The Plasmoid Thruster Experiment (PTX) operates by inductively producing plasmoids in a conical theta-pinch coil and ejecting them at high velocity. A plasmoid is a plasma with an imbedded closed magnetic fKkl structure. The shape and magnetic f d d structure of the translating pksmoids have been measured with of an array of magnetic field probes. Six sets of two B-dot probes were coostructed for measuring Bz and Be, the axial and azimuthal components of the magnetic f " .The probes are wound on a square G10 form, and have an average (calibrated) NA of 9.37 x lo-' m*, where N is the number of turns and A is the
There are a number of possible advantages to using accelerated plasmoids for in-space propulsion. A plasmoid is a compact plasma structure with an integral magnetic field. They have been studied extensively in controlled fusion research and are classified according to the relative strength of the poloidal and toroidal magnetic field (B, and Bt, respectively). An object with B, / Bt )' 1 is classified as a Field Reversed Configuration (FRC); if B, = Bt, it is called a Spheromak. The plasmoid thruster operates by producing FRC-like plasmoids and subsequently ejecting them from the device at a high velocity. The plasmoid is formed inside of a single-turn conical theta-pinch coil.As this process is inductive, there are no electrodes. Similar experiments have yielded plasmoid velocities of at least 50 km/s, and calculations indicate that velocities in excess of 100 km/s should be possible. This concept should be capable of producing Isp's in the range of 5,000 -lS,OOO s with thrust densities on the order of lo5 N/m . The current experiment is designed to produce jet powers in the range of 5 -10 kW, although the concept should be scalable to several MW's. The plasmoid mass and velocity will be measured with a variety of diagnostics, including internal and external B-dot probes, flux loops, Langmuir probes, high-speed cameras and a laser interferometer. Also of key importance will be measurements of the efficiency and mass utilization. Simulations of the plasmoid thruster using MOQUI, a time-dependent MHD code, will be carried out concurrently with experimental testing.
Ignition or thermal explosion in an oxidizing porous body of material can be described by a dimensionless reaction-diffusion equation of the form ∂ t u = ∇ 2 u + λe −1/u . Here such equations will be formulated in symmetrically shaped bounded regions , effectively reducing the mathematical formulation to that of one dimension. This is critically re-examined from a modern perspective using numerical methods. A computer algorithm is constructed and used to carry out a broad-ranging evaluation of the watershed critical initial temperature conditions for thermal ignition of nonuniform assemblies. It is then shown how the resulting mathematical structure for the ignition threshold curves can be correlated by a hyperbolic conic section with a high degree of accuracy over the full range of positive ambient temperature values. However, this sometimes over-predicts (which is bad) and sometimes under-predicts (which is good) the critical initial condition. The definition of additional dimensionless parameters is found to generate further simplification, leading to a universal correlating form capable of collapsing the entire solution space onto a single line in the plane of the new variables. In addition, this study considers the physically intuitive conjecture that spatial moments of the initial temperature profile ought to possess a direct mathematical link to the critical ignition threshold. As such, the mth-order spatial moment of the critical total energy content integrals is defined, and an empirical result is derived stating that certain orders of this moment should be insensitive to changes in ambient temperature and initial shape profile and may be considered functionally dependent on the dimensionless eigenvalue only, within some quantifiable error band. Spatial moment integrals, based on computed critical threshold conditions, are found to support this conjecture, with the best accuracy obtained for the second-order moments.2000 Mathematics subject classification: primary 80A25; secondary 35K55.
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