ALBERT

Description: Ever since the United States space program started some forty years ago, there have been many ideas on how the U.S. should proceed to explore space. Throughout the years, many innovative designs have surfaced for transfer vehicles, space stations, and surface bases. Usually the difference in designs are due to differences in mission objectives and requirements. The problem for Mars is how to choose an architecture for human travel to Mars and what kind of base construction to design for Mars that will be reliable and cost effective. Eventually, if the Space Exploration Initiative is to become a reality, NASA will have to select and fund a single mission architecture involving manned and unmanned Mars fly-by precursors, a Mars landing vehicle, and, ultimately, the plan for constructing a Mars base. The decision to commit to a single architecture is a vital one and, therefore, the design issues, the decision making process, and the analysis tools must be available to explore all of the options that are available. A large part of any space mission architecture is the Earth-to-Mars transfer vehicle. The decision on the type of transfer vehicle to design is a crucial one. The many options must take into account the constraints encountered when assembling the vehicle in earth orbit such as effective joining methods, test and evaluation methods, preventative maintenance measures, etc. Therefore, the process of trading off various designs must include every facet of that design. The on-orbit assembly/construction constraints will drive designs and architectures. This viewgraph presentation highlights the above critical issues so that designs may be evaluated from these viewpoints. Evaluating designs from the issues contained in this paper will help decision makers detect inadequate designs. Stressing these issues in the evaluation procedure will have a great impact on the decisions of future space mission transfer vehicles and consequent architectures.

Description: Propagation and damping of ion-acoustic waves have been investigated in a Q-machine plasma consisting of K(+) positive ions, SF6(-) negative ions, and electrons. The phase velocity of the ion-acoustic 'fast' mode increases with increasing epsilon, the concentration of negative ions. The wave damping decreases with increasing epsilon, and nearly disappears, for the highest wave frequencies investigated, when epsilon is more than about 0.9. Both results are in agreement with predictions from Vlasov theory.

Description: Large-amplitude (less than about 100 percent) relaxation oscillations in the plasma potential are known to be generated when the cold endplate of a single-ended Q machine is biased positively. These oscillations are associated with double layers that form near the hot plate (plasma source) and travel toward the endplate at about the ion-acoustic velocity. At the endplate they dissolve and then form again near the hot plate, the entire process repeating itself in a regular manner. By admitting a sufficient amount of neutral gas into the system, the moving double layers were slowed down and eventually stopped. The production of stationary double layers requires an ion source on the high-potential side of the double layers. These ions are provided by ionization of the neutral gas by electrons that are accelerated through the double layer. The dependence of the critical neutral gas pressure required for stationary double-layer formation on endplate voltage, magnetic field strength, and neutral atom mass has been examined. These results are discussed in terms of a simple model of ion production and loss, including ion losses across the magnetic field.

Description: In experiments on anode (ionization) double layers in nonuniform magnetic fields it has been noted that the magnetic field gradient seems to stabilize the double layer position. This effect is further investigated in a Q machine in which the magnetic field geometry could be varied. It is found that the position of the double layers, along the axis of the device, could be controlled by changing the magnetic geometry. This effect is accounted for in a physical model which takes into account the effect of ion reflection by the magnetic mirror force in the region of magnetic field nonuniformity. This model is also able to account for variation of the double layer position when the neutral gas pressure is varied.

Description: For a strong, stationary double layer to be maintained, the electron flux Phi(e) and the ion flux Phi(i) must satisfy the Langmuir condition Phi(e)/Phi(i) = sq rt m(i)/m(e). However, this condition alone does not determine whether or not the double layer will remain in stable equilibrium at a given position. A new stability condition is obtained, and the results of several laboratory double layer experiments are examined using this condition.