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m1omg
2007-Sep-29, 08:38 PM
What is the difference between a plasma and an ion engine?What kinds of plasma engine do exist?What is their spec. impulse and thrust?What are their advantages and disadvantages compared to ion engines?Or it is the same things as MPD (Magnetoplasmadynamic) thruster?

IsaacKuo
2007-Sep-29, 09:49 PM
The definitions of "ion" rocket and "plasma" rocket are a bit fuzzy, and depend on who you ask. The terms can mean more or less the same thing, but some people expect "ion" rockets to refer only to rockets where the exhaust is naturally charged (but there may also be an exhaust neutralizer). In that case, the term "plasma" rocket implies that the exhaust is naturally neutral, and no exhaust neutralizer is necessary.

However, for others, the terms "ion" rocket and "plasma" rocket are completely interchangeable.

Personally, I tend to use the term "ion" rocket only to refer to an electrostatic ion rocket of one particular type, and use the term "plasma" rocket to refer to all others. This is just a personal tendency.

There are many types of plasma rockets. Many of them, like MPD thrusters, involve electrodes that directly contact the hot exhaust plasma for purposes of running electric current through the plasma. These thrusters tend to have technical issues with electrode erosion, which might be solvable but might also be compelling disadvantages. Other thrusters, like VASIMR, are electrode-less. These pump energy into plasma via induced current or laser heating.

Generally, plasma thrusters can be categorized into either "continuous" or "pulsed" operation. The former might provide more thrust, but require more minimum power. The latter might require less power, and might be able to reach higher exhaust velocities thanks to concentrating the power into a smaller amount of propellant. The efficiency advantage could go either way; it depends on the specifics of the type of drive.

My favorite plasma thruster concepts are VASIMR for high power large scale operation, and laser ablative for low power small scale operation. But for the moment, the unquestionable king of ion/plasma drives is the electrostatic ion thruster. Despite the problems of electrode erosion and charge limits which put hard physics limits on potential performance, they're the most mature technology--proven on Deep Space One and now flying on Dawn. While other drive concepts will surpass the electrostatic ion thruster, it will be some years before those technologies are mature enough to replace it.