Experimental study of coupling 476 MHz helicon power to DIII-D plasmas

Up to ~0.5 MW of power at 476 MHz has been launched from a 30-element traveling wave antenna (TWA) into DIII-D plasmas and to date no rf-specific impurities have been detected when the antenna is powered. A fraction of the launched power is observed in the core as electron heating, motivating detailed study of the coupling process. The fraction of the power applied to the input of the antenna system emitted into the plasma is determined by the plasma conditions in the scrape-off layer, where the electron density is <~ 1 × 10¹⁹ m^-3 , and is strongly affected by the distance from the antenna face and the separatrix (TWAGAP), ELMs in H-mode discharges and by the level and location of gas puffing used to fuel the discharge. The coupling is measured with an array of rf current monitors embedded in the TWA and with directional couplers near the input of the antenna, at a time resolution of about 10 microseconds. The fraction of the coupled power in the unwanted slow mode polarization as well as the level of nonlinear activity such as parametric decay instability also depend on the plasma parameters in the SOL. Profile reflectometry, Thomson scattering, and Langmuir probes are used to constrain the electron density near the TWA.