along the direction to the guide star to average out the disturbance torques fixed to the body of the satellite and reduce the gyroscope readout noise. The roll phase is determined on the ground to high accuracy with the telemetry data from two star trackers and used in the data analysis to separate the drifts of gyroscope spin axes in the orbital plane and perpendicular to the orbital plane. The flight data shows that the roll phase is controlled to within 40 arcsec with a measurement uncertainty of 7 arcsec.
A parachute swivel mechanism (PSM) for planetary entry missions such as a Mars probe (MARSNET) or return of cometary material samples (ROSETTA mission) has been developed. The purpose of the PSM is to decouple the spin of the probe from the parachute, with low friction torque, during both the deployment and descent phases. Critical requirements are high shock loads, low friction, low temperatures, and several years of storage in the deep space environment (during the cruise phase of the probe, prior to operation). The design uses a main thrust ball bearing to cope with the load requirement and a smaller thrust ball bearing for guiding of the shaft. Except for use on the Viking and Galileo swivels, it appears that this type of bearing has very rarely been employed in space mechanisms, so that little is known of its friction behavior with dry lubrication. A slip ring assembly allows the transfer of electrical power for post-reefing of the parachute. A test program has been conducted covering the environmental conditions of Mars entry and Earth reentry. This paper describes requirement constraints, model missions of planetary entries, a bearing trade-off, analyses performed, design details, the lubrication system, and test results (friction torque versus load/spin rate). In addition, the design of the test rig is addressed.