Measurement of edge currents in DIII-D and their implication for pedestal stability

The present performance limits of tokamak discharges are strongly coupled to the stability and transport properties of the edge plasma. Both experimental and modeling efforts have shown a clear connection between the edge pressure pedestal height and core plasma confinement. The key to understanding the stability and performance limits of the pedestal revolves around an accurate knowledge of the plasma current in this region. Using the Zeeman effect in an injected 30 keV lithium beam, we have measured the currents in the edge of the DIII-D [J. L. Luxon, Nucl. Fusion 42, 6114 (2002)] tokamak for various confinement modes. This method of determining j(r) is insensitive to the large electric fields which coexist in the pedestal region and which complicate motional Stark effect measurements. For the high confinement cases, where substantial pedestal pressures exist, we find large (similar to MA/m(2)), localized (Delta R similar to 1-2 cm) currents in the pedestal region, located near the maximum in the pressure gradient. These values are consistent with calculations of edge bootstrap current using the neoclassical NCLASS [W. A. Houlberg, K. C. Shaing, S. P. Hirshman, and M. C. Zarnstorff, Phys. Plasmas 4, 3230 (1997)] and Sauter [O. Sauter, C. Angioni, and Y. R. Lin-Lin, Phys. Plasmas 6, 2834 (1999)] models and the measured pedestal density and temperature profiles. The apparent consistency of the measured j(EDGE) with neoclassical predictions occurs despite the violation of one of the fundamental tenets of the theory, namely, epsilon=rho(i)/L-P < 1, where rho(i) is the ion poloidal gyroradius and L-p is the pressure gradient scale length. The measured j(EDGE) has also been used to generate self-consistent reconstructions using the free boundary equilibrium solvers CORSICA [T. A. Casper, T. B. Kaiser, R. A. Jong, L. L. LoDestro, J. Moller, and L. D. Pearlstein, Plasma Phys. Controlled Fusion 45, 1193 (2003)] and EFIT [L. L. Lao, H. E. St. John, R. D. Stambough, A. G. Kellman, and W. Pfeiffer, Nucl. Fusion 25, 1611 (1985)]. These equilibria allow us, in conjunction with the edge localized instabilities in tokamak experiments [P. B. Snyder, H. R. Wilson, J. R. Ferron, L. L. Lao, A. W. Leonard, T. H. Osborne, A. D. Turnbull, D. Mossessian, M. Murakami, and X. Q. Xu, Phys. Plasmas 9, 2037 (2002); H. R. Wilson, P. B. Snyder, G. T. A. Huysmans, and R. L. Miller, Phys. Plasmas 9, 1277 (2002)] magnetohydrodynamic stability code, to assess the linear stability of the edge to peeling/ballooning modes. These results are then compared to the measured edge localized mode onset conditions and again good agreement is found between the experimental and model limits on the maximum permissible j(EDGE). (c) 2005 American Institute of Physics.