Thermal Propulsion
With beamed propulsion one can leave the power-source stationary on the ground, and directly (or via a heat exchanger) heat propellant on the spacecraft with a maser or a laser beam from a fixed installation. This permits the spacecraft to leave its power-source at home, saving significant amounts of mass, greatly improving performance.
Since a laser can heat propellant to extremely high temperatures, this potentially greatly improves the efficiency of a rocket, as exhaust velocity is proportional to the square root of the temperature. Normal chemical rockets have an exhaust speed limited by the fixed amount of energy in the propellants, but beamed propulsion systems have no particular theoretical limit (although in practice there are temperature limits).
In addition, microwaves can be used to heat a suitable heat exchanger, which in turn heats a propellant (very typically hydrogen). This can give a combination of high specific impulse (700–900 seconds) as well as good thrust/weight ratio (50-150).
A variation, developed by brothers James Benford and Gregory Benford, is to use thermal desorption of propellant trapped in the material of a very large microwave-sail. This produces a very high acceleration compared to microwave pushed sails alone.
Read more about this topic: Beam-powered Propulsion