Control of MHD instabilities by ECCD: ASDEX Upgrade results and implications for ITER

Abstract

_______________________________________________________________This is a preprint of a paper intended for presentation at a scientific meeting. Because of the provisional nature of its content and since changes of substance or detail may have to be made before publication, the preprint is made available on the understanding that it will not be cited in the literature or in any way be reproduced in its present form. The views expressed and the statements made remain the responsibility of the named author(s); the views do not necessarily reflect those of the government of the designating Member State(s) or of the designating organization(s). In particular, neither the IAEA nor any other organization or body sponsoring this meeting can be held responsible for any material reproduced in this preprint. Abstract. The requirements for control of MHD instabilities by ECCD are reviewed. It is shown that localised current drive is needed for control of both sawteeth and Neoclassical Tearing Modes (NTMs). In the case of NTMs, the deposition width should be smaller than the island width for efficient control. At island widths smaller than the deposition width, as is predicted to occur in ITER, theory suggests that efficient control is possible only by modulating the ECCD power in phase with the island. These predictions are experimentally confirmed in ASDEX Upgrade for NTM control. Narrow deposition has also been used to extend the operational range of NTM stabilisation in ASDEX Upgrade to lower q 95 and in the improved H-mode scenario. Our results suggest that for the ITER ECCD system, good localisation of the driven curent profile as well as the capability to modulate the ECCD in phase with rotating modes will be needed for efficient MHD control by ECCD.

The performance of improved H-modes at ASDEX Upgrade and projection to ITER

“…These discharges are prone to develop large (3,2) NTM modes, strongly deteriorating confinement, during the first few seconds when β N is increased above 2. A characteristic of (3,2) NTM activity at low q 95 < 3.5 is that the impact on confinement is generally stronger compared to improved H-mode discharges at higher q 95 [15]. In the example given before at q 95 =3.17 (Fig.…”

Electron cyclotron current drive calculated for ITER conditions using different models

“…Apart from the ECCD efficiency, the factors I cd /∆ρ (not shown here) and I cd /∆ρ 2 are also important for the choice of the launch angles [1] (∆ρ is the width of the ECCD profile with ρ as the normalized effective radius). Here, I cd /∆ρ 2 is the figure of merit for both the NTM stabilization [23] and the sawtooth period control [24].…”

Density sensitivity of intrinsic rotation profiles in ion cyclotron range of frequency-heated L-mode plasmas

“…While toroidal rotation without momentum input has been documented on a number of experiments, the scaling of this rotation to ITER and beyond is still an open area of research [9]. If the self-generated flows are insufficient, then investment in development of torque sources for large, high density, reactor-relevant tokamaks must be made, or other external control options explored [10].…”

“…While toroidal rotation without momentum input has been documented on a number of experiments, the scaling of this rotation to ITER and beyond is still an open area of research [9]. If the self-generated flows are insufficient, then investment in development of torque sources for large, high density, reactor-relevant tokamaks must be made, or other external control options explored [10].In order to have predictive capability for next-generation machines, modeling must first be shown to accurately predict flows in current devices. Alcator C-Mod [11] provides an ideal test-bed for intrinsic rotation physics as auxiliary heating, achievement of high-confinement regimes and measurement of the rotation profile are all accomplished without net momentum input.…”