All plasma rockets operate on the same type of principle: Electric fields and magnetic fields work side by side to first transform a gas – typically xenon or krypton – into plasma and then accelerate the ions in the plasma out of the engine at over 45,000 mph (72,400 kph), creating a thrust in the direction of desired travel [source: Science Alert]. There are many ways that this formula can be applied to create a working plasma rocket, but there are three types that stand out as the best and most promising [source: Walker].
Hall thrusters are one of two types of plasma engines that are currently in use regularly in space. In this device, electric and magnetic fields are set up in a perpendicular fashion in the chamber. When electricity is sent through these dueling fields, the electrons begin to whiz around super-fast in circles. As the propellant gas gets squirted into the device, the high-speed electrons knock electrons off the atoms in the gas, creating a plasma consisting of the free electrons (carrying negative charges) and the now positively charged atoms (ions) of the propellant. These ions get shot out of the back of the engine and create the thrust needed to propel the rocket forward. While the two processes of ionization and acceleration of the ions happen in steps, they occur within the same space in this engine. Hall thrusters can generate a significant amount of thrust for the input power used, so they can go incredibly fast. But there are limits on their fuel efficiency.
When NASA is looking for an engine that's more fuel-efficient, it turns instead to gridded ion engines. In this commonly used device, electric and magnetic fields are situated along the walls of the engine chamber. When electrical power is applied, high-energy electrons oscillate in and along the magnetic fields near the walls. In a similar fashion to the Hall thruster, the electrons are able to ionize the propellant gas into a plasma. In order to do the next step of creating thrust, electric grids are placed at the end of the chamber in order to accelerate the ions out. In this engine, the ionization and acceleration happen in two different spaces. While the gridded ion engine is more fuel-efficient than a Hall thruster, the downside is that it cannot generate as much thrust per unit area. Depending on the type of job they're looking to get done, scientists and aerospace engineers choose which engine suits the mission better.
Finally, there is the third type of engine: VASIMR, short for Variable Specific Impulse Magnetoplasma Rocket. This rocket, developed by former astronaut Franklin Chang Diaz, exists only in the test phase now. In this device, the ions are created via radio waves generated by an antenna to form the plasma. Another antenna further downstream adds energy that causes the ions to spin around in a circle very fast. A magnetic field provides directionality so that the ions are released out of the engine in a straight line, thereby delivering the thrust. If it works, this rocket will have enormous throttle range, something that the Hall thruster and ion gridded engine cannot achieve as easily.