Magnetic fluctuations in currentless plasmas in the ATF torsatron

This is the authors' abstract. We don't add key points for this paper.

Measurements of poloidal magnetic field fluctuations outside currentless, finite-beta (⟨β⟩ ≤ 0.5%, β 0 ≤ 3%) plasmas with peaked pressure profiles in the Advanced Toroidal Facility (ATF) torsatron reveal bands of small ( θ /B ϕ ∼ 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 B θ amplitudes increase with ⟨B⟩ for ⟨B⟩ ≲ 0.3%, and then saturate and begin to decrease with ⟨B⟩ for ⟨B⟩ ≳ 0.3%.