How large is a magnetoplasmadynamic (MPD) thruster? Are there any upcoming programs using it?

Magnetoplasmadynamic thrusters are an interesting proposed method for propulsion in space, but are there any upcoming programs that will use them? If not, might this be because there still serious and unsolved technical challenges?

Also, roughly speaking, how big might these the thruster be?

A magnetoplasmadynamic (MPD) thruster (MPDT) is a form of electrically powered spacecraft propulsion which uses the Lorentz force (the force on a charged particle by an electromagnetic field) to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or (mostly in Japan) MPD arcjet.

[...] According to Edgar Choueiri magnetoplasmadynamic thrusters have input power 100–500 kilowatts, exhaust velocity 15–60 kilometers per second, thrust 2.5–25 newtons and efficiency 40–60 percent.

• The Wikipedia article you link lists two serious technical challenges in the "development" section: the impractically high power requirements of MPD thrusters, and the rapid erosion of cathodes in many prototypes. Can you explain what other information you're after and narrow the scope of the question?
– Bear
Oct 8, 2018 at 12:19
• @Bear the question asks if there are "any upcoming programs that will use them?" (I added the word "use" just now) and then if the answer to that is no, only then, what are the challenges that are preventing that. I'm not sure the Wikipedia article states clearly that the things you've named are those reasons; there may be others.
– uhoh
Oct 8, 2018 at 15:41

Like any electric thruster, MPDs could be scaled or clustered to any desired size, but they require a great deal of power to produce even small amounts of thrust -- 20-50 kilowatts per newton:

A single MPD engine about the size of an average household pail can process about a million watts of electric power from a solar or nuclear source into thrust

This would produce only 50 newtons of thrust; the power source (solar or nuclear) would be far larger than the thruster itself.

So far the only space-flown MPD is the Japanese EPEX experiment on the Space Flyer Unit. It appears to have been successful. Thus the primary technical challenge seems to be providing MPDs with a lightweight power supply.