Pavlos G. Mikellides

Modeling and Analysis of a Megawatt-Class Magnetoplasmadynamic Thruster

The magnetohydrodynamic code MACH2 is utilized to investigate the operation and performance of a megawatt-class, quasi-steady, self-field magnetoplasmadynamic thruster. The numerical results are validated by comparisons to experimental data and offer significant insights into this regime of operation. Specifically, operation at 0.5-6 MW and 1.37 g/s shows incomplete hydrogen-propellant ionization that is dominated by electron-neutral collisions, which in turn increase the plasma voltage, but do not substantially increase thrust. Acceleration is of a hybrid nature, transitioning from primarily electrothermal to predominantly electromagnetic with increasing current level. Detailed interrogation of energy deposition shows significant deposition to internal modes, as opposed to external-circuit-element losses and electrode losses via fall voltage and thermal conduction, even at the higher power levels. These frozen-flow and radial-momentum losses can be reduced while the system still operates with minimum power penalty by proper expansion of the flow.

Design of a Fusion Propulsion System— Part 1: Gigawatt-Level Magnetoplasmadynamic Source

Optimum travel duration for manned interplanetary missions requires propulsion systems that deliver very high thrust, on the order of a thousand Newtons, in conjunction with specific impulse capabilities that exceed 10,000 s. Theoretically, rocket propellants consisting of fusion reactants intermixed with large masses of low-molecular-weight fuels can be expanded within a magnetic nozzle to meet these requirements. To produce the power levels associated with such systems, a gigawatt pulse line called Godzilla is adapted for experimental development. The megajoule-level energy available is electromagnetically deposited in cold helium gas to simulate the fusion-heated, low-molecular-weight propellant. The magnetohydrodynamic computer code, MACH2, is employed to provide guidelines in the design of this magnetoplasmadynamic plasma source. The numerical results specify the geometric configuration and operation conditions required to overcome destructive effects associated with these power levels within experimental limitations.

Development of equation-of-state and transport properties for molecular plasmas in pulsed plasma thrusters part II: A two-temperature equation of state for Teflon

The equation of state for gas phase polytetrafluoroethylene (PTFE) is calculated with a two-temperature LTE formulation. Twenty-five chemical species are included in the analysis. Sample calculations are performed for pressures from 0.001 atm to 1.0 aim and for temperature ranges of 0.1 ev to 4 ev for both heavy particle and electron temperatures. The preliminary results using the Mach2 MHD simulation program show large discrepancies between the calculated equation of state and that in the LANL SESAME tables used by Mach2. The results clearly show the importance of including complex molecular plasma effects in the simulation of the pulses plasma thrusters and related devices.

Modeling and Performance Analysis of the Pulsed Inductive Thruster

Numerical modeling of the pulsed inductive thruster exercising a time-dependent, two-dimensional, axisymmetric magnetohydrodynamics code provides bilateral validation of the thrusters measured performance and the codes capability of capturing the pertinent physical processes. Computed impulse values for helium and argon propellants demonstrate excellent correlation to the experimental data for a range of energy levels and propellant-mass values. Quantitative energy deposition analysis confirms the thrusters approximately constant-efficiency operation and captures the experimentally observed, critical-mass phenomenon. An idealized model produced a simple impulse expression for this energy range that expands the validity of the aforementioned insights to other propellants and corroborates the thrusters singular operation with ammonia propellant. The simple impulse expression produced compares well with experimental trends and can be used to guide design and optimization of operating conditions.

Modeling of the Pulsed Inductive Thruster Operating with Ammonia Propellant

Comparisons of numerical simulations to experimental data are used to validate the code and offer insights into the highest-to-date performance of ammonia propellant compared to other tested propellants. The time-dependent, two-dimensional, axisymmetric magnetohydrodynamics code used for the simulations is upgraded by a new two-temperature thermochemical model that provides ammonias thermodynamic properties including nitrogen ionization up to the fifth level. The comparisons to experimental data include magnetic field waveforms at two locations and total impulse values for a range of operating energy levels and propellant mass values. The relatively good agreement of the codes predictions and experiment allows further interrogation of the pertinent plasma flow characteristics and offers a possible explanation as to the elevated efficiency demonstrated by the thruster operated with ammonia propellant.

Application of the MACH 2 code to magnetoplasmadynamic arcjets

Numerical modeling of the Pulsed Inductive Thruster, (PIT) exercising the magnetohydrodynamics code, MACH2 aims to provide bilateral validation of the thrusters measured performance and the codes capability of capturing the pertinent physical processes. Computed impulse values for helium and argon propellants demonstrate excellent correlation to the experimental data for a range of energy levels and propellant-mass values. Quantitative energy deposition analysis confirms the thruster’s demonstrated constant-efficiency operation and captures the experimentally-observed, critical-mass phenomenon. An idealized model for these energy levels and propellants expands the validity of the aforementioned to other propellants and identifies the PIT’s unique operation with ammonia propellant.

A theoretical model for the thrust and voltage of applied-field MPD thrusters

A time dependent, 2-dimensional axisymmetric MHD simulation code, MACH 2, is adopted to model both self-field and applied-field magneto-plasmadynamic thrusters. One of the NASA Lewis MPD thruster configurations is considered with operation at 1000 amp discharge current and 0.1 g/s mass flow rate. The applied field simulations include in-plane magnetic field contributions from induced azimuthal currents generated by the plasma. Simulations of both self-field and several low applied-field cases (varying the external magnet coil current) have successfully reached steady state and depict distributions of magnetic field, velocity and temperature. Overall arcjet performance in terms of specific impulse is estimated. 8 refs.

Progress on the Godzilla Gigawatt MPD Plasma Accelerator and Nozzle for Fusion Propulsion Simulations

The new visualization of applied-field MPD thruster operation offered by previous numerical simulations with the MACH2 code, is utilized to produce an analytic model. This model provides simple expressions for the thrust and voltage drop across the plasma that capture all the trends and in most cases the magnitudes of a wide range of experimental data. It establishes that the thrust scales as the square root of the product of the applied field strength, discharge current and mass flow rate, T~VrhlB~. Voltage scales linearly with applied field strength and is independent of discharge current and mass flow rate. This model also provides additional insights that can be utilized to improve applied-field MPD thruster performance.

Multi‐Megawatt MPD Plasma Source Operation and Modeling for Fusion Propulsion Simulations

Extensive human exploration and development of space requires propulsion systems that can provide both high thrust-to-weight ratio and high specific impulse. Fusion propulsion is one of the enabling technologies considered for application in such missions. A critical and common element to many fusion propulsion concepts is the expansion of the fusion-grade plasma through a magnetic nozzle. Efforts to evaluate this essential component of a fusion propulsion system began a few years ago at The Ohio State University as part of a project sponsored by the NASA Glenn Research Center. The university houses Godzilla, a gigawatt-level, 1.8megajoule pulseline that can power an acceleration sys tem cons i s t ing (p r imar i ly ) o f a magnetoplasmadynamic (MPD) source and two magnet coils. The MPD source is designed to deliver hypersonic plasma flow to a magnetic-diffuser section for compression to a nearly stagnant state after passage through the converging-diverging guide field. The stagnated plasma will then be accelerated to supersonic speeds through a second magnet coil. The full effort encompasses both computational and experimental approaches to this device. INTRODUCTION