Centro de Fusão Nuclear

	Alfvén Cascades in JET Discharges with NBI-Heating
	S. Sharapov, B. Alper, Y. Baranov, H. Berk, D. Borba, C. Boswell, B. Breizman, C. Challis, M. de Baar, E. de la Luna, E. Evangelidis, S. Hacquin, N. Hawkes, V. Kiptily, S. Pinches, P. Sandquist, I. Voitsekhovitch, N. Young e os contributos JET-EFDA
	Resumo
	The recently developed interferometry diagnostics for detecting Alfvén eigenmodes on JET [1] reveals excitation of Alfvén cascades (ACs) in JET discharges with highpower NBI heating when the external Mirnov magnetic coils show almost no sign of ACs because of the high level of magnetic noise. With the use of interferometry, the ACs in NBI-heated plasmas are studied experimentally to assess their stability in burning plasma relevant high-density JET discharges and as a diagnostic tool. Theoretically, the particle-to-wave power transfer as a function of frequency and parallel wave-vector of ACs is analysed for the resonant interaction between ACs and NBI-produced passing ions and the conditions for the AC damping/drive due to such ions are obtained [2]. The experimentally observed excitation of ACs by sub-Alfvénic NBI-produced ions with parallel velocities as low as on JET is analysed and the comparison of a DIII-D – JET experiment on NBI-driven ACs and their effect on the beam power deposition profile [3] is discussed. New opportunities for MHD spectroscopy, which are associated with the larger number of detected ACs and with the possibility to excite the modes by sub-Alfvénic ions are demonstrated including the use of grand-cascades as indicators of triggering events for internal transport barriers in NBI-heated discharges. MHD-spectroscopy based on the interferometry detection of ACs, when no mode number can be identified, is discussed and alternative ways to obtain the necessary information on the mode numbers from the Doppler-shift effect in toroidally rotating NBI-heated plasmas for ACs are described. [1] S.E. Sharapov et al., Phys. Rev. Lett. 93 165001 (2004). [2] E.Evangelidis et al., work in progress (2005). [3] R.Nazikian et al., Fusion Energy 2004 (Proc. 20th Int. Conf. Vilamoura, 2004) (Vienna: IAEA), paper EX/5-1.