Mini-magnetospheric Plasma Propulsion
In order to reduce the size and weight of the magnet of the magnetic sail, it may be possible to inflate the magnetic field using a plasma in the same way that the plasma around the Earth stretches out the Earth's magnetic field in the magnetosphere. In this approach, called mini-magnetospheric plasma propulsion (M2P2), currents running through the plasma augment and partially replace the currents in the coil. This is expected to be especially useful far from the Sun, where the increased effective size of a M2P2 sail compensates for the reduced dynamic pressure of the solar wind. The original NASA design proposes a spacecraft containing a can-shaped electromagnet into which a plasma is injected. The plasma pressure stretches the magnetic field and inflates a bubble of plasma around the spacecraft. The current in the plasma in this case augments and partially replaces current in the coils. The plasma then generates a kind of miniaturized magnetosphere around the spacecraft, analogous to the magnetosphere that surrounds the Earth. The protons and electrons which make up the solar wind are deflected by this magnetosphere and the reaction accelerates the spacecraft. The thrust of the M2P2 device would be steerable to some extent, potentially allowing the spacecraft to 'tack' into the solar wind and allowing efficient changes of orbit.
In the case of the (M2P2) system the spacecraft releases gas to create the plasma needed to maintain the somewhat leaky plasma bubble. The M2P2 system therefore has an effective specific impulse which is the amount of gas consumed per newton of thrust. This is a figure of merit usually used for rockets, where the fuel is actually reaction mass. Robert Winglee, who originally proposed the M2P2 technique, calculates a specific impulse of 200 kN·s/kg (roughly 50 times better than the space shuttle main engine). These calculations suggest that the system requires on the order of a kilowatt of power per newton of thrust, considerably lower than electric thrusters, and that the system generates the same thrust anywhere within the heliopause because the sail spreads automatically as the solar wind becomes less dense. However, this technique is less well understood than the simpler magnetic sail and issues of how large and heavy the magnetic coil would have to be or whether the momentum from the solar wind can be efficiently transferred to the spacecraft are under dispute.
The expansion of the magnetic field using plasma injected has been successfully tested in a large vacuum chamber on Earth, but the development of thrust was not part of the experiment. A beam-powered variant, MagBeam, is also under development.
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