Numerical Investigation of Bluff-Body Stabilized Microwave Plasmas

Microwave heating of gases for space propulsion applications is addressed analytically by means of coupled solutions of the Navier-Stokes and Maxwell equations. The model is validated using experimental measurements of bluff-body stabilized microwave plasmas sustained in a resonant cavity. The size, shape, and location of the plasma are reasonably well-predicted, as are its overall thermal efficiency, coupling efficiency, and peak temperature. Parametric trends such as the effect of power and mass flow variations on thruster performance verify that the model incorporates the dominant physical processes of the plasma-fluid-dynamic interaction. In addition, the predictions indicate that the size of the plasma strongly influences the plasma temperature and its absorption characteristics. Proper design of the flow tube/bluff-body configuration enables the control of the plasma size and, hence, the plasma characteristics. This enables high-coupling efficiencies to be obtained at high power levels and, along with control of the mass flow to minimize wall heat, high thruster performance may be realized. Preliminary efforts to maximize performance resulted in specific impulse predictions of 600 s. Additional work is needed to identify methods for increasing the peak powers and to study size scale-up issues.