Ion engines are called Constant Specific Impulse (CSI) engines because their Isp does not vary during their operation. The other type engine now being developed is a Variable Specific Impulse (VSI) engine—a plasma engine. In this device, a fuel, usually hydrogen, is ionized. Very high frequency radio waves then heat it to extreme temperature to create plasma. Magnetic fields contain the plasma within the engine and direct it out the nozzle to produce low levels of thrust.
NASA was working on one of these until 2005. Its development has now been turned over to a private company—Ad Astra Rocket Co. It is called the Variable Specific Impulse Magneto-plasma Rocket, or VASIMR. It was to be tested on the ISS in 2011. Instead, some parts of it will be tested on the space station in the 2013 to 2014 time frame.
As shown in the diagram below, the VASIMR system consists of three major magnetic cells numbered in the diagram as '1', ''4', and ''6'. First, a neutral gas, typically hydrogen, is injected into cell 1 and ionized by the Helicon antenna. Next, this charged gas is heated to reach the desired density in the engine's central cell, 4, by electromagnetic waves, similar to what happens in a microwave oven. The plasma is trapped by the magnetic field that is generated by the magnetic coils. It can be heated to 10 million degrees K.
Then, the heated plasma enters the nozzle at cell 6, where the plasma detaches from the magnetic field and is expelled as exhaust to provide thrust.
VASIMR can change its thrust and Isp by changing the fraction of power sent to the Helicon antenna vs. the ICRH (ion cyclotron resonance heating) antenna. The helicon antenna is used to ionize gas injected from the gas injector. The ICRH antenna heats the gas and accelerates the particles before these particles are exhausted to space. When more power is sent to the Helicon antenna, more gas is ionized, which means more ions are ejected. But because the total system power level is constant, power sent to the ICRH antenna decreases, which means these ions exit with a lower velocity. These low-speed, large quantity ions create a high thrust, low Isp engine. On the other hand, when less power is sent to the Helicon antenna and more power is sent to the ICRH antenna, a small amount of gases are ionized and they are accelerated to a higher exit velocity. These high-speed ions produce a low thrust, high Isp rocket engine.
A recent doctoral thesis at the Georgia Institute of Technology (Georgia Tech) indicated a VASIMR type engine could cut the transit time to Mars from that nine months to one month. The diagram below illustrates the method of using VASIMR’s variable capability.The length of the lines indicate thrust levels. Note as well that the direction of the thrust varies as the spacecraft progresses along its trajectory. This is to catch up with Mars as it approaches the planet.
And you will note as well that about half way to Mars, the thrust direction reverses. This is the kicker of continuous acceleration. You can’t just keep the pedal to the metal. At the half way point you have to apply the brakes. In space, this means you have to turn the ship around and begin deceleration.