Scientists seeking to bring fusion—the power that drives the sun and stars—down to Earth must first make the state of matter called plasma superhot enough to sustain fusion reactions. That calls for heating the plasma to many times the temperature of the core of the sun. In ITER, the international fusion facility being built in France to demonstrate the feasibility of fusion power, the device will heat both the free electrons and the atomic nuclei—or ions—that make up the plasma. The question is, what will this heating mix do to the temperature and density of the plasma that are crucial to fusion production?