Potentials, E x B drifts, and fluctuations in the DIII-D boundary

Reciprocating Langmuir probes are used to investigate the structure of electrostatic potentials, E x B drifts, and fluctuations in the edge (rho < 1), scrape-off layer (SOL) and divertor of single null diverted discharges in the DIII-D tokamak. These measurements demonstrate that the X-point geometry suppresses potential fluctuations in the drift wave range of frequencies (20 kHz less than or equal to f less than or equal to 300 kHz) as predicted by theory [N. Matter and R.N. Cohen, Phys. Plasmas 2 (1995) 4042], and suppresses quasi-coherent modes in the edge of II-modes. Consequently, edge turbulence and ballooning mode stability calculations, such as those used in L-H transition and H mode pedestal theories, must incorporate realistic X-point magnetic geometry to quantitatively reproduce experimental results. In the divertor plasma, root-mean-square potential fluctuations <(phi)over tilde>(fl), are found to be larger in H mode than in L mode, and in detached versus attached divertor plasmas. These measurements have been used to benchmark turbulence simulations with two unique codes that incorporate realistic X-point magnetic geometry: the 3-D boundary turbulence code BOUT and the BAL shooting code with high-n ballooning formalism. (C) 1999 Elsevier Science B.V. All rights reserved.