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PIT is another technology that NASA is reexamining.The device relies on a rapid sequence of events that, like the MPD,sets up perpendicular electrical and magnetic fields. It begins when a nozzle releases a puff of gas (usually argon),which spreads out across the face of a flat coil of wire about 1 meter across.Then a bank of capacitors discharges a pulse of current,lasting about 10 microseconds, into the coil.The radial magnetic field generated by the pulse induces a circular electrical field in the gas,ionizing it and causing the particles to revolve in exactly the opposite direction as the original pulse of current. Because their mo tion is perpendicular to the magnetic field, they are pushed out into space. Unlike other electromagnetic drives, PIT requires no electrodes, which tend to wear out, and its power can be scaled up simply by increasing the pulse rate.In a 1-megawatt system the pulses would occur 200 times a second.

Thrust: 20 newtons

Exhaust speed: 50 kilometers per second Sample burn time: 110 days Sample fuel ratio: 14 percent

CHRISTOPH BLUMRICH; SOURCE: ROBERT VONDRA TRW, RALPH H.LOVBERG University ofCalifornia,San Diego AND C. LEE DAILEY

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