Inductive effects during second harmonic current drive experiments on T-10

Current drive during second harmonic, electron cyclotron heating experiments performed on the T‐10 tokamak have been simulated with the onetwo transport code to determine the effects of induction on the time evolution of the loop voltage and current density profile. Ray tracing shows the well focused rf power can generate centrally peaked current densities which exceed the Ohmic current densities by a factor of five, causing very peaked current profiles which will be unstable to sawteeth. A Kadomtsev model of the sawtooth shows that a limit cycle is quickly reached which maintains a broad current profile and requires generation of a negative dc component of the loop voltage localized near the magnetic axis. This negative electric field effectively reduces the measured current drive efficiency. A broader profile of driven current, as in the fundamental current drive experiment on T‐10, would not suffer this effect.