Effect of m/n = 2/1 neoclassical tearing mode on sawtooth collapse in JT-60U

Abstract

We have investigated the role of the m/n = 2/1 neoclassical tearing mode (NTM) for suppression of the sawtooth collapse in JT-60U from the viewpoint of the anomalous transport of the current diffusion, namely flux pumping. In the stabilization experiments of m/n = 2/1 NTMs by electron cyclotron current drive of JT-60U, it has been clarified that the sawtooth collapses occur during or after the stabilization of m/n = 2/1 NTMs. It is also confirmed that the minimum safety factor, q min , is nearly unity before and after the stabilization of an m/n = 2/1 NTM. While the flux pumping by ELM-NTM coupling was reported in DIII-D, the suppression of the sawtooth collapse is observed without ELMs in this study. On the other hand, it is observed that the sawtooth precursor appears during the stabilization of the m/n = 2/1 NTM accompanying the disappearance of the fluctuation of the n = 1 helical core (HC), which is induced by the m/n = 2/1 NTM. In addition, the modulation of the toroidal rotation velocity having the frequency of the n = 1 HC is observed in the core region. Because the helical flow with HCs is a possible source of the dynamo loop voltage in tokamaks, these observations suggest that the suppression of the sawtooth collapse in JT-60U is realized by the dynamo loop voltage due to the n = 1 HC induced by the m/n = 2/1 NTM. Our result indicates that n = 1 HCs induced by other MHD modes also may induce anomalous current transport in tokamaks.

Coupling among neoclassical tearing modes, edge localized modes and Alfvén eigenmodes in HL-2A high β H-mode plasmas

“…The precursor oscillation of the ST crash is dominated by the 1/1 mode. After the NTM onset, the ST crash event is not observed, consistent with the observations in JT-60U [20][21][22]. The dramatic drop of the stored energy and beta indicates the degradation of the energy confinement by the NTM.…”

“…The 'hybrid' operation scenario, which is a steady-state, high performance H-mode scenario and can operate near the n = 1 above 1, and has been realized in many devices including DIII-D [4,5], ASDEX-Upgrade [6,7], JET [8,9] and JT-60U [10,11] tokamaks. Magnetic-flux pumping [12][13][14][15][16][17][18][19][20][21][22] produced by MHD modes (especially 1/1 helical structure) is considered to play an important role in the maintaining of q min 1 and the avoidance of sawtooth (ST) crash, which (especially with long period and large amplitude) is the detrimental ingredient to drive large disruptive 2/1 neoclassical tearing mode (NTM).…”

Sawtooth suppression by flux pumping on HBT-EP

“…Sawtooth suppression by flux pumping does not depend on the source of the HC, and can occur in various beta regimes [24]. This means various sources to induce 1/1 structure in the core are possibly useful to realize sawtooth control via the dynamo effect, such as 2/1 NTM [24] and externally induced 3D field [19]. This study demonstrates the possibility to suppress sawtooth via an external mode, and suggests promising avenues for future research, including more precise sawtooth control through external mode manipulation.…”

“…In particular, we demonstrate how the stabilization of the edge mode leads to strong sawteeth. Sawtooth suppression by flux pumping does not depend on the source of the HC, and can occur in various beta regimes [24]. This means various sources to induce 1/1 structure in the core are possibly useful to realize sawtooth control via the dynamo effect, such as 2/1 NTM [24] and externally induced 3D field [19].…”

“…In DIII-D, flux pumping was observed with n = 1 helical cores (HCs) induced by the n = 1 magnetic perturbation [19], or with neoclassical tearing mode-edge localized mode (NTM-ELM) coupling [21]. JT-60U showed flux pumping through n = 1 HC induced by m/n = 2/1 NTM [22][23][24].…”