Recent Results from the Electric Tokamak
Author: Lothar W. Schmitz
Submitted: 2005-12-21 17:06:04
Co-authors: T.A Carter, P.-A. Gourdain, J,-L. Gauvreau, D. Pace, R.J. Taylor, A.E. White
University of California, Los Angeles
STR Building, 1040 Veteran Ave
Los Angeles, CA 90095-0
Ohmic plasmas in the Electric Tokamak routinely reach the density limit and approach the beta limit with beta_n ~ 2. Upgraded profile diagnostics (Thomson scattering, interferometry, and Helium line ratio measurements) have been used to conduct detailed studies of the particle balance and density limit physics. Strong central density accumulation is found experimentally, and the role of pinch contributions to the particle balance has been investigated. The density accumulation is terminated by internal MHD events (2/1 modes). Spontaneous poloidal rotation exceeding ion diamagnetic rotation is observed. This rotation is believed to be driven by ion orbit loss. With a positively biased electrode inserted past the last closed flux surface, the polarity of the radial electric field (and poloidal rotation) can be reversed, and particle accumulation is partially mitigated. The edge radial electric field, density, and poloidal/toroidal plasma rotation profiles have been obtained in the closed flux surface region (r/a >0.85) and in the scrape-off layer, using radially extended Mach probe arrays (located outboard/inboard and on top of the device). Internal poloidal rotation has been obtained by reflectometry. Radial ion temperature profiles are obtained using a two-sided ion energy analyzer. The negative edge radial electric field observed on closed flux surfaces exceeds the ambipolar neoclassical field, given by E_r = -kT_i/e (d(ln n)/dr – alpha (d(ln kT_i)/dr)). Evidence for increased radial ion orbit loss in ICRF heated plasmas is found (increased negative radial electric field and decreased central particle accumulation) The scaling of the electric field and plasma rotation with neutral and ion collisionality (0.2 < nu*_edge < 5) is investigated.