Resonant Pedestal Pressure Reduction Induced by a Thermal Transport Enhancement due to Stochastic Magnetic Boundary Layers in High Temperature Plasmas

Abstract

Good alignment of the magnetic field line pitch angle with the mode structure of an external resonant magnetic perturbation (RMP) field is shown to induce modulation of the pedestal electron pressure p e in high confinement high rotation plasmas at the DIII-D tokamak with a shape similar to ITER, the next step tokamak experiment. This is caused by an edge safety factor q 95 resonant enhancement of the thermal transport, while in contrast, the RMP induced particle pump out does not show a significant resonance. The measured p e reduction correlates to an increase in the modeled stochastic layer width during pitch angle variations matching results from resistive low rotation plasmas at the TEXTOR tokamak. These findings suggest a field line pitch angle resonant formation of a stochastic magnetic edge layer as an explanation for the q 95 resonant character of type-I edge localized mode suppression by RMPs. The impact of periodic perturbations on strongly coupled media reveal common physical features in very different physical states. In the dusty material rings around Saturn, for example, gaps are formed by gravitational resonances between the periodic motion of Saturn's moons and the strongly collisional material of the dynamic outer ring [1]. A comparable resonant mechanism is also employed for fine-tuning of the strong confining magnetic field in tokamak experiments. Here, the edge magnetic field line trajectories are perturbed by an external resonant magnetic perturbation (RMP) field with a mode structure aligned to the magnetic field line pitch angle on selected rational surfaces in the plasma edge. The periodic kicks experienced by the toroidally revolving field lines lead to the formation of an open stochastic system [2] which is a promising candidate for control of the self-organized plasma edge pedestal formed in high confinement (H-mode) plasmas [3]. In this regime, steep edge pressure gradients generate large low frequency type-I edge localized modes (ELMs) [4]. They induce transient outward heat and particle fluxes, which are expected to limit the lifetime of the divertor and first wall in the next step tomakak experiment ITER [5], potentially also degrading the plasma performance by enhanced impurity release. Therefore, the control of the ELM instabilities is a high priority physics issue for ITER. Using a periodic perturbation of the field line trajectories forming a stochastic magnetic edge layer applies a generic physics mechanism for improvement of this man-made high energy state.The complete suppression of type-I ELMs by application of small edge RMP fields, having a dominant toroidal mode number n ¼ 3, was demonstrated at DIII-D [6] and explored for ITER similar shape (ISS) plasmas with high averaged triangularity " $ 0:5 at ITER-relevant, low pedestal electron collisionality à e $ 0:1 [7]. This led to a proposal for a RMP coil set for ITER [8]; for preparation of this undertaking, plans are being made to equip practically every large tokamak in the world with RMP coils. For these projects and th...

Influence of the Resonant Magnetic Perturbation on the Plasma Boundary in DIII‐D

“…At low collisionalities the structure of the footprint patterns is in fair agreement with the "vacuum" topology, however at high collisionalities the splitting of the separatrix is much stronger than predicted, which suggests the presence of a plasma responses that contributes to an enhanced splitting of the separatrix compared to predictions based on vacuum magnetic field line modeling. In H-mode plasmas the radial pressure profile at the plasma boundary is a strong function of q 95 [31] in line with other experiments, e.g. TEXTOR [6].…”

“…A strong poloidal and toroidal modulation of plasma temperature and density has been observed on TEXTOR [6,22] and recently also in DIII-D [31]. In the last reference the T e dependence on q 95 was analyzed by means of two diagnostics: Electron cyclotron emission (ECE) and Thomson scattering as shown in figure 2.…”

Multiresonance Effect in Type-I Edge-Localized Mode Control With LownFields on JET

“…Reference [17] concluded that the resonances in the edge pressure were consistent with achieving good alignment of the magnetic field with the perturbation, suggesting the resonant formation of a stochastic layer. However, the results from JET presented in this Letter are not consistent with this interpretation, as demonstrated in Fig.…”

“…On DIII-D, strong dependence of the edge pedestal electron pressure on the edge safety factor q 95 has been observed in high confinement, high rotation plasmas during application of an n ¼ 3 field [17]. Reference [17] concluded that the resonances in the edge pressure were consistent with achieving good alignment of the magnetic field with the perturbation, suggesting the resonant formation of a stochastic layer.…”

Changes in density fluctuations as a result of resonant magnetic perturbations correlate with the density inverse scale length

“…In DIII-D, RMPs eliminate ELMs by reducing the pedestal pressure gradient below the peeling-ballooning stability limit 6 and by increasing particle transport [7][8][9] and electron thermal transport in low collisionality plasmas. 10 The density pump-out ranges from 5% to 30% as a percentage of the line averaged density in the ELMing H-mode prior to the application of the RMP coils. It is critical to understand the increase in particle transport as a result of RMPs, in order to limit the density pump-out to achieve the maximum plasma performance in future fusion devices.…”