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

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. (C) 2001 Elsevier Science B.V. All rights reserved.