Neoclassical tearing modes (NTMs) will be the principal limit on performance in ITER in the standard scenario, which has beta well below the ideal kink limit. Measurements of island size from ASDEX Upgrade, DIII-D and JET in beta rampdown experiments are used to determine the marginal size for m/n = 3/2 NTM removal. This is compared with data from ASDEX Upgrade, DIII-D and JT-60U with removal of the 3/2 NTM by electron cyclotron current drive (ECCD) at near constant beta. The empirical marginal island size is consistent in both sets of removal experiments and is found to be about twice the ion banana width. A common methodology is developed for fitting the saturated m/n = 3/2 island before (or without) ECCD in all four experimental devices, ASDEX Upgrade, DIII-D, JET and JT-60U. To this is added (and model tested to experiments) the effect of un-modulated co-ECCD on the island width due to replacing the missing bootstrap current and making the tearing stability parameter Δ′ more negative. The common model is then used to evaluate the ITER ECCD system, with or without modulation, for both the m/n = 3/2 mode, which is benchmarked here, as well as the m/n = 2/1 NTM. The ITER ECCD top launch system with 20 MW of power is found to be effective in greatly reducing the size of the islands. An m/n = 2/1 mode locking model is used to show that the rotation in ITER should be sufficient for the island reduction by ECCD to avoid locking that causes loss of H-mode and disruption.
The results of stabilizing neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD) in JT-60U are described with the emphasis on the effectiveness of the stabilization. The range of the minimum EC wave power needed for complete stabilization of an m/n = 2/1 NTM was experimentally identified for two regimes using unmodulated ECCD to clarify the NTM behaviors with different plasma parameters: 0.2 < j EC /j BS < 0.4 for W sat /d EC ∼ 3 and W sat /W marg ∼ 2, and 0.35 < j EC /j BS < 0.46 for W sat /d EC ∼ 1.5 and W sat /W marg ∼ 2. Here, m and n are the poloidal and toroidal mode numbers; j EC and j BS the EC-driven current density and bootstrap current density at the mode rational surface; W sat , W marg and d EC the full island width at saturation, marginal island width and full width at the half maximum of the ECCD deposition profile, respectively. Stabilization of a 2/1 NTM using modulated ECCD synchronized with a mode rotation of about 5 kHz was performed, in which it was found that the stabilization effect degrades when the phase of the modulation deviates from that of the ECCD at the island O-point. The decay time of magnetic perturbation amplitude due to the ECCD increases by 50% with a phase shift of ±50 • from the O-point ECCD, thus revealing the importance of the phasing of modulated ECCD. For near X-point ECCD, the NTM amplitude increases, revealing a destabilization effect. It was also found that modulated ECCD at the island O-point has a stronger stabilization effect than unmodulated ECCD by a factor of more than 2.
The use of ECRH has been investigated as a promising technique to avoid or postpone disruptions in dedicated experiments in FTU and ASDEX Upgrade. Disruptions have been produced by injecting Mo through laser blow-off (FTU) or by puffing deuterium gas above the Greenwald limit (FTU and ASDEX Upgrade). The toroidal magnetic field is kept fixed and the ECRH launching mirrors are steered before every discharge in order to change the deposition radius. The loop voltage signal is used as disruption precursor to trigger the ECRH power before the plasma current quench. In the FTU experiments (I p =0.35-0.5 MA, B t =5.3 T, P ECRH =0.4-1.2 MW) it is found that the application of ECRH modifies the current quench starting time depending on the power deposition location. A scan in deposition location has shown that the direct heating of one of the magnetic islands produced by magnetohydrodynamic (MHD) modes (either m/n=3/2, 2/1 or 3/1) prevents its further growth and also produces the stabilization of the other coupled modes and current quench delay or avoidance. Disruption avoidance and complete discharge recovery is obtained when the ECRH power is applied on rational surfaces. The modes involved in the disruption are found to be tearing modes stabilized by a strong local ECRH heating. The Rutherford equation has been used to reproduce the evolution of the MHD modes. The minimum absorbed power value found for disruption avoidance is 0.4 MW at 0.5 MA with deposition on the q=2 surface. In the similar set of experiments carried out in ASDEX Upgrade L-mode plasmas (I p =0.6 MA, B t =2.5 T, P ECRH = 0.6 MW ~ P OHM) the injection of ECRH close to q=2 significantly delays the 2/1 onset and prolongs the duration of the discharge: during this phase the density continues to increase. No 2/1 onset delay is observed when the injected power is reduced to 0.35 MW.
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