Experimental investigation of the radial structure of energetic particle driven modes

Alfv\'en eigenmodes (AEs) and energetic particle modes (EPMs) are often excited by energetic particles (EPs) in tokamak plasmas. One of the main open questions concerning EP driven instabilities is the non-linear evolution of the mode structure. The aim of the present paper is to investigate the properties of beta-induced AEs (BAEs) and EP driven geodesic acoustic modes (EGAMs) observed in the ramp-up phase of off-axis NBI heated ASDEX Upgrade (AUG) discharges. This paper focuses on the changes in the mode structure of BAEs/EGAMs during the non-linear chirping phase. Our investigation has shown that in case of the observed down-chirping BAEs the changes in the radial structure are smaller than the uncertainty of our measurement. This behaviour is most probably the consequence of that BAEs are normal modes, thus their radial structure strongly depends on the background plasma parameters rather than on the EP distribution. In the case of rapidly upward chirping EGAMs the analysis consistently shows shrinkage of the mode structure. The proposed explanation is that the resonance in the velocity space moves towards more passing particles which have narrower orbit widths.Comment: submitted to Nuclear Fusio

Coupling JOREK and STARWALL for Non-linear Resistive-wall Simulations

The implementation of a resistive-wall extension to the non-linear MHD-code JOREK via a coupling to the vacuum-field code STARWALL is presented along with first applications and benchmark results. Also, non-linear saturation in the presence of a resistive wall is demonstrated. After completion of the ongoing verification process, this code extension will allow to perform non-linear simulations of MHD instabilities in the presence of three-dimensional resistive walls with holes for limited and X-point plasmas.Comment: Contribution for "Theory Of Fusion Plasmas, Joint Varenna - Lausanne International Workshop, Villa Monastero, Varenna, Italy (27.-31.8.2012)", accepted for publication in Journal of Physics Conference Serie

Neoclassical tearing modes on AUG: improved scaling laws, high confinement at high β_N and new stabilization experiments

The accuracy of the scaling laws derived so far for the normalised beta values at the onset of neoclassical tearing modes is limited as the results depend on the presence and magnitude of seed islands. Therefore power ramp down experiments have been performed on ASDEX Upgrade, allowing to find a scaling law for the critical β value at which the NTMs disappear. For (m,n)=(3,2) NTMs these critical beta values have been found to scale nearly proportional to ρ*. As it has been recently found on ASDEX Upgrade, at high βN values there is a regime in which (3,2) NTMs cause a much smaller confinement degradation as one would expect from the β dependence of the saturated island size. The transition to this regime allows high confinement (H = 1) at high beta values ( βN > 2.3) on ASDEX Upgrade in spite of the presence of (3,2) NTMs. The plasma conditions for the transition into such a high confinement regime are discussed in detail. Furthermore, new results on NTM stabilization by localized ECCD will be presented, showing that NTMs remain stabilized even with increased heating power and thus normalized beta values well above the NTM threshold