Currently being looked at is the Plasma Beam.
NASA’s Institute for Advanced Concepts awarded $75,000 to a professor from University of Washington, Robert Winglee, and his team so that they can identify and hopefully proceed with the development of a plasma beam stations. These magnetized plasma beams are being viewed as a possibility for interplanetary flight.
One station would be at the beginning of the flight and would be the force that sends the craft toward its goal and another station would be at the other end of the flight path to slow the spacecraft. These “stations” would be run by nuclear power (or perhaps solar power). The plan is to shift the power source off of the spacecraft and onto the station thereby making the spacecraft move faster. How much faster? Winglee estimates travel at 11.7 kps.
Of course there are some details to work out. There are some problems. Andrew Coates of the Mullard Space Science Laboratory says "Beams in plasmas create instabilities, and it is possible that the beam coherence and identity would be destroyed over large distances in a naturally variable solar wind environment. There may also be problems with beam defocusing and spacecraft charging."
I think it is an interesting concept that has possibilities but also will carry some great losses. Not just loss of life or loss of cargo, but loss of time through trial & error and loss of additional time through sporadic funding. It has potential and I will keep my eyes out, but I honestly don’t think I’ll be alive for the final set up! Heck, I just hope I get to see the space elevator in my lifetime.