For missions venturing to the outer planets, what power source limits ion thruster operation?

RTGs

The operational range of an ion thruster is dictated by the electrical power it can draw. For missions close to the Sun, such as those targeting the Mars or asteroid belt, power is supplied via very large solar arrays. However, as missions travel outward toward the outer planets, sunlight becomes too weak for efficient solar power generation. In these environments, the propulsion system must rely on nuclear power, specifically Radioisotope Thermoelectric Generators (RTGs). The limitation here is that RTGs have a hard limit on the total electrical power they can continuously generate and convert into usable electricity, directly constraining the maximum thrust capability and propellant utilization rate achievable by the attached ion engine.