Electromagnetic turbulence in increased beta LAPD plasmas

The LArge Plasma Device (LAPD) at UCLA is a 17 meter long, 60 cm diameter magnetized plasma column with typical plasma parameters n_e ≈ 10¹² cm^-3, T_e ≈ 5 eV, T_i < 1 eV, and B₀ ≈ 1 kG when plasmas are produced using the primary BaO cathode source [1]. A new secondary plasma source has been installed, a 20 cm LaB₆ cathode that allows the production of much hotter (T_e ≈ 12 eV, T_i ≈ 6 eV) and denser (n_e ≈ 5 × 10¹³ cm^-3) plasmas [2]. This hundred-fold increase in plasma pressure combined with lowered magnetic field allows LAPD to be utilized to study the physics of magnetized, increased β plasmas. We will report the variation of turbulence and transport driven by edge pressure gradients in LAPD with increasing plasma β (up to ≈ 15%). As β increases, turbulence becomes more electromagnetic and magnetic fluctuations increase substantially. In particular, parallel magnetic fluctuations are seen to increase the most and are dominant at the highest β values with δ B_∥/δB_⟂ ≈ 2 and δB/B₀ ≈ 1%. The density and parallel magnetic field fluctuations are out of phase, and the magnitude of the fluctuations is consistent with pressure balance: δp ≈ -δ(B²/2μ₀) = - B₀ δ B_∥/μ₀. These observations are consistent with with the characteristics of the Gradient-driven Drift Coupling mode or GDC [3] which has been observed in gyrokinetic simulations.

[1] W. Gekelman, et al., Rev. Sci. Instr. 87 025105 (2016)

[2] C. Cooper, et al., Rev. Sci. Instr. 81 083503 (2010)

[3] M.J. Pueschel, et. al.,Plasma Phys. Control. Fusion 59 024006 (2017)