MSNW (United States)

An optimal magnetic shielding configuration for significantly reducing astronaut exposure to Galactic Cosmic Radiation (GCR) on long interplanetary missions has been realized, and is referred to as the Magnetospheric Dipolar Toroidal Magnetosphere (DTM). This configuration was shown to have the singular ability to deflect the vast majority of the GCR including High Z Energetic (HZE) ions. This external (to the spacecraft) dipolar field is created by an array of unidirectional toroidal High Temperature Superconductor (HTSC) windings mounted externally on the surface of the toroidally-shaped spacecraft habitat. In this way the spacecraft directly supports the magnetic hoop forces generated by the toroidal currents and thereby significantly reduces the structural mass requirements for the shield. The magnitude of the toroidal currents are arranged poloidally to flow so as to maintain the spacecraft shell as a constant flux boundary where the magnetic flux is zero thereby keeping the spacecraft habitat field-free. As the dipole magnetic field is perpendicular to the spacecraft habitat in all directions, the DTM provides a deflecting shield to all the incoming GCR which is nearly isotropic. In addition, the DTM shields the HTSC magnets as well thus eliminating the secondary particle irradiation hazard, which can dominate over the primary GCR for shields with closed magnetic topologies. With DTM shielding it was found that both the structural and magnet mass as well as power requirements were significantly reduced. A 3-D relativistic particle code was used to evaluate shielding effectiveness for the GCR spectrum encountered in space. Four topics that will be covered involve a direct comparison of the three principal efforts developed to date for shielding; They are: (1) Effectiveness of the magnetic shielding (2) Issues with secondaries; (3) Launch and space assembly, and (4) Advantages and other uses.

A major challenge for future space development is that Low Earth Orbit is becoming critically congested with space debris. Many solutions have been proffered with the most direct being active removal of the largest objects. There are many challenges encountered when considering Active Debris Removal (ADR). Due to the high ΔV requirements for multi-object deorbiting missions, conventional propellant-based systems become implausible. The ultimate goal of the concept described here is the development of an Air-breathing Planetary Orbiter (APO) spacecraft that can operate on the gases found in virtually every planetary atmosphere and is designed in such a way that it can also harvest the planetary atmospheres to obtain the fuel required for ADR as well as planetary exploration. At Earth the APO would be employed to deorbit large space debris as well as recover, repair or reposition other space assets. The APO is based on experimental results employing a robust plasma creation and sustainment method referred to as a Rotating Magnetic Field (RMF) generated Field Reversed Configuration (FRC). In experiments, high temperature plasmas (T ~ 40-200 eV) were formed and sustained in gases from Hydrogen to Argon at electrical input powers up to several megawatts. The transverse RMF fully ionized the gas, and in the process generated large azimuthal currents producing the FRC that both confined and heated the ions. An axial magnetic field isolated the plasma from the chamber wall and conducted the plasma exhaust out forming an on-axis plasma jet. In the spacecraft implementation, this process would efficiently convert the incoming atmosphere into the directed thrust and Isp needed for drag makeup. Based on the observed ion temperature, only a fraction of the gas ingested during the orbital transit would be required as the projected Isps ranged from 2.5 to 4 ksec. The burn chamber radius, which is the same as the spacecraft intake aperture, need be no larger than that employed in past RMF FRC experiments (0.4 m). As the APO is electrodeless and magnetically isolated, it is capable of long-term, high-power operation. The ability to operate long-term on caustic atmospheric gases, along with a large intake, minimal drag profile, the APO can perform the multi-target, high ΔV demands of ADR. The preliminary analysis and design of a prototype APO has been completed based on the drag, power processing, energy, and fuel storage requirements. The results of these analyses, as well as the mission scenario for the removal of a one-ton debris target are discussed.

Rapid missions to the outer planetary bodies require a large change in vehicle velocity upon arrival to establish stable orbits. The total required velocity change can be on the order of 10s of km/s, thus a large fraction of the spacecraft mass must be fuel, thereby limiting useful payload. Virtually all planetary bodies have an atmosphere sufficient to provide for deceleration from aerodrag. Employing a solid deflector for orbit insertion has been successful in the past, but due to frictional heating and unknown changes in atmospheric conditions, the braking must be distributed over many elliptical orbital passes into the atmosphere to achieve the required braking. The concept to be presented is the deployment of a coil set that produces a dipolar magnetic field that is then filled with plasma to provide the necessary impediment to atmospheric flow and thereby produce the desired drag. (see illustration below). It is referred to as a Plasma MagnetoShell (PMS). The advantages of PMS are many. Frictional heating is no longer a concern. The aeroshell is now composed of massless magnetic field and plasma. The scale of the magnetic barrier can be as large as desired (∼ 10–100 m), and requires less than a gram of plasma. While the dipole coil will the dominant mass, it is negligible compared to the spacecraft. With the ability to rapidly control the drag by changing field strength or plasma density, the larger braking force required for aerocapture is now feasible. The primary drag-inducing interactions between the magnetically confined ions and the neutral atmospheric particles are that of charge exchange and elastic scattering. During the interaction the neutral momentum is imparted to the magnetized ion which is then reacted onto the magnetic coils tethered to the spacecraft. A laboratory scale PMS capable of testing in LEO has been designed, built and tested. Test results and the conditions that must be satisfied for the PMS to provide the desired aerobraking on a large spacecraft will be presented.

The accurate, direct measurement of thrust or impulse is one of the most critical elements of electric thruster characterization, and it is one of the most difficult measurements to make. This paper summarizes recommended practices for the design, calibration, and operation of pendulum thrust stands, which are widely recognized as the best approach for measuring micronewton- to millinewton-level thrust and micronewton-per-second-level impulse bits. The fundamentals of pendulum thrust stand operation are reviewed, along with the implementation of hanging pendulum, inverted pendulum, and torsional balance configurations. The methods of calibration and recommendations for calibration processes are presented. Sources of error are identified, and methods for data processing and uncertainty analysis are discussed. This review is intended to be the first step toward a recommended practices document to help the community produce high-quality thrust measurements.

Nomenclature Bb, Bbias = magnitude of preexisting (vacuum) axial magnetic field B = amplitude of rf rotating magnetic field Bz = axial magnetic field change due to RMF driven currents (superscript M denotes maximum) Bext = magnitude of magnetic field external (radially) to the FRC  = plasma pressure normalized to external (vacuum) field C = capacitance value  = classical skin depth = (2/0) 1/2

Verification of large multithreaded programs is challenging. Automatic approaches cannot overcome the state explosion in the number of threads; semi-automatic methods require expensive human time for finding global inductive invariants. Ideally, automatic methods should not deal with the composition of the original threads and a human should not supply a global invariant. We provide such an approach. In our approach, a human supplies a specification of each thread in the program. Here he has the freedom to ignore or to use the knowledge about the other threads. The checks whether specifications of threads are sound as well as whether the composition of the specifications is error-free are handed over to the off-the-shelf verifiers. We show how to apply this divide-and-conquer approach for the interleaving semantics with shared variables communication where specifications are targeted to real-world programmers: a specification of a thread is simply another thread. The new approach extends thread-modular reasoning by relaxing the structure of the transition relation of a specification. We demonstrate the feasibility of our approach by verifying two protocols governing the teardown of important data structures in Windows device drivers.

A new experiment aims to form two field-reversed configurations (FRCs) and accelerate them into an interaction chamber in which they will collide, and undergo an adiabatic compression. Previous results obtained in a similar configuration showed that ion temperatures varied as a function of compression coil voltage, in the keV range. This paper outlines the physics of formation, acceleration, reconnection and compression of two FRCs.