Experimental verification of entropy cascade in two-dimensional electrostatic turbulence in magnetized plasma

The wave number spectrum (one-dimensional spectrum) of electrostatic potential fluctuations at sub-Larmor scales was measured in two-dimensional (2D) electrostatic turbulence in laboratory magnetized plasma. The spectrum at scales k_⊥ρ_i>1, where k_⊥ and ρ_i are the fluctuation wave number perpendicular to the magnetic field and ion Larmor radius, respectively, supports the existence of the k ^-10^/3 inertial range of the entropy cascade induced by nonlinear phase mixing. This indicates agreement with a theoretical prediction [A. A. Schekochihin et al., Plasma Phys. Controlled Fusion 50, 124024 (2008)PPCFET0741-3335] and the result of a 2D gyrokinetic simulation [T. Tatsuno et al., Phys. Rev. Lett. 103, 015003 (2009)PRLTAO0031-9007]. The cutoff wave numbers of the spectrum, above which the entropy cascade is smeared by collisions, observed in this experiment were consistent with those in the theory.