Consequences of plasma rotation for neoclassical tearing mode suppression by electron cyclotron current drive

Abstract

In the generalized Rutherford equation describing the nonlinear evolution of the width of the magnetic island associated with a neoclassical tearing mode, the effect of localized current drive is represented by a term ΔCD′. We investigate oscillations in ΔCD′ originating from the rotation of the island through the electron cyclotron power deposition region and their dependence on the collisional time scale on which the driven current is generated, the rotation period, the island size, and the power deposition width. Furthermore, their consequences for the island growth or the stabilization are analyzed. This work shows that the net result of the oscillations in ΔCD′ is a slight increase in the stabilizing effect of electron cyclotron current drive and consequently, a reduction in the minimum power requirement to fully suppress an island.

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

“…First of all, it should be determined whether the power required for complete NTM suppression remains within the capabilities of the system. Furthermore, it affects the decision as to whether modulation capabilities should be retained (which opens also the question of the detailed dynamical model of the ECCD process, as the island rotation frequency in the laboratory frame and the collision frequency on which ECCD establishes could be comparable [19,20]). Finally, the profile width also affects the steering accuracy required to compensate possible offsets of the deposition profile with respect to the island.…”

On the Simplification of the Modeling of Electron-Cyclotron Wave Propagation in Thermonuclear Fusion Plasmas

“…The second case concerns the effect of density fluctuations of different orders of magnitude encountered along propagation. In ITER, the spatial range of such fluctuations is expected to vary within [1,20] cm [28]. With a calculation like above, one finds the range of values of κ in the vicinity of the fluctuation to be [5.6, 99.2].…”

“…Clearly for n = 0 and 1 one obtains the intermediate equations in zero and first order, from which (7) and (9) are derived (for more details see, e.g., [10,11]). The amplitude terms of higher-order may be calculated iteratively using (20), as it is already in the formΛ ·Ā n =Ω(Ā n−1 ), or by derivation and solution of a separate transport equation for each A n , in analogy to (9). This exact computation is beyond the scope of this article and will be treated in a future work.…”

“…In some cases, it may be important to address the actual tokamak equilibrium, also in the presence of MHD instabilities which result to the appearance of magnetic islands. Since ECRH/ECCD is used for the control/suppression of such phenomena, it may be also crucial to estimate the effect of the presence of islands on the power absorption and current drive [19,20]. A second issue is related to the description of the transformation of the initially assumed Gaussian wave beam to a more complicated wave object due to localization, asymmetry and inhomogeneity in the absorption.…”

“…As an example, we briefly refer to the case of EC propagation in the presence of magnetic islands (one may find extensive studies on this topic in [19,20]). The issue of relevance here is that, in most studies of ECCD-based NTM stabilization, the analysis of the propagation and deposition has been performed in terms of the unperturbed magnetic topology, taking into account only the flux surface of interest while ignoring any effects from the islands.…”