VASIMR

Here's some stuff about the VASIMR:

Variable Specific Impulse Magnetoplasma Rocket

..consists of three major magnetic cells, denoted
as "forward," "central" and "aft." This particular configuration of
electromagnets is called an asymmetric mirror.

The forward end-cell involves the main injection of gas to be turned
into plasma and the ionization subsystem;

the central-cell acts as an amplifier and serves to further heat the
plasma.

The aft end-cell ensures that the plasma will efficiently detach from
the magnetic field. Without the aft end-cell, the plasma would tend
to follow the magnetic field and provide only a small amount of
thrust.

With this configuration, the plasma can be guided and controlled over
a wide range of plasma temperatures and densities.

Neutral gas, typically hydrogen, is injected at the forward end-cell
and ionized.

It is heated to the desired temperature and density in the central-
cell, by the action of electromagnetic waves, similar to what happens
in microwave ovens.

After heating, the plasma enters a two-stage hybrid nozzle at the aft
end-cell where it is exhausted to provide modulated thrust.

Two parameters are varied during a typical operation: thrust and the
velocity of the particles leaving at the rocket exhaust. This latter
is called the specific impulse. As the ship accelerates on its
journey, the thrust decreases and the specific impulse increases. The
opposite is also true as the ship slows down at its destination.

Advantages
1.) Variable Specific Impulse and Thrust at maximum power.
2.) Electrodeless design with magnetic insulation.
3.) High power density.
4.) Continuous acceleration (very low artificial gravity).
5.) High efficiency Ion Cyclotron Resonance Heating, high voltage and
low current.
6.) Capability for powered mission aborts.
7.) Propulsion system is adaptable to slow, high payload robotic
cargo missions as well as fast, lower payload human transfers.

source:
http://spaceflight.nasa.gov/mars/technology/propulsion/aspl/vasimr.htm
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George
http://cygo.com/