Bruce E. MacNeal

Azimuthal angular rotation (ψ̄̇) in domain walls during radial motion of bubble domains

Changes in bubble chirality are investigated using transient photography in order to determine the rotation of the azimuthal angle, either ψ or ψ, during radial motion. The critical rotation required to change chirality is determined experimentally to be near ψ=π. The Walker model, which predicts a critical rotation of π/2, cannot be used to describe radial motion. Instead, the horizontal Bloch line model, in which chirality changes occur during HBL punch‐through when ψ reaches nπ, must be used. The first seven values of the minimum step amplitude, Hn, required to produce exactly n chirality changes during a single bias step are given, within ±0.2 Oe, by Hn=[2vsH′ (γ)−1]1/2(nπ)1/2+Hs, where vs is the constant wall velocity, H′, is an effective field gradient, and Hs is an effective drag field. A 30‐nsec pause in the wall motion, observed when ψ reaches π, is thought to be due to the large effective gyrotropic field produced during punch‐through. Contrary to the HBL stacking mechanism, punch‐throug...

Wall oscillation and overdamped motion in magnetic bubble domains

Bubble‐wall oscillation, in response to a step change in bias field, is reported in four bubble garnet samples with different compositions and characteristics. The wall velocity during oscillatory motion is found to be independent of the instantaneous drive on the domain wall, but does depend on the initial drive on the wall. The experimental relationship between the wall velocity vs and the initial drive Ha is given by vs=0.86 m/sec+(2.4 m/sec Oe) Ha for expanding motion and by vs =1.6+2.6Ha for collapsing motion. The half‐period of oscillation, typically between 100 and 300 nsec, increases linearly with initial drive for low drive fields (∼1 Oe) and approaches a constant value at higher fields (≳2 Oe). Wall motion is analyzed in terms of Slonczewski’s formulation of wall mechanics and a velocity‐momentum relationship of the form ?=a+b? is found to agree with the observed motion for a=3.2 m/sec and b=1.5 m/sec. Underdamped motion is observed in two ranges of initial drive field separated by a region in w...

Erratum: Wall oscillations in the presence of in‐plane fields in magnetic bubble materials

Wall oscillations after an abrupt change in bias field are observed in parallel‐stripe domains as a function of both the magnitude and direction of an external in‐plane field. Two distinctly different types of oscillations are found. For small values of in‐plane field (Hip<10 Oe) nonlinear oscillations are observed. The wall velocity is saturated, resulting in oscillations with a characteristic triangular shape. The saturation velocity varies from 6.8 m/sec with Hip=0 to 12 m/sec with Hip=10 Oe. The half‐period of the oscillations, τ/2, increases with increasing bias pulse amplitude Ha from τ/2=50 nsec with Ha=2.0 Oe, to τ/2=140 nsec with Ha=6.0 Oe. Increases in the angle β between the in‐plane field and the domain walls cause increases in τ/2. For larger values of the in‐plane field (Hip≳30 Oe) linear oscillations are observed, in which the wall velocity changes with the instantaneous drive field Hextz, resulting in sinusoidal oscillations. The frequency of the oscillations, ν, increases with increasing ...

Exo-atmospheric telescopes for deep space optical communications

For deep space optical communications, optical telescopes located above the Earths atmosphere would have significant performance advantages over telescopes mounted on the Earths surface. Link outages due to cloud cover would be eliminated, atmospheric attenuation would be eliminated, and signal degradation due to stray light would be reduced. A study has been conducted to compare various exo-atmospheric platforms for the Earth end of the optical link. The three most promising platforms among many initially considered were selected for detailed study: satellites, free-flying airships and tethered airships. System configurations were compared that would have data rate capability comparable to a 6-m to 10-m diameter ground-mounted telescope, 100 percent line-of-sight coverage to a deep space spacecraft in the ecliptic, and at least 80 percent coverage in the event of failure of one Earth terminal. Based upon technical feasibility and readiness, life-cycle cost, performance and risk, a satellite platform is recommended. However, it is noted that airship technology may be advanced in the next decade or so to a level where airships should be reconsidered. Finally, this study provides a basis for a future study to compare systems using Earth-mounted and exo-atmospheric telescopes

Form Follows Function: A Pragmatic Approach to Access-To-Space for Space Technology Experiments

The form of access-to-space for NASA space missions is often dictated before they are designed. Typically, solicitations specify the form - i.e., the launch vehicle to be used. The investigator then tries to design a mission within the given form and cost limits. This study adopts a more pragmatic point of view: that the form of access should follow from the function of the mission. Given the function, the form of space access is then optimized. The functions of a broad range of space technology experiments and current forms of space technology experiments and current forms of space access were analyzed. Results show that there might be a more effective approach to access-to-space.

