Magnetic fluctuations in currentless plasmas in the ATF torsatron

Abstract

Measurements of poloidal magnetic field fluctuations outside currentless, finite-beta (⟨β⟩ ≤ 0.5%, β0 ≤ 3%) plasmas with peaked pressure profiles in the Advanced Toroidal Facility (ATF) torsatron reveal bands of small (B̃θ/Bϕ ∼ 10−5), coherent fluctuations in the frequency range 5-60 kHz. The geometrical structure of these fluctuations shows n = 1 toroidal mode symmetry, with inferred poloidal mode numbers of m = 2 and 3. The coherent Bθ amplitudes increase with ⟨B⟩ for ⟨B⟩ ≲ 0.3%, and then saturate and begin to decrease with ⟨B⟩ for ⟨B⟩ ≳ 0.3%.