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A potential technique for suppressing edge localized modes is theoretically analyzed. Recent experiments have shown that externally generated resonant magnetic perturbations (RMPs) can stabilize edge localized modes (ELMs) by modifying the density profile [T. E. Evans et al., Nat. Phys. 2, 419 (2006); Y. Liang et al., Phys. Rev. Lett. 98, 265004 (2007)]. Driving toroidally asymmetric current internally through the scrape-off layer (SOL) plasma itself can also generate RMPs that are close to the required threshold for ELM control. Ion saturation current densities can be achieved by producing potential differences on the order of the electron temperature. Although the threshold is uncertain in future devices, if driven coherently through the SOL, the upper limit for the resulting perturbation field would exceed the present experimental threshold. This analysis provides the tools required for estimating the magnitude of the coherent SOL current and RMP generated via toroidally asymmetric biasing of the target. Flux expansion increases the perturbation near the X-point, while phase interference due to the shearing of field lines near the X-point reduces the amplitude of the effective SOL perturbation and makes the result sensitive to both toroidal mode number n and the phasing at the target plate. If the current density driven at the target plate decays radially, the amplitude over the useful coherence width of the current profile will be reduced. The RMP can still exceed the present threshold at low n if the radial location and width of the biasing region are optimally chosen. [ABSTRACT FROM AUTHOR]

The structure of the magnetic field perturbations due to nonaxisymmetric field-aligned currents in the tokamak scrape-off layer (SOL) are analytically calculated near the X-point. Paper I [I. Joseph et al., Phys. Plasmas 16, 052510 (2009)] demonstrated that biasing divertor target plates in a toroidally asymmetric fashion can generate an appreciable toroidally asymmetric parallel current density in the SOL along the separatrix. Here, the magnetic field perturbation caused by a SOL current channel of finite width and stepwise constant amplitude at the target plate is derived. Flux expansion amplifies the magnetic perturbation near the X-point, while phase interference causes the SOL amplitude to be reduced at large toroidal mode number. Far enough from the current channel, the magnetic field can be approximated as arising from a surface current near the separatrix with differing amplitudes in the SOL and the divertor leg. The perturbation spectrum and resonant components of this field are computed analytically asymptotically close to the separatrix in magnetic flux coordinates. The size of the stochastic layer due to the applied perturbation that would result without self-consistent plasma shielding is also estimated. If enough resonant field is generated, control of the edge pressure gradient may allow stabilization of edge localized modes. [ABSTRACT FROM AUTHOR]

A critical issue for fusion-plasma research is the erosion of the first wall of the experimental device due to impulsive heating from repetitive edge magneto-hydrodynamic instabilities known as 'edge-localized modes' (ELMs). Here, we show that the addition of small resonant magnetic field perturbations completely eliminates ELMs while maintaining a steady-state high-confinement (H-mode) plasma. These perturbations induce a chaotic behaviour in the magnetic field lines, which reduces the edge pressure gradient below the ELM instability threshold. The pressure gradient reduction results from a reduction in the particle content of the plasma, rather than an increase in the electron thermal transport. This is inconsistent with the predictions of stochastic electron heat transport theory. These results provide a first experimental test of stochastic transport theory in a highly rotating, hot, collisionless plasma and demonstrate a promising solution to the critical issue of controlling edge instabilities in fusion-plasma devices. [ABSTRACT FROM AUTHOR]