Plasma Magnet

Plasma Magnet

Spacecraft plasma propulsion has long held the promise of yielding exhaust velocities, and therefore propulsion efficiencies, beyond the reach of chemical propulsion. Recently, the idea of using the solar wind for spacecraft propulsion has been put forth. The solar wind flows out from the sun at velocities greater than 105 m/s and provides a dynamic pressure of rough ly 2x10-9 N/m2 at 1 AU. To intercept a relevant amount of thrust for a spacecraft mission (0.1 - 1 N), a barrier to the solar wind must be 4-10 km in radius. One possible barrier is a magnetic field of 50nT at or beyond 4 km. To create such a field using an electromagnet re quires very large-scale engineering, for example a circular electromagnet 300m in radius carrying 105 amp-turns. While such an electromagnet is not impossible, it would likely be so massive that the relatively modest thrust coupled from the solar wind would provide acceleration too slow to be of interest. To keep the mass of the system low and thus yield a concept that is applicable to a near-term spacecraft demonstration (few 100 kg spacecraft mass, few kW power ), the \223RMD/Plasma Magnet\224 has been proposed.

Plasma magnet uses phased antennas operating in the radio frequency range to produce a rapidly rotating magnetic field. This rotating magnetic field preferentially accelerates electrons within a plasma to produce a DC current that can lead to the generation of a steady state magnetic field in space very much larger than can be possibly sustained by electromagnets. In the image at left, the two circular magnetic field coils located inside the blue plasma are driving roughly 1000 Amp currents in the blue plasma. This Plasma Magnet has come into equilibrium with an externally applied magnetic field.

This research may lead to a system where a modest spacecraft could create a very large (many kilometers), steady state magnetic field around itself. This magnetic field would act as a barrier to the solar wind plasma that flows out from the sun at more than 100 km/s with a dynamic pressure of roughly 2nPa. If this could be achieved, spacecraft propulsion would become drastically faster and more efficient. At left is an artist conception of what a spacecraft using Plasma Magnet propulsion might look like.

The gif at the left shows a computer simulation of a rotating magnetic dipole field (solid lines) dragging electrons with it. The electrons bounce around the dipole field faster than the field rotates, but the rotating magnetic field still imparts an average motion in the direction of rotation. This simple model of particle motion bodes well for the Plasma Magnet concept.

The gif at the left shows a cartoon of how the Plasma Magnet works. The rotating (blue lines) magnetic field produced by the spacecraft drives the (red dots) electrons. The driven electrons then produce their own static magnetic field (red lines), which is much larger in extent than the field produced by the rotating magnetic field coils. It is this (red) magnetic field that acts as a large-scale barrier to the solar wind.

The gif at left shows a collection of still images taken at various external magnetic field strengths. The external field provides an inward pressure to balance the outward pressure of the expanding Plasma Magnet. As the external field drops, the blue plasma of the Plasma Magnet expands, just as a sealed balloon expands as it moves from the surface of the earth to the lower pressures of the upper atmosphere. Because of the optical access to our existing vacuum chamber, it is difficult to watch the expansion when the external field drops below 44 Gauss.