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 n_e and temperature T_e and fluctuations in density (ñ_e) and plasma floating potential ( \ ̃gf_f) are made in ECH plasmas at 1 T. 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 n_e = (3-6) × 10¹² cm^-3. Relative fluctuation levels, as the FRLP is moved into core plasma where T_e > 20 eV, are n ̃_e n_e ≈ 5%, and e \ ̃gf T_e = 2 n ̃_e n_e about 2 cm inside the LCFS. The observed fluctuation spectra are broadband (40-300 kHz) with kρ_s ≤ 0.1, where k is the wavenumber of the fluctuations and p_s is the ion Larmor radius at the sound speed. The propagation direction of the fluctuations reverses to the electron diamagnetic direction around r a < 1. The phase velocity and the electron drift velocity are comparable (v_ph ~ v_de). The fluctuation-induced particle flux is comparable to fluxes estimated from the particle balance using the H_α spectroscopic measurements. Many of the features seen in these experiments resemble the features of ohmically heated plasmas in the Texas Experimental Tokamak (TEXT).