An ion thruster ionizes a propellant like xenon and accelerates it through an electric field set by the beam voltage, reaching an exhaust velocity of tens of kilometers per second — far beyond any chemical engine. Thrust is just beam current times the ionized mass flow times that exhaust velocity, so even a fully charged beam only produces a few tens of millinewtons. How well electrical power turns into that tiny push is total efficiency, three multiplied factors: electrical efficiency in the power supply, mass-utilization efficiency (how much of the injected propellant actually gets ionized and thrown out the back), and beam-divergence efficiency (how much of the beam stays pointed straight along the axis instead of spraying sideways). Raising beam voltage raises specific impulse but lowers thrust-to-power — the tradeoff behind every electric-propulsion design. Because so little propellant has to move so fast, a small xenon load spread over a long burn adds up to an enormous Δv. Set the beam voltage, beam current, ion mass, mass utilization, electrical efficiency, and beam divergence, then watch it all play out live. It all runs on your device.
You are in the Physics lab.
Electrical power in, three efficiency losses out — and how far a little xenon can push.
What the thruster schematic, the loss-split bar and the two plots are telling you — then what the model says about voltage.
The schematic shows neutral propellant ionized in the discharge chamber and accelerated by charged grids into a high-speed exhaust beam, with a neutralizer injecting electrons so the beam and the spacecraft don't build up opposite charge. The gauge beside it tracks total efficiency — how much of the electrical power actually survives as beam thrust power once electrical, mass-utilization and divergence losses are subtracted (the same split the loss bar shows).
The loss-split bar breaks electrical power into three losses that never show up as a single number — electrical loss, mass-utilization loss, and beam-divergence loss — plus what's left as delivered beam thrust power. Widen any slice and total efficiency falls, since it's the product of electrical, mass-utilization, and divergence efficiency together.
The first plot tracks Δv accumulated and propellant remaining as a long burn runs from throttle-up to depletion. The second plot draws specific impulse and thrust-to-power against beam voltage and marks where the current design sits, showing why high-Isp designs trade away thrust density.
Three multiplied efficiency factors, a velocity that trades against thrust, and why so little propellant goes so far.
Thrust is the ionized mass flow rate — beam current times the ion's mass-to-charge ratio — multiplied by exhaust velocity, the speed a beam voltage gives each accelerated ion. Because exhaust velocity is enormous but mass flow is tiny, the product comes out in millinewtons: an ion thruster trades raw thrust for extreme efficiency per kilogram of propellant.
Total efficiency is electrical efficiency times mass-utilization efficiency times beam-divergence efficiency. Electrical efficiency is the power supply's conversion loss; mass-utilization efficiency is how much injected propellant actually gets ionized instead of drifting out neutral; beam-divergence efficiency is how much of the beam's momentum stays pointed straight back instead of spraying off-axis. Each factor is a real, physical loss, and losing ground on any one lowers the whole product.
Exhaust velocity, and with it specific impulse, rises with the square root of beam voltage, but the power needed to drive the beam rises linearly with voltage — so thrust-to-power falls as voltage climbs, the central design tradeoff of electric propulsion. Because exhaust velocity is already so high, the rocket equation's Δv — exhaust velocity times the log of how much the spacecraft's mass falls as propellant burns away — adds up fast from very little propellant, which is why a small xenon load can push a spacecraft for months and still deliver a Δv a chemical rocket could never match with the same mass.
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