For a launch loop payload with an exit velocity of 11 km/sec, the first 30% of the velocity gained generates 44% of the heat in the rotor, because the slip is high and almost all the 14 km/sec rotor drive power is being turned into heat. What if that could be reduced, by increasing the efficiency of the energy coupling from rotor to payload? This would increase payload efficiency and reduce energy cost
30% of exit velocity occurs 9% of the way down the track. Enhancing this much of the track with extra hardware and weight is possible - see LowerWestIncline. A mass driver would be too expensive, but similar results might be achievable by magnetically coupling the payload to the track with velocity reduction coils, loops of ordinary wire with perhaps a 1x to 3x horizontal ratio over the first 9% of the track.
The alternating magnetic field moves 3 times as fast in the lower section - this might be the rotor moving at 14 km/s. The field moves slower in the upper section, in this case 4.67 km/s . So if the payload is moving at 3km/s, and slipping behind the field at 1.67 km/s, it will appear to be moving at 9 km/s and slipping at 5 km/s to the rotor.
The rotor should be optimized for accelerating the payload close to exit velocity; with long field coils, it will probably drive less optimally. However, significant efficiency gains are possible - that means more payload launched per megajoule, less heat into the rotor, and significantly higher throughput.
MORE LATER - needs analysis.
Motor thrust and power loss is proportional to "slip", the vehicle drive power is the thrust times the velocity. The slip and power loss can be high because both rotor and stator are only heated for milliseconds per segment, while the velocity (and total power) is enormous. I presume launch sleds will be in the range of 10 to 100 meters long, mass limited by the weight of the magnets in the Halbach arrays on the sled. Much depends on circumventing parasitic inductance in the motors and arrays; I can't afford the weight of large capacitors on the track, and they would heat up far too much on the sled.
This can be applied in the opposite direction, for accelerating vehicles moving faster than rotor speed. This may have applications for some versions of the space cable, using 3 km/sec bolt streams to launch 9 km/sec vehicles. However, if the rotor and vehicle are moving at the same speed, the relative speed is zero and it will be difficult to transfer power from the relatively stationary rotor to the vehicle.