Axi-symmetrization of radiation pattern and two wave heating of fundamental ECRH in GAMMA 10

Abstract In GAMMA 10, the radiation pattern of a heating wave of fundamental ECRH for potential formation was not axi-symmetric but vertically elongated on the resonance surface. The non-axisymmetric radiation pattern caused a non-axisymmetric potential profile and likely induced a radial loss. Then, the radiation pattern has been axi-symmetrized. Accordingly, an almost axi-symmetric potential profile has been obtained and a large density increase during ECRH has been observed in the central cell. This implies that radial losses associated with ECRH have been substantially reduced. A new antenna has recently been installed for additional plug electron heating (second ECRH). With this antenna, a larger power and/or a longer pulse width have become available. In a most recent experiment, we have succeeded in axial plugging with a duration up to 0.15 s by sequential use of the usual ECRH and the second ECRH. Moreover, there is an indication that the second ECRH is more desirable for further reduction of the radial loss.