Enabling Affordable Communications for the Burgeoning Deep Space Cubesat Fleet

The low costs of development and launch, coupled with new propulsive technologies, have made cubesats increasingly popular for use in science investigations beyond geosynchronous orbit. As this deep space cubesat fleet grows in size, the challenge of trying to provide affordable communications for it grows commensurately. The mass, power, and volume constraints inherent to cubesats limit the antenna size and transmit power that they can use to close the deep space link. As a consequence, cubesats need to rely more heavily on ground antennas that are characterized by large aperture, low noise temperatures, and relatively high-power transmitters. Such antennas are not in great abundance, nor are they inexpensive to build. For this reason, NASA’s Deep Space Network has been advocating a three-pronged approach to meeting anticipated cubesat demand: development of simultaneous, shared-beam multi-spacecraft communications capabilities, development of large-antenna cross-support arrangements with other agencies and universities, and development of less uplink-intensive navigation techniques. This paper focuses on the pursuit of simultaneous, shared-beam multi-spacecraft communications capabilities. While the Multiple Spacecraft per Antenna (MSPA) technique has existed for over a decade, it has generally been limited to supporting downlink for just two in-beam spacecraft at a time. This limitation has largely been a function of the number and cost of available receivers. A relatively new technique that potentially overcomes this limitation is Opportunistic MSPA (OMSPA). Instead of relying on additional receivers, OMSPA makes use of a digital recorder at each ground station that is capable of capturing the intermediate frequency (IF) signals from every spacecraft in the antenna beam within the frequency bands of interest. When cubesat projects see one or more opportunities for their cubesat(s) to intercept the traditionally scheduled antenna beam of a “host” spacecraft, they can arrange for the cubesat(s) to transmit open loop during those opportunities. Via a secure Internet site, the cubesat mission operators can then retrieve the timeand frequency-relevant portions of the digital recording for subsequent demodulation and decoding, or subscribe to a service that does it for them. This “opportunistic” use of a host spacecraft’s ground antenna beam potentially enables cubesat projects to make use of large ground antennas for downlink without having to compete with bigger, better-funded missions for antenna time in the formal scheduling process. In so doing, it also potentially enables cubesat projects to avoid the aperture fees associated with formally scheduled downlink time – fees that factor into the “bottom-line” of competitivelybid NASA missions and that actually get charged to non-NASA missions. Taking advantage of these potential OMSPA benefits, however, will require cubesat projects to pursue mission designs that ensure at least periodic in-beam operations relative to a “host” spacecraft. In the case of a constellation of cubesats with inter-spacecraft distances that do not extend outside of the beam-width of the desired ground antenna at the given range, one cubesat can serve as the “host” and have a formally scheduled downlink while the rest of the cubesats can downlink essentially for “free” via OMSPA. Deep space cubesats, of course, will need uplink in addition to downlink. Beyond commanding, this need is driven by the use of two-way ranging and Doppler for navigation. While OMSPA may not directly facilitate uplink, it does have the potential to free up antennas for those spacecraft that periodically require formally scheduled links for commanding and two-way radio metrics. NASA is also exploring the physical feasibility of an in-beam, simultaneous multi-spacecraft uplink technique. As with OMSPA, if successful, it will require little new equipment, further enabling affordable deep space cubesat communications.

Influence of mission requirements and technology on NASA SCAN antenna asset architecture: Antenna-Sharing

Currently, NASAs Space Communications and Navigation program (SCaN) is responsible for three networks: the Space Network (SN), the Near-Earth Network (NEN), and the Deep Space Network (DSN). Though this asset architecture is consistent with current mission requirements it is difficult to predict future trends.1 2 Certain trends in mission requirements and technology development could dramatically influence SCANs asset architecture. Several of these trends are identified, and qualitative assessments of their effects on architecture are discussed. A quantitative analysis of one architecture change is presented: cross-network asset sharing. Significant opportunities exist for sharing of NEN antennas, particularly in X-band. Results show that existing 11.3-m antennas could meet the requirements of 62 cruise operation segments (Op Segs) at separation distances out to the inner planets. With X-band enhancements, the 18-m antennas could meet requirements of 107 Op Segs. S-band opportunities exist, but the numbers are not significant due to the transition away from S-band by future missions. A few opportunities also exist in the K-bands as more missions transition toward higher frequencies. Issues with frequency compatibility, physical design, scheduling and cost could limit or preclude effective sharing.