The steps in producing thrust are as follows:
• The propellant ( hydrogen) is first ionized by electromagnetic waves.
• Then it is transferred to the central chamber. This chamber is threaded with magnetic fields. The ions spiral around the magnetic field lines with a certain natural frequency. By bombarding them with radio waves of the same frequency, the system heats the ions to 10 million Kelvin's.
• A magnetic nozzle converts the spiraling motion into axial motion, driving the hydrogen ions out the back of the rocket and producing thrust.
There is a magnetic choke using which the rocket exhaust can be controlled. When we close the choke, the number of ions getting forced out of the nozzle decreases. Thus the thrust is less. But the temperature is kept constant, thus increasing the specific impulse.
Conversely, by opening the choke, more ions are forced out of the nozzle increasing the thrust. As temperature is constant, specific impulse decreases.
A spacecraft would use low gear to climb out of planetary orbit, and high gear for interplanetary cruise.
Thus VASIMR can work in both high gear and low gear unlike all the present technologies
This technique is currently not being used as very high temperatures are required for generation of plasma. Next propulsion technique is electrostatic propulsion.