The role of edge resonant magnetic perturbations in edge-localized-mode suppression and density pump-out in low-collisionality DIII-D plasmas

Abstract

Two-fluid nonlinear MHD simulations using the TM1 code demonstrate that the formation of magnetic islands at the top and bottom of the H-mode pedestal, together with the strong screening of resonant fields in the gradient region of the pedestal, can account for ELM suppression and density pump-out by n = 2 Resonant Magnetic Perturbations (RMPs) in lowcollisionality DIII-D ITER Similar Shape (ISS) plasmas. Using experimentally relevant transport coefficients, neoclassical resistivity, electron collisionality, and RMP amplitudes, nonlinear MHD simulations reproduce the observed level of density reduction (density pumpout) in DIII-D due the formation of narrow magnetic islands and resulting enhanced collisional transport in the resistive foot of pedestal. For large amplitude RMPs (Br/Bt>1´10 -4 ) simulations predict field penetration and pressure reduction at the top of the pedestal consistent with experimental observations at the onset of ELM suppression. The predicted reduction in the height and width of the pedestal by magnetic island enhanced transport provides a quantitative mechanism for the stabilization of the Peeling-Ballooning Mode (PBM). Importantly, these simulations predict strong screening of resonant fields in the steep gradient region of the pedestal due to strong E´B and diamagnetic flows. However, if the plasma resistivity is made artificially larger (~10X) than neoclassical, the simulations predict magnetic stochasticity throughout the plasma edge and the collapse of the pedestal due to the reduction in the penetration threshold with increasing resistivity. A scaling relation for resonant field penetration at the pedestal top, using several hundred nonlinear simulations, reproduces the density and E´B dependence of the ELM suppression threshold observed in DIII-D. Simulations using ITER model equilibria suggest that the penetration threshold at the top of the ITER pedestal will be much lower than in DIII-D due to the anticipated lower perpendicular flow velocities in ITER, resulting in the weaker screening of resonant fields.

Effect of bootstrap current on the onset of neoclassical tearing mode

“…It is known theoretically that RMPs can increase (decrease) the local electron density of the m/n=0/0 component around the resonant surface for the plasma rotation in the ion (electron) drift direction [23,24], as also observed in experiments [25,[27][28][29][30]]. In agreement with numerical calculation results, the quasi-linear formula shows that [23] rs(ne)ne0=d1…”

Wide Operational Windows of Edge-Localized Mode Suppression by Resonant Magnetic Perturbations in the DIII-D Tokamak

“…Ref. [20]), we estimate that Bth/BT decreases by ≈ 50% going from high to low pedestal density in Fig. 1.…”

“…Some progress has been made in understanding the role of magnetic islands on the q95 windows of ELM suppression based vacuum field analysis [7,18]. However, such calculations typically produce extensive regions of magnetic stochasticity that are inconsistent with the high edge temperatures characteristic of H-mode [19,20]. It is now understood that models must take into account the screening response [11,[21][22][23] of high temperature fusion plasmas and the associated transport [24][25][26] in order to establish consistency with experimental observations.…”

“…Using nonlinear two-fluid MHD simulations, it was recently shown that the observed density and EB rotational dependence of ELM suppression and density pump-out in DIII-D ITER Similar Shape (ISS) plasmas can be understood from the conditions required for resonant field penetration and the resulting collisional transport. [20,28]. These simulations, based on the TM1 code, used experimental profiles and plasma parameters taken before the RMP was applied, and predicted the profile evolution resulting from the penetration and screening of resonant fields.…”

Pedestal electron collisionality and toroidal rotation during ELM-crash suppression phase under n = 1 RMP in KSTAR

“…e;ped > 0.9 plasmas. Recent nonlinear two-fluid MHD simulation using TM1 27,28 suggests that the reduced jx ? ;e j in an edge region of high rotation plasma can allow achieving the ELM-crash suppression in high collisionality (or density) conditions.…”