MHD modes in regular and reversed shear scenarios and possibilities for their control through current drive

Abstract

The onset conditions for neoclassical tearing modes are shown to be over a wide range of experiments in good agreement with predictions of the polarization current model. In the presence of sufficiently large seed islands, this model extrapolates unfavourably to reactorgrade devices operating with peaked current profiles. The expected decrease of the seed islands produced by sawteeth at the (3, 2) and (2, 1) resonant surfaces may counteract this trend, but extrapolation of this effect has large uncertainties. Active control of this mode on ASDEX Upgrade and COMPASS-D through modulated or unmodulated current drive at the resonant surface has been demonstrated to stabilize this mode, in the electron cyclotron current drive (ECCD) case with a small fraction of driven current (2% of I p ). In advanced scenarios, magnetohydrodynamic (MHD) modes can contribute to the stationarity of the current profile. Fishbones can clamp the current profile development near the q = 1 or q = 2 surface, without significantly deteriorating energy confinement, whereas double-tearing modes, acting in a similar form, lead to substantial confinement losses. First experiments on ASDEX Upgrade with application of central electron cyclotron resonance (ECR)-heating and current drive to discharges with an internal transport barrier show a substantial effect on MHD stability, affecting the passage of the q-profile through q min = 2, and degrading or prolonging the core reversed shear phase, depending on the current drive direction.

Dynamics of resistive double tearing modes with broad linear spectra

“…Off-axis sawteeth were observed on Tokamak Fusion Test Reactor (TFTR) when the minimum of the safety factor, q min , is near or below 2 [6]. Resistive and neoclassical q s = 2 DTMs were investigated in Axisymmetric Divertor Experiment (ASDEX)-Upgrade, one motivation being the possible interaction * Electronic address: abierwag@uci.edu † Electronic address: benkadda@up.univ-mrs.fr ‡ Electronic address: hamaguch@ppl.eng.osaka-u.ac.jp § deceased ¶ Present address: Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA of these modes with internal transport barriers (ITBs) [7,8]. In Japan Atomic Energy Agency Tokamak Upgrade (JT-60U) q s = 3 DTMs are thought to play a crucial role in disruptions, as indicated by experimental observations and numerical results [9,10,11].…”

Numerical studies on the stabilization of neoclassical tearing modes by radio frequency current drive

“…24 -26 It has been shown on ASDEX upgrade ͑axially symmetric divertor experiment upgrade͒ and other tokamaks that localized ECCD can fully stabilize NTM's. [27][28][29][30][31] Theoretical analysis based on the generalized Rutherford equation suggests that the modulated current drive to deposit the rf current around the o point of the magnetic island is favorable for the stabilization, assuming the rf current density to be a function of the helical magnetic flux. 32,33 In Ref.…”

“…The simulation results essentially agree with experimental ones and the analytical prediction. 25,[27][28][29] With the improvement of the experimental technology, feedback control of the rf wave deposition location is possible by changing the launching mirror angle of ECCD, the radial plasma position, or the toroidal magnetic field, based on the real time measurement of magnetic perturbations. 30 It is desirable to use the rf current to stabilize the NTM at a small amplitude once it is observed to minimize its effect on plasma confinement.…”

Self-Organized Stationary States of Tokamaks

“…These sawtooth-free discharges have been correlated with a 3/2 tearing mode interacting with ELMs [1] and alternatively with the presence of fishbone modes [2,27,28]. However, a theory for how these MHD modes maintain the current profile has previously been lacking.…”