= Ion propulsion
<Ion propulsion> produces <thrust> by using electrical <energy> to accelerate ions out of a <spacecraft>. In a gridded ion engine, a discharge creates ions from propellant such as <xenon>, electric fields accelerate them through grids, and a neutralizer supplies electrons so the departing beam does not leave the spacecraft progressively charged. The exhaust carries momentum; no surrounding air is required.<Ion Propulsion by NASA>[NASA Facts, ion generation, accelerator grids and neutralizer]{ref}
For an ideal directed exhaust with <mass flow rate> $\dot m$ and <exhaust velocity> $v_e$, neglecting the pressure term,
$$
T=\dot m v_e,\qquad \eta P=\frac12\dot m v_e^2,\qquad T=\frac{2\eta P}{v_e}.
$$
Here $P$ is electrical input power and $\eta$ the fraction converted into directed exhaust kinetic energy. Thus high exhaust speed saves propellant for a given momentum change but reduces thrust at fixed power. At $P=1\,\mathrm{kW}$, $\eta=0.6$ and $v_e=30\,\mathrm{km\,s^{-1}}$, the ideal thrust is $0.04\,\mathrm N$.
Ground tests operate the engine in a pumped vacuum chamber, measuring beam current, power, propellant flow and thrust. A bright plume alone cannot establish efficiency or delivered impulse. NASA's Deep Space 1 and Dawn missions demonstrated sustained ion propulsion in flight, where small thrust accumulated over long operating times.<Ion Propulsion by NASA>[NASA Facts, ground testing and Deep Space 1/Dawn history]{ref}
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