There are a few technically feasible approaches to get about a megawatt of power in space using nuclear power or solar power. Russia has a project to develop about a megawatt nuclear reactor for space applications.
NASA Glenn Research Center, GRC, has several programs to advance near-term photovoltaic array development. One project is to design, build, and test two 20 kW-sized deployable solar arrays, bringing them to technology readiness level (TRL) 5, and through analysis show that they should be extensible to 300 kW-class systems (150 kw per wing). These solar arrays are approximately 1500 square meters in total area which is about an order-of-magnitude larger than the 160 square meters solar array blankets on the International Space Station (ISS).
Orbital ATK has a promising lightweight and compact solar array structure. The MegaFlex
Deployable Space System, DSS, developed a roll-out array, ROSA, EDU that employs an innovative stored strain energy deployment to reduce the number of mechanisms and parts. The elastic structure maintains stiffness throughout deployment for partially deployed power generation. The rectangular design can be configured in many ways by either lengthening the booms, adjusting the length and width, or attaching several winglets onto a deployable backbone. Lengthening and/or shortening the booms provides power scaling without changing any of the subsystems or stowed configuration. See below for a fully deployed ROSA array.
* four 150 kilowatt wings would be 600 kilowatts in power. The new wings are easy to deploy and do not involve astronauts.
* eight 150 kilowatt wings would be 1.2 megawatts
Power at the hundreds of kilowatts and megawatt levels would be great for magnetoplasmadynamic thrusters
In theory, magnetoplasmadynamic (MPD) thrusters could produce extremely high specific impulses (Isp) with an exhaust velocity of up to and beyond 110,000 m/s (0.03% of light speed), triple the value of current xenon-based ion thrusters, and about 25 times better than liquid rockets. MPD technology also has the potential for thrust levels of up to 200 newtons (N) (45 lbF ), by far the highest for any form of electric propulsion, and nearly as high as many interplanetary chemical rockets. This would allow use of electric propulsion on missions which require quick delta-v maneuvers (such as capturing into orbit around another planet), but with many times greater fuel efficiency
CGI rendering of Princeton University’s lithium-fed self-field MPD thruster (from Popular Mechanics magazine)
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.
Generally, a gaseous material is ionized and fed into an acceleration chamber, where the magnetic and electrical fields are created using a power source. The particles are then propelled by the Lorentz force resulting from the interaction between the current flowing through the plasma and the magnetic field (which is either externally applied, or induced by the current) out through the exhaust chamber. Unlike chemical propulsion, there is no combustion of fuel. As with other electric propulsion variations, both specific impulse and thrust increase with power input, while thrust per watt drops.
There are two main types of MPD thrusters, applied-field and self-field. Applied-field thrusters have magnetic rings surrounding the exhaust chamber to produce the magnetic field, while self-field thrusters have a cathode extending through the middle of the chamber. Applied fields are necessary at lower power levels, where self-field configurations are too weak. Various propellants such as xenon, neon, argon, hydrogen, hydrazine, and lithium have been used, with lithium generally being the best performer.
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.
One potential application of magnetoplasmadynamic thrusters is the main propulsion engine for heavy cargo and piloted space vehicles (example engine a^2 for Manned mission to Mars)
A plan, proposed by Bradley C. Edwards, is to beam power from the ground. This plan utilizes 5 200 kW free electron lasers at 0.84 micrometres with adaptive optics on the ground to beam power to the MPD-powered spacecraft, where it is converted to electricity by GaAs photovoltaic panels. The tuning of the laser wavelength of 0.840 micrometres (1.48 eV per photon) and the PV panel bandgap of 1.43 eV to each other produces an estimated conversion efficiency of 59% and a predicted power density of up to 540 kW/m2. This would be sufficient to power a MPD upper stage, perhaps to lift satellites from LEO to GEO