ATF edge plasma turbulence studies using a fast reciprocating Langmuir probe

Electrostatic turbulence on the edge of the Advanced Toroidal Facility (ATF) torsatron is investigated experimentally with a fast reciprocating Langmuir probe (FRLP) array. Initial measurements of plasma electron density ne and temperature Te and fluctuations in density (ne) and plasma floating potential (φf) are made in ECH plasmas at 1 T. At the last closed flux surface (LCFS, r/anti a∝1), Te≅20-40 eV and ne≅1012 cm-3 for a line-averaged electron density anti ne=(3-6)x1012 cm-3. Relative fluctuation levels, as the FRLP is moved into core plasma where Te>20 eV, are ne/ne≅5%, and eφf/Te≅2ne/ne about 2 cm inside the LCFS. The observed fluctuation spectra are broadband (40-300 kHz) with anti kρs≤0.1, where anti k is the wavenumber of the fluctuations and ρs is the ion Larmor radius at the sound speed. The propagation direction of the fluctuations reverses to the electron diamagnetic direction around r/anti a<1. The phase velocity and the electron drift velocity are comparable (vph∝vde). The fluctuation-induced particle flux is comparable to fluxes estimated from the particle balance using the Hα spectroscopic measurements. Many of the features seen in these experiments resemble the features of ohmically heated plasmas in the Texas Experimental Tokamak (TEXT). (orig.)