EX/P6-24 Experimental Study of Fast Wave Absorption Mechanisms in DIII-D in the Presence of Energetic Ions

DIII-D experiments have studied interaction between injected 80 keV deuterons and fast waves (FWs) at 60 MHz (up to 1.4 MW), 90 MHz (up to 2 MW), and at 116 MHz (up to 1.6 MW), at ion cyclotron harmonic numbers between 4 and 8. While strong absorption of FWs at the 4th harmonic of injected deuterons has been clearly observed, ion absorption under the same conditions at 6th and 8th harmonics is seen to be weak. Data on fast wave absorption on the injected beam is obtained from confinement analysis and equilibrium reconstruction, from fusion neutron rate measurements, and from fast ion Dα (FIDA) spectroscopy. Strong acceleration of the beam above the injected energy in the 4th harmonic case, weak acceleration in some of the 6th harmonic cases, and no acceleration in the 8th harmonic cases is observed with the FIDA diagnostic. The experimental results also indicate clearly the importance of edge losses in cases in which the central absorption mechanisms are weak. Therefore quantitative modeling must take the edge losses into account. Core absorption mechanisms will be much stronger in ITER, so that edge losses should be much less significant than in DIII-D, but the competition between core damping mechanisms will be similar. Hence, establishment of a quantitative model of the partitioning of FW power among competing absorption processes is important in both cases.