Plasma-buoyancy dynamics in microwave electrothermal thrusters: experimental insights

Abstract 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, TM $$^z_{011}$$ cavity thruster operated with nitrogen over 200–2000 sccm and at 80–600 W. Three dimensionless surrogates—a modified Bond number $$B_\mathrm{o}$$ (buoyancy), a swirl group $$\Theta$$ , and an electromagnetic anchoring group $$\Pi_{\mathrm{EM}}$$ —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 $$\dot{m}$$ , $$P_{\mathrm{in}}$$ , and the stagnation-pressure ratio $$P_{0,\mathrm{h}}/P_{0,\mathrm{c}}$$ with a single exponential nonlinearity in $$P_{0,\mathrm{h}}/P_{0,\mathrm{c}}$$ . The inferred elasticities show displacement grows with mass flow while remaining only weakly sensitive to small, local changes in $$P_{\mathrm{in}}$$ or $$P_{0,\mathrm{h}}/P_{0,\mathrm{c}}$$ ; the dominant response is a sharp departure once $$P_{0,\mathrm{h}}/P_{0,\mathrm{c}}$$ moves away from its near-optimal neighborhood. Orientation controls performance: with the nozzle upward, buoyancy helps anchor the discharge at the throat and $$P_{0,\mathrm{h}}/P_{0,\mathrm{c}}$$ often approaches $$\sim3{:}1$$ (peaking near 3.3–3.5), whereas in horizontal/downward tests buoyancy displaces the bubble off-axis, reducing $$P_{0,\mathrm{h}}/P_{0,\mathrm{c}}$$ by $$\sim20-40\%$$ . 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 $$I_{\mathrm{sp}}$$ models for MET design.