Spatiotemporal changes in the pressure-driven current densities on DIII-D due to magnetic islands

Petty, C. C.; Jayakumar, R. J.; Makowski, M. A.; Holcomb, C.T.; Humphreys, D.A.; Haye, R.J. La; Luce, T. C.; Politzer, Peter; Prater, R.; Wade, M. R.; Welander, A.S.

doi:10.1088/0029-5515/52/1/013011

Cited by 6 publications

(4 citation statements)

References 39 publications

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“…Although poloidal magnetic flux pumping robustly broadens the current profile in the hybrid scenario, experiments in DIII-D find that localized current drive can affect small changes in the local current density. One example of this is the ability of localized ECCD to directly stabilize neoclassical tearing modes (NTM) by replacing the 'missing' bootstrap current at rational q surfaces [37]. Figure 9 shows another example, where strong central ECCD in the RMP ELM-suppressed, steady-state hybrid scenario slightly reduces q min according to MSE-constrained EFIT equilibrium reconstructions.…”

Section: Sawtooth Activitymentioning

confidence: 99%

Advances in the steady-state hybrid regime in DIII-D—a fully non-inductive, ELM-suppressed scenario for ITER

et al. 2017

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The hybrid regime with beta, collisionality, safety factor and plasma shape relevant to the ITER steady-state mission has been successfully integrated with ELM suppression by applying an odd parity n = 3 resonant magnetic perturbation (RMP). Fully non-inductive hybrids in the DIII-D tokamak with high beta ( ⩽ 2.8%) and high confinement (H98y2 ⩽ 1.4) in the ITER similar shape have achieved zero surface loop voltage for up to two current relaxation times using efficient central current drive from ECCD and NBCD. The n = 3 RMP causes surprisingly little increase in thermal transport during ELM suppression. Poloidal magnetic flux pumping in hybrid plasmas maintains q above 1 without loss of current drive efficiency, except that experiments show that extremely peaked ECCD profiles can create sawteeth. During ECCD, Alfvén eigenmode (AE) activity is replaced by a more benign fishbone-like mode, reducing anomalous beam ion diffusion by a factor of 2. While the electron and ion thermal diffusivities substantially increase with higher ECCD power, the loss of confinement can be offset by the decreased fast ion transport resulting from AE suppression. Extrapolations from DIII-D along a dimensionless parameter scaling path as well as those using self-consistent theory-based modeling show that these ELM-suppressed, fully non-inductive hybrids can achieve the Qfus = 5 ITER steady-state mission.

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Section: Sawtooth Activitymentioning

confidence: 99%

Advances in the steady-state hybrid regime in DIII-D—a fully non-inductive, ELM-suppressed scenario for ITER

et al. 2017

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“…Magnetic islands flatten the pressure profile at the O-point when the parallel transport inside the island dominates over the gradient restoring cross-field transport. This flat spot gives rise to a bootstrap current perturbation [4][5][6] that reinforces the NTM growth, as described by the modified Rutherford equation [7,8]. In the presence of small-scale turbulence, the NTM stability could be changed.…”

Section: Introductionmentioning

confidence: 97%

Effect of magnetic islands on profiles, flows, turbulence and transport in nonlinear gyrokinetic simulations

Navarro

Bardóczi

Carter

et al. 2017

Plasma Phys. Control. Fusion

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Neoclassical tearing modes have deleterious effects on plasma confinement and, if they grow large enough, they can lead to discharge termination. Therefore, they impose a major barrier in the development of operating scenarios of present-day tokamaks. Gyrokinetics offers a path toward studying multi-scale interactions with turbulence and the effect on plasma confinement. As a first step toward this goal, we have implemented static magnetic islands in nonlinear gyrokinetic simulations with the GENE code. We investigate the effect of the islands on profiles, flows, turbulence and transport and the scaling of these effects with respect to island size. We find a clear threshold island width, below which the islands have little or no effect while beyond this point the islands significantly perturb flows, increase turbulence and transport. Additionally, we study the effect of radially asymmetric islands on shear flows for the first time. We find that island induced shear flows can regulate turbulent fluctuation levels in the vicinity of the island separatrices. Throughout this work, we focus on experimentally relevant quantities, such as rms levels of density and electron temperature fluctuations, as well as amplitude and phasing of turbulence modulation. These simulations aim to provide guidelines for interpreting experimental results by comparing qualitative trends in the simulations with those obtained in tokamak experiments.

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“…The m/n = 2/1 neoclassical tearing mode (NTM) is a major impediment in the development of operational scenarios of present and future tokamaks, as it degrades plasma confinement and, if large enough, often locks to the wall, slows down the plasma rotation and leads to plasma termination [1,2] (m/n are the poloidal/toroidal mode numbers). NTMs are destabilized by a helical bootstrap current perturbation (δj BS ) arising due to 'missing' pressure gradient (∇p) at the magnetic island O-point [3][4][5]. NTM stabilization has been achieved by electron cyclotron current drive (ECCD) in various H-mode scenarios by replacing δj BS at the mode rational surface [6][7][8][9].…”

Section: Introductionmentioning

confidence: 99%

Controlled neoclassical tearing mode (NTM) healing by fueling pellets and its impact on electron cyclotron current drive requirements for complete NTM stabilization

et al. 2019

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Controlled partial stabilization of core m/n = 2/1 Neoclassical Tearing Modes (NTMs) by fueling deuterium pellets is reported in DIII-D and KSTAR H-mode plasmas (m/n are the poloidal/toroidal mode numbers). Analyses of DIII-D data exploring possible physics origins show that an explanation is offered by NTM-turbulence multi-scale interaction, triggered by a sudden increase of local gradients near q = 2 caused by the pellet. Pellet injection from the high-field side allows deep fueling which reaches the island region. In turn, low-k turbulent density fluctuations ( n) increase by 30% in the island region. This n can drive transport across the island separatrices, reducing the pressure flat spot at the O-point and diminishing the NTM drive. The Mirnov probe array detects the reduction of the 2/1 magnetic amplitude by up to 20%. Causality between elevated gradients outside of the island, turbulence spreading into the island and reduced NTM drive is qualitatively supported by non-linear gyrokinetic turbulence simulations. These show increased penetration of ion-scale n from the background plasma to the O-point region when the background gradient is increased. This interaction has potentially far reaching consequences as it can lead to a reduction of the required electron cyclotron current density (j ECCD ) for NTM suppression by 70%, as predicted by the modified Rutherford equation. This beneficial effect of fueling pellets can be important as j ECCD is the anticipated active NTM control technique for the International Thermonuclear Experimental Reactor (ITER), but its efficiency will be lowered by third harmonic absorption in Pre-Fusion Power Operation-1 (PFPO-1) at half magnetic field.

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Spatiotemporal changes in the pressure-driven current densities on DIII-D due to magnetic islands

Cited by 6 publications

References 39 publications

Advances in the steady-state hybrid regime in DIII-D—a fully non-inductive, ELM-suppressed scenario for ITER

Advances in the steady-state hybrid regime in DIII-D—a fully non-inductive, ELM-suppressed scenario for ITER

Effect of magnetic islands on profiles, flows, turbulence and transport in nonlinear gyrokinetic simulations

Controlled neoclassical tearing mode (NTM) healing by fueling pellets and its impact on electron cyclotron current drive requirements for complete NTM stabilization

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