so far these propulsion devices are mostly aimed for cruising propulsion, e.g. its to maintain a constant speed.
these devices simply doesn't provide enough thrust for accelerating a payload.
Absolutely false. You are, apparently, stuck thinking how it is at the bottom
of a gravity well, in a thick atmosphere. That's simply not how it works in free fall, in a vacuum.
First off, you don't need to "maintain" speed in a vacuum, except vs. other acceleration vectors. In other words, the solar wind, for example, makes a difference, albeit very small for a non-light sail craft, so heading directly into it would decelerate your craft somewhat (although less than the acceleration given here by a long shot, much less the Sun's gravity), but all in all, if you push an object in space, it will take a VERY, VERY long time to decelerate from "friction" with the vacuum. We're talking millions
of years from a single push by a human hand, assuming the object doesn't fall into a gravity well or somesuch.
Second, these low-impulse engines provide plenty
of thrust to accelerate a payload. As I mentioned above, an ion thruster
has, on the average, under a milligee of acceleration (25-250 millinewtons of thrust). That is projected to enable trips to Mars in 70 days or less! Even the tiniest delta-V adds up, when applied indefinitely over time in an almost completely frictionless medium ("hard" vacuum). In fact, as soon as you can apply ANY sustained thrust, no matter how small, you are no longer confined to least-energy orbits on interplanetary trips, as chemical rockets certainly would be.
The power densities required for LARGE amounts of acceleration from either "EM" or ion thrusters would be very difficult to satisfy, certainly, and you'd never be able to use them for launching from the bottom of any major gravity well, but that's also not particularly necessary. Solar power can work (although the inverse-square law is a problem
) but there are other solutions, such as fission power, that are certainly technically (although not necessarily socially or politically) feasible. And you simply don't try to launch from a planet's surface using these types of thrusters ^^' .
NASA isn't researching these techs for giggles, you know; they're FAR more useful and efficient for deep space exploration than chemical thrusters ever will be; plasma and ion thrusters are already in use for this very reason. The added bonus of ZERO reaction mass requirements for EM thrusters makes them even more useful; for instance, it makes interstellar travel suddenly quite possible, assuming you can solve the life-support side of that equation