DARPA isn't building anything. If you read the site you'll see that it's just a study in how to develop a path to develop the technologies needed to eventually build a ship.
Correct, it's a technology symposium - DARPA's just organizing it.
Are you allowed to tell the general public (schmucks like the rest of us on this board) a little bit about your proposal? I'm curious.
We take a helicon (which is a tube of plasma excited to extremely high ionization by a specifically tuned RF signal wound around a coil of electromagnets surrounding the tube), use it as a plasma source and inject that plasma into an IEC (which is an experimental fusion device). The motivation was that while fusion does not currently take place to any appreciable degree inside an IEC, it has a mode that's known to act as a plasma jet. In physics research they use what's called the 'star mode' of operation - and jet mode was always avoided because it destroys the internals of the IEC if you don't modify it to allow for the jet to exhaust. If you build a channel (as we have) to allow the jet to exhaust, then it can potentially produce thrust.
In the current evolution, it isn't a candidate for interstellar propulsion - but it is
most definitely a candidate for satellite, trans-lunar injection and interplanetary propulsion. Isp is in the 4000-200,000 range depending on power levels and efficiencies we still have to work out. Nobody has modeled the electrostatic field in jet mode in an IEC before because it was of no interest, so that's a mystery right now as well. But at present the leading electric propulsion thruster is the Hall Effect Thruster and we think we can out-perform it - plus, the Hall thruster almost always has to use Xenon, which is becoming increasingly expensive and is absolutely not an option for interstellar travel because of the extremely large quantities it would require.
Actually just last week we got it to fire in jet mode briefly. There are still a lot of unknowns regarding the right conditions to fire in jet mode. Man it was a thing of beauty!
In the future, it may be possible to further develop the IEC concept such that you get fusion taking place and you get a much, much, MUCH hotter plasma coming out (i.e., higher velocity - higher thrust and ISP) in which case it is a candidate for interstellar propulsion.
Minus the fusion aspect, the helicon->IEC coupling is similar to VASIMIR but a lot mechanically simpler.
Here's a picture
but this was just before we got it into jet mode. What you're seeing is a ball of argon plasma (the blue ball) inside a concentric electrostatic grid (the atom looking metal rings), with a hole to allow the plasma to escape the potential well in a specific direction. Unfortunately, it's not quite a jet there, it's diverging into a diffuse spray. As a jet it's a very narrow tight beam with what we believe may be shock patterns like you see in compressible flow.