On the interplay between neoclassical tearing modes and nonlocal transport in toroidal plasmas

Abstract

This Letter presents the first observation on the interplay between nonlocal transport and neoclassical tearing modes (NTMs) during transient nonlocal heat transport events in the HL-2A tokamak. The nonlocality is triggered by edge cooling and large-scale, inward propagating avalanches. These lead to a locally enhanced pressure gradient at the q = 3/2 (or 2/1) rational surface and hence the onset of the NTM in relatively low β plasmas (β N < 1). The NTM, in return, regulates the nonlocal transport by truncation of avalanches by local sheared toroidal flows which develop near the magnetic island. These findings have direct implications for understanding the dynamic interaction between turbulence and large-scale mode structures in fusion plasmas.Over the past years, a number of experiments which study transient transport events in magnetically confined plasmas have revealed a provocative phenomenon [1][2][3][4][5][6][7] . A rapid change of local thermal transport in response to a change of plasma parameters occurs at a location distant from the perturbation, i. e., the so-called "nonlocal" transport, which cannot be explained just by the standard diffusive local transport paradigm. Although several theoretical models [8][9][10][11][12] have been proposed which try to interpret this nonlocal effect, the underlying physical mechanisms remain unclear, due to limited experimental data.On the other hand, to achieve economic viability for a future fusion reactor, a crucial task is to operate the plasma at relatively high β values, where β is the ratio of plasma pressure p to the magnetic field (B) pressure defined as β = p/(B 2 /2μ 0 ). However, the maximum achievable β can be limited by resistive MHD instabilities, in particular, the neoclassical tearing mode (NTM) [13][14][15][16][17] . The NTM is driven by a loss of a large bootstrap current related to a large plasma pressure gradient within a magnetic island. The occurrence of NTMs may considerably increase parallel heat transport across the width of the island, degrade energy confinement and even cause plasma disruptions. Thus, the NTM will prevent plasmas moving into a high-β state if it appears in the low-β case. As a consequence, understanding the physical processes for the onset of NTMs, as well as their prevention, is also one of major challenges for fusion.In this Letter, we present the first experimental observation of self-regulation of nonlocal transport events by NTMs generated during transient nonlocal transport events. The nonlocal effect is excited by edge cooling and propagates inward by avalanche events. These cause a local increase of the pressure gradient at the inversion surface, and thus the onset of the NTM in relatively low β plasmas (β N < 1). The presence of the NTM, results in the development of sheared flows at the magnetic island. The dual role of low-order rationals as both damping 18 and drive mechanism of steady-state 19 and fluctuating 20 E r × B flows has been identified, thus the magnetic topology is an important regulator of radial ...

On the correlation between ‘non-local’ effects and intrinsic rotation reversals in Alcator C-Mod

“…These effects have been extensively documented in many tokamaks and helical devices, namely in TEXT [4][5][6], TFTR [7,8], Tore Supra [9,10], RTP [11][12][13], ASDEX-U [14], JET [15], LHD [1,[16][17][18][19][20][21][22][23], HL-2A [24][25][26][27][28], Alcator C-Mod [29,30] and, more recently, in KSTAR [31]. Despite the reproducibility of these phenomena in very different plasma conditions, the underlying mechanism of the core response is still unclear.…”

A review of supersonic molecular beam injection for plasma fueling and physical studies in magnetic fusion devices

“…It can be observed that the Hurst parameters, self-similarity in turbulence, and large-scale radial correlation are all enhanced with the growing inward propagation of turbulent activities. In the transient NLT event, the interaction between the nonlocal transport and the neoclassical tearing mode (NTM) was observed (Ji et al 2016). A typical experiment for the interaction study is shown in Fig.…”

“…Fig. 29 Typical discharge waveforms with an onset of a 3/2 NTM during the nonlocal transport induced by SMBI (vertical yellow bar) in ECRHheated plasma at HL-2A(Ji et al 2016) …”

Progress of HL-2A physical experiments

“…离子驱动的阿尔芬级联模(ACs) [32] , 见图24 [33,34] . [36] . 此外, 也观察到了由电子鱼骨模触发的非局 域热输运过程, 研究结果表明磁扰动、长程相关、介 观尺度结构以及剪切流对非局域输运起到重要作 用 [37] .…”