CHARACTERISTICS OF EDGE PLASMA TURBULENCE ON THE ATF TORSATRON

Measurements of electrostatic fluctuations on the edge of the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] are used to study the role of the edge turbulence in the particle transport in this current-free magnetic configuration. Spatial profiles of the plasma electron density n(e), temperature T(e), and fluctuations in density (n approximately e) and in the plasma floating potential (PHI-approximately f) are measured at the edge in electron cyclotron heated plasmas using a Langmuir probe array. At the last closed flux surface (LCFS), r/a almost-equal-to 1, T(e) almost-equal-to 20-40 eV, and n(e) almost-equal-to 10(12) cm-3 for a line-averaged electron density nBAR(e) = (3-6) X 10(12) cm-3. The relative fluctuation levels decrease as the probe is moved into the core plasma. For T(e) > 20 eV, n approximately e/n(e) almost-equal-to 5%, and e-phi approximately f/T(e) almost-equal-to 2n approximately e/n(e) at r/a = 0.95. The measured fluctuation spectra are broadband (40-300 kHz) with kBAR-rho-s almost-equal-to (0.05-0.1), where kBAR is the average wave number of the fluctuations and rho-s is the ion Larmor radius at the sound speed. Near the LCFS, the density fluctuations can be approximated by n approximately e/n(e) almost-equal-to 0.4/kBAR(L)n, where L(n) is the gradient scale length of n(e). The propagation direction of the fluctuations reverses to the electron diamagnetic direction at r/a < 1. The phase velocity of the fluctuations and the electron drift velocity are comparable. The particle flux estimated from the fluctuations is consistent with fluxes obtained from the particle balance using the H-alpha spectroscopic measurements. Many of the features seen in the ATF edge fluctuations resemble those of Ohmically heated plasmas in the Texas Experimental Tokamak (TEXT).