ICRH power absorption measurements from analysis of RF sinusoidal low-frequency modulation experiments in TEXTOR

Modulation of the ICRH power amplitude has been performed sinusoidally in TEXTOR over a wide modulation frequency range in an (H)-D plasma. From an analysis of reproducible discharges in which the ICRH is modulated at a relatively high modulation frequency Ω/2π (50-300 Hz), we observe that only a fraction (about 50%) of the ICRH power is transferred to the plasma with a good energy confinement time. We also show that the analysis of a single modulated discharge already provides part of the information. The analysis is supported by a careful investigation of the non-stationary energy response obtained from the diamagnetic look and from the equilibrium measurement. The eigenfunction expansion of the transport equation for global energy in which each successive mode has a particular absorption coefficient and a decreasing confinement time indicates that about 50% of the ICRH power is coupled to the fundamental mode with the best confinement time. The remainder of the power goes to other modes with decreasing confinement times. Such a time-dependent analysis allows us therefore to distinguish between the fraction of the power absorbed by the core of the plasma (good confinement) and the fraction absorbed by the edge (bad confinement). The conclusions of this modulation analysis are qualitatively similar for high-field side (HFS) antennae with predominantly direct electron heating and for low-field side (LFS) antennae with predominantly minority-ion heating. In the second scenario, a detailed analysis has been provided to account for the anisotropic contribution to the energy of the minority ions. (author)