Plasma-Buoyancy Dynamics in Microwave Electrothermal Thrusters: Experimental Insights

Ground tests of microwave electrothermal thrusters (METs) are conducted under 1 g, where buoyant forces distort the plasma morphology and bias performance data. We investigate these effects in a 2.45-GHz, TMz011 cavity thruster operated with nitrogen over 200–2000 sccm and at 80–600 W. Three dimensionless surrogates — a modified Bond number Bo (buoyancy), a swirl group θ, and an electromagnetic anchoring group ΠEM — bring all experimental points onto a single, self-consistent scaling curve. Guided by these surrogates, we then recast the horizontal-nozzle data into a centered, reduced-order surrogate: a log-linear form in ṁ, Pin, and the stagnation-pressure ratio P0,h/P0,c with a single exponential nonlinearity in P0,h/P0,c. The inferred elasticities show displacement grows with mass flow while remaining only weakly sensitive to small, local changes in Pin or P0,h/P0,c; the dominant response is a sharp departure once P0,h/P0,c moves away from its near-optimal neighborhood. Orientation controls performance: with the nozzle upward, buoyancy helps anchor the discharge at the throat and P0,h/P0,c often approaches ~3:1 (peaking near 3.3–3.5), whereas in horizontal/downward tests buoyancy displaces the bubble off-axis, reducing P0,h/P0,c by ~20-40%. Equal top–bottom injection maximizes swirl confinement and suppresses buoyant drift. The surrogate offers a compact path to correct 1-g measurements toward flight-like conditions and to couple directly with thrust and Isp models for MET design.