Reduction or elimination of edge localized modes (ELMs) while maintaining high confinement is essential for future fusion devices, e.g., the ITER. An ELM-free regime was recently obtained in the National Spherical Torus Experiment, following lithium (Li) evaporation onto the plasma-facing components. Edge stability calculations indicate that the pre-Li discharges were unstable to low-n peeling or ballooning modes, while broader pressure profiles stabilized the post-Li discharges. Normalized energy confinement increased by 50% post Li, with no sign of ELMs up to the global stability limit.

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m=n 3=1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m=n 2=1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer. Helical magnetic field perturbations are introduced in tokamak plasmas to study, on the one hand, the ergodic divertor concept [1,2] and, on the other hand, the interaction of such perturbations with the magnetohydrodynamics (MHD) stability of the plasma [3,4]. Recent experiments, for instance, suggest a control method to mitigate edge localized modes while maintaining the pedestal pressure and thus plasma confinement [5][6][7]. However, open questions remain, in particular, with regard to the influence on the momentum transport of the plasma. Indeed, one motivation to equip the tokamak TEXTOR with the Dynamic Ergodic Divertor (DED) [8] was to be able to study the interaction between helical magnetic field perturbations and plasma transport and stability.The DED consists of 16 magnetic perturbation coils (four quadruples), plus two additional coils for the compensation of the magnetic field imperfections at the feeder regions of the coils. The coils wind helically around the inner side of the torus (major radius: R 1:75 m; minor radius of the circular plasma cross section typically a 0:47 m) with a pitch corresponding to the magnetic field lines of the magnetic flux surface with a safety factor of q 3. Depending on the choice of coil connections to the power supplies, base modes with different poloidal and toroidal mode numbers can be produced. For the DED these are m=n 12=4, 6=2, and 3=1. The penetration depth into the plasma strongly depends on the mode numbers: While the m=n 12=4 affects the edge plasma only, the m=n 3=1 mode reaches into the plasma center (the maximum radial magnetic field component achievable by the DED at the q 2 surface is 10 ÿ3 of the total magnetic field).In this Letter we present results obtained by the m=n 3=1 mode operation. Covering about one-third of the poloidal cross section of the torus, the mode spectrum of the DED does not contain many sidebands. For the m=n 3=1 configuration the three dominant resonant components inside the plasma are m 1, 2, and 3. In Fig. 1 their strengths at the respective resonances are PRL 94, 015003 (2005) P H Y S I C A L

Plasma-wall interaction leads to the release of impurities and neutrals of the working gas, which contribute significantly to the energy losses from the plasma edge, and therefore, crucially affects the development of thermal instabilities in fusion devices. An analytical model for impurity radiation is proposed, which takes into account the erosion mechanisms of wall material and the motion of impurity particles across magnetic surfaces. The temperature dependence of radiation losses is found to be very different from that predicted by the coronal approximation often used in considering thermal instabilities. The consequences for the development of poloidally symmetric detachment and multi-faceted asymmetric radiation from the edge ͑MARFE͒ are analyzed. It is demonstrated that the MARFE threshold principally depends on the mechanism by which working gas neutrals are released from the wall and on the neutral's properties, e.g., their ionization rate.

A significant influence of the dynamic ergodic divertor (DED) on the density limit in TEXTOR has been found. In Ohmic discharges, where without DED detachment normally arises at the density limit, a MARFE (multifaceted asymmetric radiation from the edge) develops when the DED is operated in a static regime. The threshold of the MARFE onset in the neutral beam heated plasmas is increased by applying 1 kHz ac DED at the high-field side. The theoretical predictions based on the parallel energy balance taking poloidal asymmetries into account agree well with the experimental observation.

ABSTRACT:The robustness of impurity-seeded radiative mantle solutions for ITER to uncertainties in several physics and operating parameters is examined. Our results indicate that -50 -90 % of the input power can be radiated h m inside the separatrix with Ne, Ar and Kr injection, without significant detriment to the core power balance or collapse of the edge temperature profile, for a wide range of conditions on the impurity pinch velocity, edge temperature pedestal, and plasma density.