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Suppression of Large Edge Localized Modes with a Stochastic Magnetic Boundary in High Confinement DIII-D Plasmas

Description: Large sub-millisecond heat pulses due to Type-I ELMs have been reproducibly eliminated in DIII-D for periods approaching 7 energy confinement times with small dc currents driven in a simple magnetic perturbation coil. The current required to eliminate all but a few isolated Type-I ELM impulses during a perturbation coil pulse lasting several seconds is less than 0.4% of plasma current. Based on vacuum magnetic field line modeling, perturbation fields from the coil resonate strongly with plasma flux surfaces across most of the pedestal region (0.9 {le} {psi}{sub N} {le} 1.0) when q95 = 3.7{plus_minus}0.2 creating small remnant magnetic islands surrounded by weakly stochastic field lines. Under the best ELM suppression conditions, the stored energy, {beta}N and H-mode quality factor are unaffected by the perturbation field along with the electron pressure profile, radial electric field and poloidal rotation across the pedestal. Consequently, the H-mode transport barrier and global energy confinement time is also unaltered in these cases. Although some isolated ELM-like events typically occur during the perturbation coil pulse, long periods free of large Type-I ELMs ({Delta}t > 4-6 {tau}{sub E}) have been reproduced numerous times, on multiple experimental run days. Several Type-I ELM suppression and modification behaviors have been identified and studied over a range of discharge conditions including those matching the ITER scenario 2 flux surface shape and aspect ratio scaled down by a factor of 3.5 to fit in the DIII-D vacuum vessel. Since large Type-I ELM impulses represent a severe constraint on the survivability of the divertor target plates in future fusion devices such as ITER, a proven method of eliminating these impulses is critical for the development of tokamak reactors. Results presented in this paper suggest that non-axisymmetric edge magnetic perturbations could be a promising option for controlling ELMs in future tokamaks such as ITER.
Date: October 18, 2004
Creator: Evans, T E; Moyer, R A; Watkins, J G; Osborne, T H; Thomas, P R; Becoulet, M et al.
Partner: UNT Libraries Government Documents Department

Edge Localized Mode Control in DIII-D Using Magnetic Perturbation-Induced Pedestal Transport Changes

Description: Edge localized mode (ELM) control is a critical issue for ITER because the impulsive power loading from ELMs is predicted to limit the divertor lifetime to only a few hundred full-length pulses. Consequently, a technique that replaces the ELM-induced transport with more continuous transport while preserving the H-mode pedestal height and core performance would significantly improve the viability of ITER. One approach is to use edge resonant magnetic perturbations (RMPs) to enhance pedestal transport enough to reduce the pedestal pressure gradient {del}p{sub ped} below the stability limit for Type I ELMs. In DIII-D, n = 3 RMPs have been used to eliminate Type I ELMs when the edge safety factor is in the resonant window q95 {approx} 3.5 without degrading confinement in H-modes with ITER-relevant pedestal collisionalities v*{sub e} {approx} 0.2. The RMP reduces {del}p{sub ped} as expected, with {del}p{sub ped} controlled by the RMP amplitude. Linear peeling-ballooning (P-B) stability analysis indicates that the ELMs are suppressed by reducing {del}p{sub ped} below the P-B stability limit. The {del}p{sub ped} reduction results primarily from an increase in particle transport, not electron thermal transport. This result is inconsistent with estimates based on quasi-linear stochastic diffusion theory based on the vacuum field (no screening of the RMP). The particle transport increase is accompanied by changes in toroidal rotation, radial electric field, and density fluctuation level {tilde n} in the pedestal, suggesting increased fluctuation-driven particle transport.
Date: September 27, 2006
Creator: Moyer, R A; Burrell, K H; Evans, T E; Fenstermacher, M E; Joseph, I; Osborne, T H et al.
Partner: UNT Libraries Government Documents Department