Cross–machine benchmarking for ITER of neoclassical tearing mode stabilization by electron cyclotron current drive

Abstract

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.

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

“…Mðt; DÞ represents the power modulation and it is written in terms of a Heaviside function as Mðt; DÞ ¼ Hðcosðx rot tÞ À cosðDpÞÞ; (12) where D is the power on-time fraction. It is assumed here that the power is modulated exactly at the island rotation frequency and that the power-on phase is exactly centered around the O-point passage through the ECCD beam.…”

“…3 NTM suppression experiments have been performed successfully using both ECRH and ECCD. [4][5][6][7][8][9][10][11][12][13][14] The generalized Rutherford equation (GRE) 15,16 provides a theoretical model for the growth and stabilization of tearing modes. It describes the nonlinear temporal evolution of the full width w of the associated magnetic island as a function of different stabilizing and destabilizing mechanisms.…”

ECE for NTM control on ITER

“…[8] For both equatorial ECE and a line-of-sight ECE system, the required ECE channel layout to attain a detection accuracy of 5 mm and 7 mm for the 2/1 and 3/2 NTM is investigated, which is equal to the required deposition accuracy for full suppression. [4] A video bandwidth of 2 kHz is used which suffices for the detection of the 2/1 and 3/2 NTM based on a model for mode rotation by La Haye et al [9] The detection latency is determined for seeded islands, growing in accordance with the polarization model for the saturation of the bootstrap island growth at small island widths.…”

“…The evolution of island width and phase is described by a combined model by Van den Brand et al, which combines island growth described by the Generalized Rutherford Equation and a model for island rotation as put forward by La Haye et al [4][1] [9] The island width and phase, as a function of time, and radial position serve as input for the NOn-Thermal ECE Code (NOTEC), which calculates noise-free thermal ECE detected by an antenna viewing the ITER plasma. [10] The NTM perturbs the temperature profile which depends on all three coordinates (r c , θ, φ), where r c is the plasma minor radius in cylindrical coordinates, φ the toroidal and θ the poloidal angle.…”

Development and testing of a fast Fourier transform high dynamic-range spectral diagnostics for millimeter wave characterization

“…Electron cyclotron heating and current drive ͑ECH&CD͒ has demonstrated the ability to suppress NTMs on tokamaks. [3][4][5][6] ECH&CD and electron cyclotron emission ͑ECE͒ take place at localized areas in the plasma where the wave frequency is resonant with the local electron cyclotron frequency or one of its harmonics. In current tokamaks this frequency is in the order of 100 GHz.…”