The multi-fluid plasma model only assumes local thermodynamic equilibrium within each fluid, e.g. ion and electron fluids for the two-fluid plasma model. Derivation of the MHD model involves several asymptotic and simplifying assumptions that can limit its applicability. Therefore, the two-fluid plasma model more accurately represents the appropriate physical processes. Physical parameters indicate the importance of the two-fluid effects: electron to ion mass ratio m e /m i , ion skin depth δ i , and ion Larmor radius r L . The MHD model assumes m e /m i = 0, δ i = 0, and r L = 0. Asymptotic approximations of the two-fluid model, Hall-MHD, has an unbounded Whistler wave that requires artificial dissipation. No unbounded waves exist in the two-fluid model. An algorithm is developed for the simulation of plasma dynamics using the two-fluid and multi-fluid plasma models. The algorithm implements a discontinuous Galerkin method that uses an approximate Riemann solver[1] to compute the fluxes of the fluids and electromagnetic fields at the computational cell interfaces. The two-fluid plasma model has time scales on the order of the electron and ion cyclotron frequencies, the electron and ion plasma frequencies, the electron and ion sound speeds, and the speed of light. Such disparate time scales motivate a semi-implicit time-stepping scheme to overcome the severe time step restrictions of explicit schemes. The algorithm is validated with several test problems including the GEM challenge magnetic reconnection problem [2] and the generation of dispersive plasma waves which are compared to analytical dispersion diagrams. The algorithm is applicable to study advanced physics calculations of plasma dynamics including magnetic plasma confinement and astrophysical plasmas. Three-dimensional solutions of the Z-pinch and the field reversed configuration (FRC) magnetic plasma confinement configurations are presented.
A fusion space thruster based on the flow-stabilized Z-pinch may be possible in the near-term and provide many advantages over other fusion-based thruster concepts. The Zpinch equilibrium is classically unstable to gross disruption modes according to theoretical, numerical, and experimental evidence. However, a new stabilization mechanism has been discovered that can stabilize these modes with plasma flow. The stabilizing mechanism was developed for a Z-pinch plasma equilibrium which has an axial velocity profile that is linear in radius. When the velocity shear exceeds a threshold, the plasma modes are stabilized. The magnitude of the peak velocity is dependent on the mode wavelength but is sub-Alfvénic for the wavelengths of experimental interest, vmax > 0.1VAka where VA is the Alfvén speed, k is the axial wave vector, and a is the characteristic pinch radius. The flow Z-pinch experiment ZaP has been built at the University of Washington to experimentally verify the sheared flow stabilizing mechanism. The experiment has achieved plasma flow velocities of 10 5 m/s and stability for almost 2000 growth times. For more information the reader is encouraged to visit http://www.aa.washington.edu/AERP/ZaP. The extension of the flow Z-pinch to a space thruster is straight forward. The plasma in a flow Z-pinch would already be moving axially, fusing, and releasing a tremendous amount of nuclear energy. The end of the Z-pinch can be left open to allow the escape of the energetic plasma. Specific impulses in the range of 10 6 s and thrust levels of 10 5 N are possible.
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