Editor's Note: Sessions 4-6 are for advanced learners. We recommend that learners follow these sessions in sequence. Electrothermal systems have limited utility for a number of deep-space missions with large ΔVs because of performance constraints placed on them by excessive frozen flow and electrode losses; that is, 20% of the input power is deposited into the electrodes as heat. The specific impulse and thrust efficiency of arcjets operating on standard space-storable propellants (e.g., hydrazine) are limited to less than 700 s and 41%, respectively. Recent US Air Force arcjet tests have demonstrated specific impulses of over 800 s on ammonia, which is also space storable, at 30% thrust efficiency. Researchers in Germany have shown that arcjets can produce specific impulses of 2,000 s with hydrogen as propellant, but also at relatively low efficiency. Ironically, it was the lack of high performance in electrothermal systems that may have first led to the development of one kind of electromagnetic engine; the magnetoplasmadynamics (MPD) thruster. The MPD thruster was "invented" by accident. Arcjet researchers were investigating the effect of mass flow rate on thrust. They noted that while the thrust of the arcjet initially dropped with decreasing mass flow rate as expected, the thrust began to increase with decreasing flow rate once a sufficiently low flow rate was reached. This seemingly impossible result actually marked the transition from electrothermal heating to electromagnetic acceleration as the flow rate decreased. Electromagnetic devices pass a large current through a small amount of gas to ionize the propellant. Once ionized, the plasma is accelerated by an electromagnetic body force called the Lorentz force,
which is created by the interaction of a current (j) with a magnetic field (B). The current is provided between the energized positive and negative electrodes, while the magnetic field is either induced by (created from) the current itself, applied externally via an electromagnet or both. The strength of the Lorentz force for an MPD thruster with a self-induced magnetic field is roughly proportional to the ratio J2 /  , where J is the total thruster current and is the propellant mass flow rate. While gas-phase propellants like hydrogen and lithium (after vaporization) can be used, solid propellants can also be used in pulsed electromagnetic accelerators called pulsed plasma thrusters (PPTs).
Regents of the University of Michigan | | Figure EM1: Operating principles of the magnetoplasmadynamics (MPD) thruster. |
PPTs use solid Teflon propellant to deliver specific impulses in the 900-1,200 s range and very low, precise impulse "bits" (10-1,000 μNs) at low average power (< 1 to 100 W). While PPTs are inherently inefficient (ηt ~5%), their simplicity and low impulse bits provide them with a niche for space propulsion; for example, precision-flying of a spacecraft constellation. The PPT consists of a coiled spring that feeds the Teflon propellant bar, an igniter plug to initiate a small-trigger electrical discharge, a capacitor, and the electrodes through which current flows. Plasma is created by ablating the Teflon from the discharge of the capacitor across the electrodes (figure EM2). The plasma is then accelerated to generate thrust by the Lorenz force that is established by the current and its induced magnetic field. While PPTs have flown on both American and Soviet/Russian spacecraft since the 1960s, a recently developed PPT was used to maintain fine pitch attitude control for the NASA New Millennium Program's Earth Observing-1 mission that was launched in 2000 (figure EM3). 
Image courtesy of Aerojet | | Figure EM2: Operating principles of the pulsed plasma thruster (PPT). |
MPD thrusters can be operated in pulsed mode like PPTs or continuously. While MPD thrusters have never been used in space for propulsion, a Japanese MPD thruster was operated in the space shuttle bay for basic plasma physics experiments. The MPD thruster's high specific impulse (> 4,000 s) and high power density make it an excellent candidate for high-power, high-ΔV missions of the future. MPD thrusters do suffer from relatively low efficiency (<50%) due to frozen flow losses and electrode deposition. NASA has partnered with the Moscow Aviation Institute (MAI) to engage in MPD thruster research. Figure EM4 shows a 30 kW MPD thruster operating on lithium propellant at Princeton University. MAI has operated lithium-propellant MPD thrusters at power levels above 200 kW. 
Courtesy of the Electric Propulsion and Plasma Dynamics Lab, Princeton University. | | Figure EM4: A MAI Lithium MPD thruster operating at 500 A-20 V and at a propellant flow rate of 20 mg/s at Princeton University. |
NASA | | Figure EM3: The EO-1 pulsed plasma thruster. |
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