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 (ñ_e) and in the plasma floating potential (φ̃ _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≈1, T_e≈20-40 eV, and n_e≈10¹² cm^-3 for a line-averaged electron density n̄_e = (3-6)×10 ¹² cm^-3. The relative fluctuation levels decrease as the probe is moved into the core plasma. For T_e>20 eV, ñ_e/n_e≈5%, and eφ̃_f/T _e≈2ñ_e/n_e at r/a = 0.95. The measured fluctuation spectra are broadband (40-300 kHz) with k̄ρ_s≈ (0.05-0.1), where k̄ is the average wave number of the fluctuations and ps is the ion Larmor radius at the sound speed. Near the LCFS, the density fluctuations can be approximated by ñ_e/n_e ≈0.4/k̄L_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_α spectroscopic measurements. Many of the features seen in the ATF edge fluctuations resemble those of Ohmically heated plasmas in the Texas Experimental Tokamak (TEXT) [Phys. Rev. Lett. 62, 1844 (1989)].